TGA100 TRACE GAS ANALYZER USER AND REFERENCE MANUAL LAST REVISION: 2 August 2004 COPYRIGHT © 1992 - 2004, CAMPBELL SCIENTIFIC, INC.
TABLE OF CONTENTS 1 OVERVIEW 12 1.1 System Components 12 1.2 Theory of Operation 13 1.2.1 Optical System 13 1.2.2 Laser Scan Sequence 14 1.2.3 Concentration Calculation 15 1.3 Trace Gas Species Selection 15 1.4 Dual Ramp Mode 1.5 User Interface 1.6 Micrometeorological Applications 17 15 16 1.6.1 Eddy Covariance 17 1.6.2 Flux Gradient 1.6.3 Site Means 1.6.4 Absolute Concentration / Isotope Ratio Measurements 20 1.7 Specifications 18 19 21 1.7.
3.4 Real Time Screen 28 3.4.1 Screen Layout 3.4.2 Navigating and Editing 3.4.3 Run Mode 3.4.4 Dynamic Parameters 31 3.4.5 Detector Video 3.4.6 Functions 3.4.7 Graph Selections 32 3.4.8 Graph Display Limits 33 3.4.9 Quick Keys 33 3.5 29 30 30 32 32 Parameter Change Menu 35 3.5.1 Standard Parameter Screens 36 3.5.2 File Output Selection Screen 37 3.5.3 Analog Output Screen 38 3.5.4 Gradient and Site Means Screens 3.6 TGA Files 39 39 3.6.1 Parameter Files 39 3.6.
4.3 Finding the Absorption Line 52 4.4 Laser Mapping 53 4.5 Optimizing Laser Parameters 55 4.5.1 Laser Temperature 4.5.2 Zero Current 58 4.5.3 High Current 59 4.5.4 Omitted Data Count 4.5.5 Laser Modulation Current 63 4.5.6 Laser Maximum Temperature and Laser Maximum Current 63 4.5.7 Laser Multimode Correction 4.6 55 62 63 Optimizing Detector Parameters 64 4.6.1 Detector Gain and Offset 64 4.6.2 Detector Temperature 65 4.6.3 Detector Linearity Coefficients 65 4.
6 EDDY COVARIANCE MEASUREMENTS 6.1 Overview 6.2 Flow Rate and Tubing Size 86 86 88 7 AUXILIARY INPUTS AND OUTPUTS 90 7.1 Reading Data from a CSAT3 Sonic Anemometer 7.2 Reading Data from a CR9000 90 7.3 Sending Concentration Data to a CR9000 91 7.4 TGA Analog Inputs 91 7.5 PC Analog Inputs 7.6 Analog Outputs 92 7.7 Digital Outputs 93 7.8 Excitation Source 90 92 93 8 TGA100 OPTIONS 94 8.1 Laser Cooling 94 8.1.1 LN2DEWAR TGA100 LN2 Laser Dewar 8.1.
10 TROUBLESHOOTING 102 10.1 Fiber Optic Diagnostics 102 APPENDIX A.
1 OVERVIEW The TGA100 Trace Gas Analyzer measures trace gas concentration in an air sample using tunable diode laser absorption spectroscopy (TDLAS). This technique provides high sensitivity, speed, and selectivity. The TGA100 is a rugged, portable instrument designed for use in the field. It can measure one of a large number of gases by choosing appropriate lasers and detectors.
1.2 Theory of Operation 1.2.1 Optical System The TGA100 optical system is shown schematically in Figure 1-2. The optical source is a lead-salt tunable diode laser that operates between 80 and 140 K, depending on the individual laser. Two options are available to mount and cool the laser: the TGA100 LN2 Laser Dewar and the TGA100 Laser Cryocooler System. Both options include a laser mount that can accommodate one or two lasers. The LN2 Laser Dewar mounts inside the analyzer enclosure. It holds 10.
1.2.2 Laser Scan Sequence The laser is operated using a scan sequence that includes three phases: the zero current phase, the high current phase, and the modulation phase, as illustrated in Figure 1-3. The modulation phase performs the actual spectral scan. During this phase the laser current is increased linearly over a small range (typically +/- 0.5 to 1 mA). The laser’s emission wavenumber depends on its current.
1.2.3 Concentration Calculation The reference and sample detector signals are digitized and averaged over 50 consecutive scans. The average reference and sample scans are then corrected for detector offset and nonlinearity, and converted to absorbance. A linear regression of sample absorbance vs. reference absorbance gives the ratio of sample absorbance to reference absorbance.
1.5 User Interface The TGA100 includes a computer that provides the user interface. It displays the data in real time, allows the user to modify control parameters, and saves data to the hard disk. The real time graphics screen is presented in Figure 1-4. In the upper left corner is a box which displays the TGA software version, the laser and detector temperatures, and the time. Beneath the time and temperature display is a blank area used for information and error message display.
1.6 Micrometeorological Applications The TGA100 is ideally suited to measure fluxes of trace gases using micrometeorological techniques. In addition to its rugged design that allows it to operate reliably in the field with minimal protection from the environment, it also incorporates several hardware and software features to facilitate these measurements. 1.6.
1.6.2 Flux Gradient The TGA100 also supports the measurement of trace gas fluxes by the gradient method. The TGA100 automatically controls gradient switching valves and computes the mean concentration at each of the two intake heights. Timing parameters are entered by the user to control the gradient valves, typically switching between intakes every 5 to 20 s. The results are displayed on the TGA100 PC in real-time and stored on the hard disk. Figure 1-6 illustrates a typical gradient application.
