Instructions

UM-0085-B09 DT80 Range User Manual Page 325
RG
Strain is a dimensionless quantity, as it is a ratio of length to length. Strain is often expressed in ppm form (i.e. the length
ratio is multiplied by 1,000,000), in which case it has the units of µStrain (microstrain).
To convert the DT80’s ppm bridge readings to strain, use the formula:
 =
4
. 
. 


where
• B
out
is the DT80's bridge channel (BGV or BGI) result (ppm)
• G is the gauge factor
• N is the number of active gauges in the bridge
The conversion can be done in the DT80 by applying a polynomial as a channel option (see Polynomials (Yn) (P66)):
Y1=0,k"uStrain" 'Polynomial definition
3BGV(Y1) 'Bridge channel
where k = 4 / GN
For the 3-wire BGI "simulated bridge" configuration (see B5 – 3-Wire BGI Input (P314
)) the formula to convert ppm
reading to strain is slightly different:
 =
2
. 
. 


Strain Gauges – Vibrating Wire
DT80G/ 85G GeoLogger only
Vibrating wire strain gauges (VWSGs) are widely used for measuring strain in geotechnical applications. A gauge
consists of a tensioned steel wire whose resonant frequency changes as deflections causes the wire’s tension to change.
The DT80G/85G GeoLoggers include special hardware to support VW gauges. This works by generating a short
electrical pulse to "pluck" the gauge, then measuring the resulting frequency as it resonates. The strain experienced by
the gauge is proportional to the square of the resonant frequency minus the square of the "at rest" resonant frequency.
The FW channel type is used to measure the frequency of a VWSG. This operates in a similar way to the regular
frequency channel type (
F), except that the defaults for some options are different.
Channel Options
The following channel options are relevant for the FW channel type:
• the channel factor – this is the sample period (gate time) in ms (default is 200ms)
• MD (measurement delay) is important, as described below (default is 350ms)
Note that channel options relating to the DT80's instrumentation amplifier, e.g. GLx (gain lock) and A (attenuators), are
not relevant for
FW channels because much of the normal input circuitry is bypassed when performing a VWSG
measurement.
VS1 – Vibrating Wire Strain Gauge and Thermistor
VW gauges can be connected to the DT80 as for any other voltage input. For example, up to three gauges could be
connected to one channel using the configuration shown in V1 – Shared Terminal Voltage Inputs (P305).
Often the gauge will include an integral thermistor, to allow temperature corrections to be made. In this case it is often
convenient to read both the thermistor and the gauge using a single DT80 analog channel, as shown below.
Figure 150: Wiring for vibrating wire strain gauge with thermistor
To measure
Use the command
strain
1FW
temperature
1*YS01 (for example)