Instructions

UM-0085-B09 DT80 Range User Manual Page 324
RG
Channel Options
Useful channel options for F channels are:
• the channel factor – this is the sample period (gate time) in ms (default is 30ms)
• 2V – this option will offset input signal by -2.5V. This effectively changes the threshold point from 0V to approx.
+2.5V, which is useful for TTL level inputs
• GLx, ESn, as for voltage measurements
Note: For DT80/81 Series 1 loggers, the 2V channel option is not supported for differential measurements (measurements between
the + and – terminals, e.g. 1F)
For frequency measurements, the maximum gain range (30mV full scale) is not available. The
GL30MV option is therefore not
valid for this channel type.
The range of frequencies that can be measured depends on the configured sample period (channel factor). For the
default setting of 30ms, this range is approximately 33Hz – 20kHz. If the input frequency is too low to be measured,
UnderRange will be returned.
To measure lower frequencies, the sample period should be increased. For example
3F(1000)
will measure down to 1Hz (upper limit is still 20kHz), while
3F(10000)
will allow frequencies down to 0.1Hz to be resolved.
The drawback to selecting a long sample period is that the measurement will take a long time to complete. This may
delay the execution of other schedules.
Period Measurement
The period of a signal can be measured by taking the reciprocal of a frequency measurement, e.g.:
RA5S 3+F(2V,1000,F1,"Period~s",FF4)
will return the period, in seconds, of an TTL-level logic signal connected between 3+ and 3#. Given the 1000ms sample
period, the maximum period that can be returned will be approximately 1.0s. The F1 option applies intrinsic function #1
(1/x).
Strain Gauges – Bridge
Bridge Wiring Configurations
Strain gauges change resistance when stretched or compressed. They are commonly wired in a bridge configuration,
because bridges are well suited to measuring small changes in resistance; see Bridges (P311).
Each of the arms of the bridge can be either:
• an active element: a strain gauge which is subjected to the forces acting on the structure
• a temperature compensation element: a strain gauge which is not stressed but which has similar temperature
coefficient to the active gauge. This is used to cancel out any changes to the active gauge's resistance due to
temperature effects.
• a completion resistor: a precision resistor whose resistance is equal to the nominal (at rest) strain gauge
resistance.
A full bridge has four active gauges, typically two on one side of a structure (in tension) and two on the other (in
compression). A half bridge has two active gauges; the other two arms are temperature compensation gauges or
completion resistors. Finally a quarter bridge has only one active gauge.
Calculating Strain
The strain-to-resistance relationship is
 =


=
1



where:
• L is the original length
• ΔL is the length change
• R is the original gauge resistance
• ΔR is the gauge resistance change
• G is the gauge factor, a measure of the sensitivity of the gauge (typical foil gauges have a gauge factor of 2.0,
which means that if they are stretched by 1% their resistance changes by 2%)