1.6.3 Site Means The TGA100’s site means sampling mode is similar to the flux gradient mode in that it controls switching valves and calculates mean concentrations for each intake. The difference between the two sampling modes is that the gradient mode considers the sample intakes in pairs, switching several times between an upper and lower intake before moving to another site, but the site means mode considers all of the intakes as one group.
1.6.4 Absolute Concentration / Isotope Ratio Measurements The TGA100 can be configured for highly accurate measurements of trace gas concentrations by performing frequent calibration. The TGA100 has a small offset error caused by optical interference. This offset error changes slowly over time, with a standard deviation roughly equal to the short-term noise.
1.7 Specifications 1.7.1 Measurement Specifications Sample Rate: 10 Hz Averaging Period: 0.1 sec Sample cell volume: 480 ml Frequency Response (@ 4.8 liter/sec actual flow rate): 3 Hz The TGA100 frequency response is determined by the averaging time (0.1 s) and the time for a new sample to fill the sample cell. The frequency response was measured at 14.4 slpm flow rate and 50 mbar sample pressure (4.8 actual l/s) by injecting 1 µl of N2O into the sample stream.
Typical performance for isotope ratio measurements is given in delta notation. For example, the δ13C for CO2 is given by: Rs − 1 × 1000 RVPDB δ 13C = where Rs is the ratio of the isotopomer concentrations measured by the TGA100 (13CO2/12CO2) and RVPDB is the standard isotope ratio (13C/12C). δ13C is reported in parts per thousand (per mil or ‰). The 10 Hz noise is the square root of the Allan variance with no averaging.
2 INSTALLATION The basic components required to operate the TGA100 are shown in Figure 2-1. Other components, such as a sample air dryer, valves to switch between multiple intakes, calibration gases, etc. may also be required, depending on the user’s application. These optional components will be discussed in other sections. TGA100 Analyzer TGA100 PC Fiber Optic Cable Reference Gas Connection Suction Hose Sample Intake Reference Gas Sample Pump Figure 2-1.
3) Connect the sample intake to the sample gas inlet. The sample intake should be filtered to remove particulates (10 µm maximum pore size) and should have an appropriate needle valve or fixed orifice to control the sample gas flow and pressure. 4) Connect power. For older units, connect a user-supplied, regulated 12 Vdc supply with at least 5 ampere capacity to the system enclosure POWER IN connector.
b) Newer TGAs have a transputer board with a single “D” connector, and a single cable assembly to make this connection. 5) Connect the 7996 I/O terminal board (if needed) to the optional 7996 I/O board in the TGA PC. 2.3 Routine Operation Once the TGA100 has been set up, it should be checked periodically to verify proper operation, download data files, and fill the laser dewar with liquid nitrogen, if necessary. This section gives suggestions for routine operating procedures. 2.3.
3) Exit the Real Time display mode. 4) Exit the TGA program. 5) Shut off power to the TGA PC and monitor. 6) Shut off the TGA sample pump. 7) Shut off power to the TGA enclosure. 8) Shut off the reference gas supply. 9) Shut off the air gap purge supply, if applicable. 10) Shut off calibration gas supplies, if applicable. If the TGA100 is not to be operated for an extended period, allow the laser to warm up.
3 TGA SOFTWARE 3.1 General The TGA software runs on the TGA PC. It provides the user interface to the TGA100, allowing the user to view the operation of the TGA, set parameters, and collect data. The TGA program actually is a set of three programs that run concurrently on three computers, communicating in real time. The first computer is the TGA PC itself. It runs the user interface and data storage functions of the TGA software.
The functions available at the main menu are described below. R) Real Time TGA Program Turns the TGA on and displays the real time screen. This is the normal operating mode. See section 3.4 for additional information. T) TGA on/off Toggles the TGA on or off. When the TGA is on, all current and temperature controls are active and concentration calculations are being made, but the real time screen is not displayed, and no data are saved to the hard disk. L) Laser on/off Toggles the laser on or off.
3.4.1 Screen Layout The real time graphics screen is presented in Figure 3-2. In the upper left corner is a box which displays the TGA software version, the laser and detector temperatures, and the time. Beneath the time and temperature display is a blank area used for information and error message display. The rest of the top of the screen has five menu columns: run mode, dynamic parameters, detector video, special function enable/disable, and graph selections. Figure 3-2.
3.4.2 Navigating and Editing The keys are used to cycle through the following menus: RUN MODE, PARAMETER, DET VID, FUNCTION, GRAPH 1, GRAPH 2, GRAPH 3, DETECTORS, Graph 1 display scale, and Graph 2 display scale. The heading for the current menu is highlighted. The active option within each menu is also highlighted. The keys are used to select a specific option (marked with an asterisk “*”) within the selected menu and the key is used to activate the option.
3.4.4 Dynamic Parameters The next menu column, labeled “PARAMETERS”, provides access to the dynamic parameters, i.e. those that may be changed in real time. The keys are used to scroll through the list of dynamic parameters (listed in Table 3). If the Run/Edit mode is selected, the value of the selected dynamic parameter may be changed using the keys.
3.4.5 Detector Video This next menu column, labeled “DET VID”, is used to select the display mode of the processed detector data in the two bottom-right displays. The display mode may be selected either by pressing the corresponding Quick key or by highlighting the selection using the keys and pressing . Each option is discussed below. Magnify displays the reference and sample transmittance, scaled to the maximum and minimum of the data used in the concentration calculation (i.e.
