Measurement Computing WBK Options - Manual (Page 77)

WBK16, Strain-Gage Module 949794 WBK16, pg. 37
(6) Provide adequate input power to each WBK16 in a system.
Providing the proper level of input supply voltage is very important. Insufficient input voltage can cause
the WBK16 to exhibit channel-to-channel excitation interaction. All individual channels can be set
properly and then begin to lose voltage as additional channels are connected. It is imperative that the
WBK16 not be “starved” for input voltage. This can very easily happen if more than one WBK16 is
powered from the supplied TR-40U power adapter or some other smaller and inadequate source. A
WBK16 can require as much as 25 watts of input power if configured for eight channels of 120 ohm
bridges at 10 volts of excitation. The 15V, 2.4A (36 watts) output of the TR-40U is not sufficient for two
WBK16’s. A variation of this problem can occur if a group of WBK16’s is daisy-chained together with
an insufficient wire size feeding the group. Voltage drop in the wiring can also starve the WBK16’s to a
greater degree as distance from the source increases. It is strongly recommended that individual WBK16
units each operate from the TR-40U provided or from an individual power lead from an adequately sized
source such as a large battery or power supply.
(7) Handle channel configuration headers carefully.
The 16-position, machined-pin IC sockets [into which the bridge completion headers are inserted] have
demonstrated a tendency to become unreliable if the headers are rocked sideways to remove them or if
resistors with larger leads than those we supply are plugged directly into them. The unreliability manifests
itself with widely fluctuating readings, especially if touched, or if the WBK16 enclosure is subjected to
shock or vibration. Cold solder joints on the headers have similar symptoms.
(8) Install internal reference node resistors if you plan to use half or quarter bridges
There are internal 1 Meg ohm bias resistors [located between the excitation rails] that create a very high
impedance “reference node voltage” in the WBK16 without installing the recommended resistors. 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 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.