User manual

1-14 Signal Management 886995 DBK Option Cards and Modules
Calculation of Scale and Offset
Using the above information, calculate the values of scale (m) and offset (b) that will compensate for the
measurement errors (RA1 and RA2). This is possible because the correct and actual readings are related by
the (mx + b) equations:
RC1 = m(RA1) + b
and
RC2 = m(RA2) + b
Substituting in the values noted in the above calibration process:
RC1 = m(RA1) + b
0 = m(2.1) + b
RC2 = m(RA2) + b
100 = m(104) + b
Solving for “m” results in:
100 = m(104 -2.1)
m = 100/(104-2.1) = 0.9814.
Substituting this value of “m” to solve for the value of “b”:
0 = 0.9814 (2.1) + b
b = -2.0608.
Implementing the Scale and Offset Constants in DaqView
To implement the scale and offset constants in DaqView, first make sure that the acquisition process is
turned off. Then, click on the cell in the Units column for channel 2. The engineering units pull-down
menu above the grid becomes active; click on the down arrow and select the “mx + b” option.
After doing that you have the ability to enter new numbers for m and b. Perform those entries and click on
OK to save them. You can then place the thermocouple back in the ice and fluidized sand baths, observe
the new readings, and note the improvement in their accuracy.
If you want optimum accuracy of all channels in the temperature measurement system, you should perform
the two-point calibration process for each channel.
Converting Degrees from Celsius to Fahrenheit
Once you have performed the two-point calibration process and determined the scale and offset values for
units of degrees Celsius, you can use this information to find the corresponding scale and offset values for
other temperature units. In the previous example, the calibration process produced the following equation
for “mx + b”, using units of degrees Celsius:
RC (°C) = 0.9814 * RA (°C) - 2.0608
To convert to degrees Fahrenheit, use the relationship:
RC (°F) = 1.8 * RC (°C) + 32
Substituting the latter equation into the former yields:
RC (°F) = 1.8 * [0.9814 * RA (°C) - 2.0608] + 32
RC (°F) = 1.7665 * RA (°C) + 28.29
So the new values of m and b are:
m' = 1.7665
b' = 28.29
These are the scale and offset values that you should enter. Note that RA is still in degrees Celsius since the
raw reading produced by the data acquisition software is in degrees Celsius.