Table 4. Detector Graph Display Options 3.4.8 DETECTORS Description Ramp A Ramp A, reference and sample detectors Ramp B Ramp B, reference and sample detectors Alt A&B Alternate between Ramp A and Ramp B, reference and sample detectors RefDet A&B Reference detector, ramp A and B. SmpDet A&B Sample detector, ramp A and B. Graph Display Limits The Y-axis limits for graphs 1, 2, and 3 are displayed in the upper right and lower right corners of each graph.
Table 5. Quick Key Summary Key Function I Turn line locking on/off for ramp A Alt-I Turn line locking on/off for ramp B. Available only when dual ramp mode is on.
3.5 Parameter Change Menu The system parameters are stored in the file, TGAPARM.CFG, which is read when the program is loaded. TGAPARM.CFG is updated at the end of real time operation to maintain a current parameter set, and a new file MMDDHHMM.gas (gas is a parameter) is written when data collection is started, maintaining a history for future reference (see section 3.6.1). The parameter change menu may be used to change system parameters.
3.5.1 Standard Parameter Screens Most of the parameter screens have three columns, containing the parameter name, value, and allowable range, as shown in Figure Figure 3-4. The selected parameter value is highlighted and a corresponding prompt is displayed at the bottom of the screen. Use the keys to select the parameter to be edited. To change the selected parameter’s value, type the new value and press or the keys.
3.5.2 File Output Selection Screen The File Output Selection screen selects which data will be included in the 10 Hz data file. It has four columns, containing the on/off indicator ([X] to save data, or [ ] to skip), the description, the present value, and the units. The present value and units are displayed only if the TGA is on. An example of the File Output Selection screen is shown in Figure Figure 3-5. To toggle whether the selected parameter should be saved or not, type , , or ‘X’.
3.5.3 Analog Output Screen The Analog Output screen allows the user to configure the analog output channels (see section 0). Use the keys, or the and keys to move to the field to be changed. To change which data will be output, highlight the desired channel, and type . A new menu will appear that will show the options available for output.
3.5.4 Gradient and Site Means Screens The Site Means and Gradient screens allow the user to edit valve switching parameters (see section 5). These screens are organized in rows and columns, with one row for each site, as shown in Figure 3-7. The user can navigate to each field using the keys, or the and keys. Type the number and press or an arrow key to change a selected field. Figure 3-7. Example Site Means Screen 3.
Three functions are available at the Parameter File Operations screen to help the user manage parameter files: S Save parameters to user-specified file R Read user-specified parameter file D Document parameters (create tgaparm.doc file) The Save and Read functions allow the user to store and recall a particular TGA setup. The Document function creates file tgaparm.doc, which documents the parameters in a format very similar to the parameter editing screens.
Table 6. Gradient File Contents Day Day of year, with January 1 written as 1 Time Time of day, in 24 hour format. For example 6:15:01 AM is written as 06:15:01, and 6:15:01 PM is written as 18:15:01. Site Site number, from 1 to 18. M/S ID Master/slave identifier. This column includes a number, 0 through 4, to identify data from the master TGA100, which always controls the sampling system, and slave TGA100s, which may share a sampling system as described in section 5.3.
Table 7. Site Means File Contents Day Day of year, with January 1 written as 1 Time Time of day, in 24 hour format. For example 6:15:01 AM is written as 06:15:01, and 6:15:01 PM is written as 18:15:01. Site Site number, from 1 to 18. M/S ID Master/slave identifier. This column includes a number, 0 through 4, to identify data from the master TGA100, which always controls the sampling system, and slave TGA100s, which may share a sampling system as described in section 5.3.
3.6.6 Header Files Any time a concentration, gradient, or site means file is created, a header file called MMDDHHMM.hdr is also created, unless a header file is already open. The file may already open, for example, if 10 Hz concentration data collection is started, and then either the gradient or site means mode is started. In this case the same header file is used, with additional lines written to give information for the second data collection mode.
4 DETAILED SETUP INSTRUCTIONS When the TGA100 is first installed, or if it is reconfigured (with a new laser, for example) the operational parameters must be set for optimal performance. This section gives detailed instructions to set up the TGA100. If the TGA100 has already been configured, see section 2.3.1 for routine startup instructions. 4.
Table 9. Suggested Reference Gas Concentration Gas Species Concentration (ppm) Methane (CH4) 10,000 – 20,000 Nitrous Oxide (N2O) 1500 – 2500 Ammonia (NH3) 4000 – 6000 Carbon Dioxide (CO2) isotopic ratios 50,000- 100,000 Other Contact Campbell Scientific Generally, any value in the range given should be acceptable. A high concentration is recommended if a relatively weak absorption line is used, or if the TGA is to be operated at very low pressure.
4.1.4 Air Gap Purge For isotope ratio applications, the air gap between the dewar and lens and the short sample cell should be purged as shown in Figure 4-1. This is not required for most trace gas applications, where the ambient concentration is very low, and there is very little absorption. The sample cell is at low pressure, making the sample absorption very narrow compared to the pressure-broadened ambient absorption.
4.2 Optical Alignment The TGA100 has a simple, robust optical design that makes it easy to adjust and maintain its optical alignment. The optical system, illustrated in Figure 4-2, includes the laser, a collimating lens in front of the laser, a beamsplitter to reflect some of the laser’s energy onto the reference detector, and two focusing lenses mounted in front of the sample and reference detectors. Reference detector Figure 4-2.
