Omega Engineering OMB-DBK-34A, User Manual

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Example
The following examples perform selected steps for a typical setup. There will be strain gage and load cell
examples. Referring to the typical setup procedure, step 3 says to determine the maximum voltage from
the transducer at maximum load or strain.
A strain gage example:
Most strain gages come with Gage Factors (GF) used to calculate the approximate output of the bridge
circuit with a typical strain value. The formula is:
( ) Excitation Voltage)(Gage Factor)(Strain in strain units
*Bridge circuit output voltage
4
=
*linear estimate, some strain gages are not linear, refer to strain gage theory for more information.
If we have a 120 ohm strain gage with a gage factor of 2.1 and excitation voltage of 5 V (due to the current
limitation of the excitation regulator on the DBK16, it must be less than 6 V) and we are measuring 4000
microstrain:
Bridge output voltage for 4000 microstrain =
(5)(2.1)(4000 10 )
4
10.5 mV
-6
×
=
A load cell example:
Load cells come with a mV/V specification which means for each volt of excitation at maximum load, the
load cell will output a specific millivolt level.
Consider a 3000 pound load cell rated at 2.05 mV/V using 10 V of excitation (assume it is a 350 ohm load
cell). When 3000 pounds are applied, the voltage out of the load cell is:
(10)(2.05×10
-3
) = 20.5 mV
For 1000 pounds, it would be (1000/3000)(10)(2.05×10
-3
) = 6.833 mV
Now that we know what our full-scale voltage will be from our sensor, we can calculate what the voltage
gain should be on the DBK16 so the full-scale sensor output will correspond to the full-scale Daq device
input. Full-scale Daq device inputs are:
-5 to +5 V for DaqBook and DaqBoard [ISA type] in bipolar mode
0 to +10 V for DaqBook, DaqBoard [ISA type], and DaqBoard/2000 in unipolar mode
-10 to +10 V for DaqBoard/2000 in bipolar mode and Daq PC-Card
To calculate the needed DBK16 gains, use the maximum voltage you want to represent the full-scale of
the sensor (usually full-scale Daq device input with a little space to prevent saturation) divided by the full-
scale sensor output.
Example: The strain gage above at 4000 microstrain will output 10.5 mV. If using a DaqBook in bipolar
mode, full-scale is +5 V. Also, we will allow 0.5 V space on the full-scale input. The DBK16 gain should
be:
4.5 V/10.5 mV = 428.6
Now we must determine how to distribute the gain in the DBK16 (typical setup procedure step 4). There is
the input amplifier that has a gain range of ×100 to ×1250, the filter gain (×1 or ×2) and the scaling
amplifier that has a range of ×1 to ×10. If we round off our gain to ×420, any of these possible settings
will work.
Input Gain ×420 ×100 ×240 ×300
Filter Gain (enabled) No Yes (×2) Yes (×1) No
Scaling Gain ×1 ×2.1 ×1.75 ×1.4
Total DBK16 Gain ×420 ×420 ×420 ×420
After we determine how all our gains will be distributed, the sensor can be hooked up to the DBK16, the
bridge completion resistors installed, and the excitation voltage set. Then we can proceed to set the gains.
We will use DaqView for this.
DBK16, pg. 12
879895
DBK Option Cards and Modules