Measurement Computing StrainBook/616 - Manual (Page 26)

2-12 Operation Reference 967794 StrainBook/616
(8) Install internal reference node resistors if you plan to use half or quarter bridges.
Internal 1 Meg ohm bias resistors [located between the excitation rails] create a very high impedance
“reference node voltage” in the StrainBook and WBK16 when the recommended resistors are not installed.
Do not attempt measurements using these default resistors , even though it seems to work. Install the
previously recommended 1000 ohm components and use a calibration method which compensates for the
slight bridge imbalance.
(9) Do not neglect the excitation regulator remote sense leads.
The remote sensing feature of the StrainBook [and WBK16] will compensate for voltage drop in long lead
wires to provide accurate excitation levels at the terminals of full-bridge and half-bridge configurations.
If the remote sense lines are not used be sure to tie them to their respective output lines to minimize
excitation noise.
(10) Spend your resistor dollars wisely.
For the widely used 3-wire quarter bridge configuration purchase the lower bridge completion resistor (R
A
)
with the best available temperature coefficient and sufficient power rating as to minimize self-heating. The
tolerance of the resistance is not as critical, but it should be 0.1% or better.
The internal bridge completion locations for the reference node (R
G
and R
H
) have about 50 milli-ohms
resistance between their midpoint connection pads and the tap to the amplifier is at the lower end of this
resistance. This resistance nullifies the benefit of using bridge completion resistors with better than 0.1%
resistance tolerance because offset nulling will still be necessary.
If using shunt calibration purchase high-precision shunt calibration resistors with micro-strain values
appropriate to your application. These are the closest to “standards” short of a very high precision strain
calibrator.