Horizontal adjustment screw Tighten only when transporting Horizontal lock screw Axial lock screw Figure 4-3. Alignment Hardware - Detector End Horizontal adjustment screw Horizontal clamping screws Vertical adjustment screw Axial clamping screw Vertical clamping screw Vertical clamping screw Figure 4-4.
If the TGA is equipped with an iris in front of the focusing lens, open it fully (this is recommended for normal operation – see section 4.5.2). 3) Start the TGA program and make sure the laser and detector parameters are set appropriately for the laser. If the laser has not been changed this normally means just using the parameters that were loaded automatically at startup. If switching to another laser, this normally means reading in a parameter file that was previously used with the laser.
the long sample cell back (away from the dewar) about 5 mm from the center of its adjustment range and retighten the axial clamping screw. Alternately adjust the horizontal and vertical adjustment screws. 6) If no detector response can be found, perform the following checks: • If two lasers are installed, verify you are aligning to the correct laser. • Verify the dewar cable is installed correctly. If two or more lasers are installed, verify you are using the correct cable.
4.2.4 Focus Adjustment The optical system includes the long sample cell, with the lens holder at the dewar end, and the beamsplitter and detectors at the other end. To focus the system, this entire assembly is moved closer or farther away from the dewar. 1) To adjust the focus, first note the sample detector signal at the current focus position. Then loosen the axial clamping screw, slide the optical assembly either forward or back a short distance (~2 mm), and retighten the axial clamping screw.
Finding the Absorption Line The TGA’s spectral scan must be locked onto a selected absorption line. When the TGA is restarted, its spectral scan position is set by the parameters stored in the default parameter file. After the laser temperature has stabilized, the previously selected absorption line should be visible in the detector response. Note that it may take a minute or two for the actual laser temperature to stabilize completely after the displayed temperature is stable.
If a new laser is being tested, or if there is any doubt about the identity of the absorption lines, first turn the dual ramp mode off, and find the ramp A absorption line as described above. Then start dual ramp mode and select the “RefDet A & B” detector display option.
Return to the Main Menu, select the Laser Mapping Menu, and then select the File Format screen. • Set the File Format to ASCII or binary, as desired. • Set the Concentration File Decimation Factor to 1 • Set the Interval to start new data files (min) to 0 3) Return to the Laser Mapping Menu and select the File Output Selection screen. Select the following data to be saved to the 10 Hz file (additional data may be selected if desired).
When the upper current is reached, the data file will be closed and the laser temperature will be incremented. A new file will be collected for each laser temperature, and each file will contain a scan of the DC current. The data files can then be processed by the user to make a plot of reference detector signal vs. DC current. 4.4 Optimizing Laser Parameters Normally the laser parameters are adjusted only when a new laser is installed, or after transporting the system or warming and recooling the laser.
Table 10. Example Laser Temperature Optimization Data Laser Laser DC Reference Temperature (K) Current (mA) Transmittance (%) Concentration Noise (ppb) Sample Signal (mV) 103.7 482.6 65.9 7 103.9 477.4 66.2 8 104.1 472.1 66.4 9 104.3 466.6 67.3 10 104.5 460.8 69.1 13 104.7 454.9 72 20 104.9 448.7 78.2 45 105.1 442.2 90.5 350 Transmittance and noise much worse - try going down. 103.5 487.7 65.8 7 103.3 492.6 67.7 6 103.1 497.5 77.8 15 102.9 502.9 89.
Reference Transmittance (%) Next, look at the reference detector transmittance as a function of temperature. The transmittance should have a minimum at the (optimum) laser temperature. It should be higher at temperatures above and below the optimum temperature. This increased transmittance results from an increased fraction of the laser’s energy at undesired frequencies (multimode operation). Again, it is usually not necessary to plot the data, but Figure 4-7 shows a typical example.
Sample Signal (mV) In some cases the minimum concentration noise may be at a different laser temperature than the minimum reference transmittance. First, if the DC current is near the laser threshold current, the laser’s optical power output may be reduced significantly at higher laser temperatures (lower DC current). This can be verified by looking at the sample detector signal as a function of laser temperature. This is shown in Figure 4-9 for our example. 45 40 35 30 25 20 15 10 5 0 102.5 103 103.
response again increases, the algorithm reduces the zero current to just below the threshold current. After the algorithm terminates, reenable automatic control of detector offset and gain, and restart line lock. For dual ramp mode, the same value of the zero current is used at the start of ramp A and ramp B. The process described above should set the zero current to a value that is acceptable for both ramps. 4.4.
The goal of the high current adjustment procedure is to make the movement of the absorption line symmetrical about its center line as possible, for equal changes in the DC current. The undesired asymmetry is caused by the laser temperature perturbation. As the laser warms up at the start of the ramp, it warms more quickly at first, and then more slowly as it approaches equilibrium.
8) Press “I” to start line lock again. Watch the reference detector display as the lines to come to the center of the spectral scan.
Ramp B Offset Laser Current Detector Response 13 CO 2 12 Ramp A CO 2 Ramp B Figure 4-12. Dual Ramp Scan Sequence To set the ramp B high current, first set the (ramp A) high current as described in section 4.4.3. Then repeat the process for the ramp B high current. When evaluating the ramp B high current setting,, observe the ramp B line instead of the ramp A line, and use both the ramp A line lock and the ramp B line lock to center the absorption lines.
