Measurement Computing DBK Part 2 - Manual (Page 57)

DBK Option Cards and Module 899892 DBK43A & DBK43B, pg. 37
4. Select Mode = Bridge from the drop down list above the grid. This selection commands the
calibration multiplexer to route the transducer voltage through the analog path.
5. Turn off all the channels in the system, except for those DBK43A [or DBK43B] channels that
are to be adjusted.
6. Click the Download button to send the current configuration to the LogBook.
7. Select Indicators \ Enable Input Reading Column from the menu bar to display the offset
values for each channel.
Adjust the Offset
For the associated channel, apply the first calibrated load to each gage (typically no-load) and set the
voltage to 0.0V for each transducer. This is accomplished by adjusting the trimpot labeled OFFSET. If the
first point is actually a calibrated load, you will need to move the load to each gage, one at a time, to adjust
its associated offset.
Adjust the Input and Scale Amplifier Gain
Complete the following steps to adjust the channel gain.
1. Apply the second load to each gage channel. The value of this load should approximate that
of the maximum expected load. For the best results, a gain should be selected so that the
bridge’s maximum output equals 90% of the A/D’s input range.
2. Calculate the desired voltage for the second point using the following equation:
V
D
= (L
A
/L
M
) * V
I
* 90%
Where: V
D
= Desired voltage for 2 nd point of calibration
L
A
= Applied load used in calibrating the 2 nd point
L
M
= Maximum load expected during usage
V
I
= Input voltage range
Example: The load standard that will be applied to the gage as the 2 nd point in the 2-Point
calibration is 100lbs. The maximum expected load during usage is 150lbs.
The programmable input range of the data acquisition system is set for +5V.
The desired output voltage of the strain gage signal conditioning electronics is:
V
D
= (100/150) * 5 * 0.90 = 3V
In this example, we should adjust the GAIN and SCALE trimpots until a value of 3V is
measured.
If 150 lbs is applied to the gage, a voltage of 4.5V will be measured.
V
D
= (150/150) * 5 * 0.90 = 4.5V
3. Apply the second calibrated load to each gage and set the voltage to V
D
, as derived in step 2.
Do this for each transducer by adjusting the trimpots labeled GAIN and SCALE for the
associated channel. Note that the GAIN trimpot provides most of the amplification (course
adjustment), while the SCALE trimpot allows for fine-tuning.
Repeating the Process
Since adjusting the gain for the first time will have an affect on the offset, it is recom-
mended that offset and gain adjustment be performed twice for each channel.