4.4.5 Laser Modulation Current The laser modulation current parameter controls the width of the spectral scan. In most cases, the spectral scan will include a single absorption line. This is generally preferred, but is not required for proper operation. If a group of two or more lines is used, make sure the line locking is enabled, and then adjust the modulation current as needed to include the entire group of lines.
if there is another absorption line near by. Second, too much absorption will increase the chances of absorbing the multimode power in some other absorption lines of the gas. To achieve the optimum amount of absorption, first note the reference transmittance with reference gas in the reference cell only (the normal configuration.) Normally the reference gas concentration is chosen to give approximately 50% absorption. However, for this test, it is best to have between 75% and 85% absorption.
same group as the sample detector gain, all that is needed is to select the desired gain within the group. Therefore, the reference detector gain parameter has a range of 0 to 7, and a gain of 1 will provide an actual gain of 278, 698, 1396, etc., depending on the setting of the sample gain. The detector offset provides 0 to 40 mV of offset at the input of the detector preamplifier and is used to center the detector signal in the input range, allowing the detector gain to be maximized.
transmitting most of the optical power to the sample detector and reflecting less than 10% onto the reference detector, and 2) the reference detector is adjusted (see section 4.5.2) to give a relatively low response. The reference detector may have a small amount of nonlinearity, but this tends to be cancelled by setting the sample detector linearity coefficient so that the sample detector matches the reference detector, as described below.
5 SAMPLING SYSTEM CONTROL The TGA software can control sampling system switching valves and process the 10Hz concentration data to calculate the mean concentration for multiple air sample intakes. There are two different sampling system control modes: gradient mode and site means mode. In the gradient mode, the air sample intakes are always considered as pairs (two levels at one or multiple sites).
Figure 5-1 illustrates a typical flux gradient measurement at one site. Two intake assemblies are mounted at different heights on the measurement mast. Tubing connects each intake assembly to a gradient valve assembly that selects one of the intakes at a time. The air sample from the selected intake flows through the sample air dryer, which filters and dries the air sample. A needle valve at the outlet of the dryer sets the sample flow rate, typically 5 to 10 slpm.
the volume of the sample tubing and the sample flow rate, and can be several seconds. The user must set the Shift Samples parameter to accommodate this time delay. If the sampling system includes multiple sites, the Shift Samples may be different for each site, because the flow rate and/or sample tube length (volume) may be different.
discarded. The Discard Scans column of the gradient parameter menu can be adjusted to discard as many level scans as necessary. More information on setting up multi-site gradient parameters may be found in section 5.1.7.3. The mean concentration, standard deviation of concentration, and mean pressure are calculated using all of the valid data for each level (taking into account the shifted samples, the omitted samples, and the discarded scans).
Example 1: The gradient valve assembly uses a pair of nonlatching two-way valves. The level 1 intake is connected to the level 1 valve, the level 2 intake is connected to the level 2 valve, and the output ports of both valves are connected by a tee to the TGA100 sample inlet. If the control signal to a valve is low, it will be closed; if the control signal is high, it will be open. This example is illustrated in Figure 5-3.
Example 2 illustrates the use of the Pulse Samples and Invert digital output bits parameters (see Figure 5-4). For this example the gradient valve is a latching type three-way valve. The level 1 intake is connected to valve inlet port 1, the level 2 intake is connected to valve inlet port 2, and the valve output port is connected to the TGA100 sample inlet.
that site’s gradient valve assembly to a corresponding inlet on the site selection assembly located near the TGA100. The site selection assembly consists of four nonlatching three-way valves, one for each site. The outlets of the four valves are connected to a manifold that is connected to the TGA100 sample inlet. The TGA100 controls the site selection system using timing parameters supplied by the user. Normally each site is measured for 15 to 60 minutes before switching to the next site.
2) In the Gradient Mode Parameters screen, put the following values in the Site Time column: sites (rows) 1, 2, 3, and 4 should be set to 15 minutes, and all other sites (rows) should be set to zero. This configures the system to cycle through the four sites once each hour, sampling from each site for 15 minutes. 3) In the Site I/O Bits column, set site (row) 1 to xxxxxxxxxx0001xx, where ‘x’ means it can be either 0 or 1.
The top row of the Gradient Mode Parameters screen has three parameters that control the timing of the gradient valve switching for both levels and all sites. The first parameter, Samples/Level, is the time spent at each of the levels (one and two) before switching to the other level. It is entered as the number of 10 Hz samples, so the actual duration in seconds will be 0.1 x Samples/Level. This parameter applies to both level 1 and 2 - the time spent at each of the two levels will always be the same.
5.1.7.1 Flow Rate The sample flow rate should be high enough to minimize the equilibration time after valve switching, but low enough to avoid a possible bias between sample intakes. Two issues that can affect the equilibration time are the time to replace the sample in the sample cell, and mixing in the tubing. The time to replace the sample in the sample cell depends on the actual flow rate and the volume of the sample cell: t = v/q where t is time, v is volume, and q is actual flow rate.
1000 12 900 10 8 700 Time (s) Pressure (mB) 800 600 500 6 4 400 300 2 200 0 100 0 10 20 30 40 50 60 70 80 90 0 100 10 20 30 40 1 slpm 2 slpm 3 slpm 50 60 70 80 90 100 Tube Length (m) Tube Length (m) 5 slpm 1 slpm 7 slpm 2 slpm 3 slpm 5 slpm 7 slpm Figure 5-7. Pressure Drop and Travel Time for 0.125" (3.
1000 60 900 50 40 700 Time (s) Pressure (mB) 800 600 500 30 20 400 300 10 200 0 100 0 50 100 150 200 250 300 350 400 0 50 100 5 slpm 7 slpm 11 slpm 16 slpm 150 200 250 300 350 400 Tube Length (m) Tube Length (m) 5 slpm 26 slpm 7 slpm 11 slpm 16 slpm 26 slpm Figure 5-10. Pressure Drop and Travel Time for 0.375" (9.5 mm) ID Tubing Two examples are given as a guide in selecting tubing for gradient applications.
inlet end of the sample tube and approximately 7 s travel time. The travel time is nearly the same for the two tubing sizes, so either one should give acceptable results. The smaller tubing has the advantage of being near the optimum Reynolds number, making the travel time relatively insensitive to changes in flow rate, but it is at the very limit of being able to carry the full 4 slpm.
7) If multiple sites are used, the Shift Samples must be set individually for each site. The Omit Samples and Samples/Level parameters are common to all sites, so they must be set large enough to accommodate all sites. An additional complication may be introduced by switching from one site to another. The site valves will switch from one site to another at the start of a level scan, when the gradient valve assemblies switch from level 2 to level 1.
Digital Control Cable Purge Dryer Sample Dryer Sample Sample Sample Intakes Site Selection Sampling System Dryer Purge Bypass Figure 5-11. Example Site Means Application: 8-level Vertical Profile 5.2.2 Site Means Calculations The TGA100 calculates the mean concentration, the standard deviation of the concentration, the concentration rate of change (slope), and the mean pressure for each site.
Valve Switch Site 1 Valid Site 4 Site 2 Valid Site 3 Valid Site 4 Valid Concentration Valve Switch Valve Status Valve Switch significant bit (bit 0) is on. At the solid vertical line near the left edge, the valve status changes to 2, indicating bit 0 has turned off and the next bit (bit 1) has turned on. These digital output bits are used to control the site selection valves. In this example it is assumed that bit 0 selects site 1, bit 1 selects site 2, etc.
5.2.3 Real Time Display When the site means mode is active, vertical lines are drawn on graph 1 and graph 2 to mark the time of critical events (see Figure 5-12). When a sampling system valve switches at the start of a new site, a solid vertical line marks the time. A vertical dotted line marks the end of the shift samples, and a vertical dashed line marks the end of the omitted samples.
5.2.5 Site Means Parameters Many parameters must be set to control the site means sampling system and calculations. These parameters are edited at the Site Means Mode Parameters screen, shown in Figure 5-13. Each of these parameters is discussed in this section. Figure 5-13. Example Site Means Mode Parameters Screen The top row of the Site Means Mode Parameters screen has the Output Interval parameter. This is the interval, in minutes, for calculating and saving data to the site means file.
of the table. They are also affected by the Invert digital output bits parameter in the Miscellaneous Valve Control Parameter screen, which defines whether an active bit will have a high voltage or a low voltage. Pulse Samples: The duration (number of samples at 10 Hz) of the site means switching pulse. For nonlatching valves, enter "-" to keep the bits energized during the entire site time. This parameter applies to all of the Site I/O Bits (bits 0 through 15). 5.
Two parameters must be set for proper master/slave operation. First, each TGA100 must be identified as the master or as slave 1 to 4. This is done by entering the Miscellaneous Valve Control menu and setting the Master/Slave designation parameter as appropriate (0 for the master, 1 for slave #1, 2 for slave #2, etc.) This parameter must be set in each TGA100. If there are no slaves attached (standalone operation), set this parameter to 0. Second, set the Number of slaves attached to this TGA parameter.
CSAT3 Cable Figure 6-1.
6.2 Flow Rate and Tubing Size It is important to maintain the high frequency response for eddy covariance measurements. The frequency response of the analyzer itself is discussed in section 1.7.1. The other consideration is to avoid the loss of high frequencies by mixing in the tubing from the air sample intake to the analyzer. Using a high flow rate and a minimum length, small diameter sample tube will help to preserve the high frequency variations in the trace gas concentration.
27 900 24 800 21 700 18 Time (s) Pressure (mB) 1000 600 500 15 12 400 9 300 6 200 3 0 100 0 40 80 120 160 200 240 280 320 0 40 80 13 slpm 17 slpm 22 slpm 28 slpm 120 160 200 240 280 320 Tube Length (m) Tube Length (m) 35 slpm 13 slpm 17 slpm 22 slpm 28 slpm 35 slpm Figure 6-5. Pressure Drop and Travel Time for 0.375" (9.
7 AUXILIARY INPUTS AND OUTPUTS The TGA100 has three options for digital communication with other devices: sending concentration data to a CR9000 data logger, reading data from a CSAT3 sonic anemometer, and reading data from a CR9000 data logger. It also has four analog inputs and a 5V excitation output inside the analyzer enclosure. The optional 7996 Input/Output Board can be installed in the PC to provide eight additional analog inputs, two analog outputs, and sixteen digital outputs.
To read data from a CR9000, complete the following steps: 1) Connect a cable (CSI P/N 10847) from the RS422 Link connector on the TGA 9030 CPU card to the TLink connector on the CR9000 9031 CPU card. 2) Start the TGA program and from the Main Menu, select the Parameter Change Menu and then the File Format screen. 3) Set the Output conc data to additional device? parameter to 0. 4) Set the Read data from additional device? parameter to 2. Every 0.
TGA analog input four is configured for a pressure transducer to measure the pressure in the vacuum manifold at the outlet of the sample and reference cells. Its measured voltage is converted to pressure using the parameters in the Pressure Calculations screen. The measured pressure may be viewed on the real time screen by selecting Pressure for display in graph 1, 2, or 3, and it will be written to the 10 Hz file if selected for file output on the File Output Selection screen. 7.
v = −10 + (1.8 − 0) (10 − (−10)) = −2.8 (5 − 0) This voltage can be converted back to a concentration as follows: x = 0 + ((−2.8) − (−10)) (5 − 0) = 1.8 (10 − (−10) It is extremely important when using the analog outputs to set the data value limits carefully. If the data value exceeds the voltage range, the analog output will be invalid. However, if the data value limits are set too wide, the 12-bit voltage resolution may not preserve the resolution of the data value.
8 TGA100 OPTIONS 8.1 Laser Cooling The laser must be cooled to as low as 80 K, depending on the individual laser. Two options are available to mount and cool the laser: the TGA100 LN2 Laser Dewar and the TGA100 Laser Cryocooler System. Both options include a laser mount that can accommodate one or two lasers. An optional second laser mount is available if more than two lasers are to be mounted in the same dewar. Contact Campbell Scientific for details on the second laser mount 8.1.
8.2 Lasers The TGA100 uses a lead-salt tunable diode laser. These lasers are available from 1000 to 3250 cm-1. Each gas species has a unique set of absorption lines, and tunable diode lasers have limited tuning ranges. Therefore, in most cases a different laser is required for each gas species to be measured. The laser dewar can accommodate one or two lasers, allowing the user to select a different gas without opening the dewar to install a different laser.
9 TGA100 ACCESSORIES 9.1 TGA100 Insulated Enclosure Cover The TGA100 insulated enclosure cover is recommended when the TGA100 is operated in the field without additional shelter. The cover has a rain-proof, white exterior to reflect the sun’s heat, and additional insulation to dampen diurnal temperature fluctuations. It fits over the TGA100, attaching with integral hook-and-loop fasteners.
30 25 Flow Rate (slpm) 20 60 Hz 50 Hz 15 10 5 0 0 10 20 30 40 50 60 70 80 Pressure (mbar) Figure 9-1. RB0021 Sample Pump Flow Rate 9.4 Sample Air Dryers Accurate measurements of trace gas fluxes by eddy covariance or gradient techniques require that variation in water vapor concentration be eliminated either by drying the sample gas before it is measured or by correcting the trace gas flux (Webb, E.K., Pearman, G.I. and Leuning, R.
The PD625 is similar to the PD1000, but it is designed for lower flow rates. Its 50-tube, 24" Nafion® dryer element has a drying capacity one eighth that of the PD1000. The PD625's inlet filter, tubing connections, and purge flow meter range are also smaller than for the PD1000. See Table 14 to compare specifications for the two dryers. The PD625 is normally used in the two-dryer configuration, so it does not include the sample flow meter or sample needle valve included in the PD1000. Table 14.
Figure 9-3. Purge connection for horizontal installation (left) and vertical installation (right) 9.4.3.2 Sample Inlet Filter Install a filter element (10 µm maximum pore size) in the filter holder at the sample inlet. The PD1000 uses a 47 mm membrane filter holder, and is shipped with a box of 100 spare filter elements. Be sure to use the filter elements (white) and not the separator papers (light blue). Additional filter elements can be ordered from a laboratory supply vendor (Pall Corp.
The purge flow (actual flow rate) must be at least twice the sample flow to achieve a very dry sample. The following equation gives the minimum purge flow (standard flow rate) to meet this requirement: Vp ≥ 2 Pp Pt Vt Where V p is the purge flow rate, Vt is the total flow rate, Pt is the pressure of the total flow through the dryer, and Pp is the purge pressure. The flow rates in this equation are standard flow rates (at one atmosphere) and the pressures are in absolute units.
Purge dryer Ambient air in (2.5*X slpm) 0.5*X slpm Open (or remove) needle valve From manifold outlet Sample dryer Sample air in (X slpm) Cap this outlet (or remove tee) 2*X slpm To vacuum pump Open (or remove) needle valve To TGA100 sample inlet Figure 9-4. Two-Dryer Configuration The PD1000 dryer comes from the factory configured for the split-sample mode.
10 Troubleshooting If for some reason the PC transputer cannot communicate with the chassis electronics, a message similar to the one below will be displayed. Link open pserver.exe -v tga transputer reset booting from tga.run pserver: timeout booting tga.run after 0x44a bytes 1Kb booted Exit code = 0 1 Program not loaded, aborting attempt If this occurs at startup: Try to start again. Verify the analyzer 12 V power supply voltage.
Note that the loopback program simply checks a closed-loop path for communication effectiveness. The above steps are given to help the user troubleshoot the entire cable system, but any fiber optic cable may be attached to the link adapter to check for damage. For example, if the orange part of the long outdoor cable is suspected, simply connect up both ends of the orange line to the PC link adapter and run pserver loopback, link 1. Also, a crude check may be done with a flashlight (or sunlight).
Appendix A. Options for File Save and Real Time Display Display Option Description Notes 10 Hz Conc Trace gas concentration, in ppm. 1 Mean Conc Mean concentration measurements (ppm). If in site means or gradient mode, the mean concentration is calculated at each valve switch. Otherwise the averaging period is specified by dynamic parameter #12 (see section 3.4.4). 1 Conc StDev Standard deviation of the concentration (ppb), calculated over the period specified by dynamic parameter #12.
Display Option Description dynamic parameter #12. (see section 3.4.4). Notes IsotopeRatio Isotope ratio (‰) calculated from concentrations for ramp A and ramp B. Note that if Rstandard is set to zero, then the isotope ratio is set to zero. 2 Mean Ratio Isotope ratio (‰), calculated from mean concentration for ramp A and ramp B 2 Ratio StDev Standard deviation of the isotope ratio (‰), where the averaging period is specified by dynamic parameter #12 (see section 3.4.4).
Display Option Description Notes M/S 3 Conc Trace gas concentration (ppm) of TGA slave #3 1, 5 M/S 4 Conc Trace gas concentration (ppm) of TGA slave #4 1, 5 M/S 1 Conc B Trace gas concentration (ppm) of TGA slave #1, ramp B 2, 5 M/S 2 Conc B Trace gas concentration (ppm) of TGA slave #2, ramp B 2, 5 M/S 3 Conc B Trace gas concentration (ppm) of TGA slave #3, ramp B 2, 5 M/S 4 Conc B Trace gas concentration (ppm) of TGA slave #4, ramp B 2, 5 Notes: 1) If in dual ramp mode the value will
Appendix B: Default Parameter File TGA Parameters File -----------------------------------------TGA100 Version #: 6.07 Parameter File Version #: 9.00 Parameter Menu: Laser Parameters --------------------------------------------Laser operating temperature (K) 0.00 [0..310] Laser DC current (mA) 0.00 [0..1000] Laser Modulation current (mA) 0.00 [0..100] Laser Zero current (mA) 0.00 [0..1000] Laser High current offset (mA) 0.00 [-200..
REF thermistor gain REF thermistor offset 4369.0000 [1000..4000000] 339.7490 [-10000..10000] REF PID control gain 0.0250 [-10..10] REF PID control tau 4.0000 [-100..100] Parameter Menu: Ramp B Parameters --------------------------------------------Dual ramp control: 0=off, 1=on 0 Ramp B Gas mnemonic GasB Ramp B Reference gas conc (ppm) 0.00 Standard isotope ratio Heavy isotope in Ramp A (0) or B (1) [0..1] [0..9999999] 0.0000000 [0..1] 1 [0..1] Ramp B offset current (mA) 0.
Parameter Menu : Gradient Data --------------------------------------------Gradient mode samples/level 100 [10..3000] Gradient mode omit samples 10 [1..3000] - [0..3000] Gradient valve pulse samples Site Number Site Time [0..1440] Discard Scans Shift Level Site Valve Pulse Samples Bits Bitmasks Samples [1..3000] [0..3000] [0..864000] 1. 1 1 0 11 0000000000000000 - 2. 0 1 0 11 0000000000000000 - 3. 0 1 0 11 0000000000000000 - 4.
Parameter Menu H : Site Means --------------------------------------------Site Means output interval (min) Site Number Site Samples [0..3000] 10 [1..1440] Omit Shift Site Valve Pulse Samples Samples Bitmasks Samples [1..3000] [0..3000] [0..3000] 1. 100 1 0 0000000000000000 - 2. 0 1 0 0000000000000000 - 3. 0 1 0 0000000000000000 - 4. 0 1 0 0000000000000000 - 5. 0 1 0 0000000000000000 - 6. 0 1 0 0000000000000000 - 7. 0 1 0 0000000000000000 - 8.
Parameter Menu: File Format Parameters --------------------------------------------File format: ASCII (0) or binary (1) 1 [0..1] Concentration File Decimation Factor 1 [1..864000] Interval to start new data files (min) 0 [0..10080] European date format (0=no, 1=yes) 0 [0..1] Site Means and Gradients to printer 0 [0..1] Printer Code 0 [0..3] Read data from additional device? 0 [0..2] Output conc to additional device? 0 [0..
Detector Data to Save to Disk ----------------------[ ] Ref Det Signal (graph: 0.00 to 60.00 mV ) [ ] Ref Det Trans (graph: 0.00 to 100.00 [ ] Ref Det Temp (graph: 0.00 to 60.00 øC ) [ ] Ref Det Peltier (graph: 0.00 to 13.00 [ ] Ref Det Gain (graph: 0.00 to 0.00 ) [ ] Ref Det Offset (graph: 0.00 to 0.00 ) [ ] Smp Det Signal (graph: 0.00 to 60.00 mV ) [ ] Smp Det Trans (graph: 0.00 to 100.00 [ ] Smp Det Temp (graph: -25.00 to 0.00 øC ) [ ] Smp Det Peltier (graph: 0.
Other Device Data to Save To Disk, Page 5 ----------------------[ ] Other Device 1 (graph: 0.00 to 10.00 ) [ ] Other Device 2 (graph: 0.00 to 10.00 ) [ ] Other Device 3 (graph: 0.00 to 10.00 ) [ ] Other Device 4 (graph: 0.00 to 10.00 ) [ ] Other Device 5 (graph: 0.00 to 10.00 ) [ ] Other Device 6 (graph: 0.00 to 10.00 ) [ ] Other Device 7 (graph: 0.00 to 10.00 ) [ ] Other Device 8 (graph: 0.00 to 10.00 ) [ ] Other Device 9 (graph: 0.00 to 10.
Parameter Menu: Serial Number Parameters --------------------------------------------TGA100 S/N Laser S/N Dewar S/N Sample detector S/N Reference detector S/N Site name/description Parameter Menu: Pressure Calculation Parameters --------------------------------------------Pressure transducer zero output (V) 0.000 [-5..5] Pressure xdcr full-range output (V) 5.000 [-5..5] Press xdcr full-range pressure (psia) 50.00 [1..50] Units for pressure measurement 2 [1..
