Measurement Computing DBK Part 2 - Manual (Page 50)

DBK43A & DBK43B, pg. 30 899892 DBK Option Cards and Modules
11. DBK43A users: In LogView, open the LogBook Hardware Configuration Window (hardware
tree) and select NORM for each DBK43A.
DBK43B users: In LogView, open the LogBook Hardware Configuration Window (hardware
tree) and select RUN for the CAL1 switch and select CAL for CAL2.
This completes the section entitled: “Procedures Common to All Calibration Steps (Required)”
Nameplate Calibration and Manual Calibration
To properly calibrate a strain gage channel using the Nameplate method, the required gain must first be
calculated. If the desired gain and offset are already know [as in the Manual calibration method] skip to
the section, Determining the Gain of Each Amplification Stage.
The following examples outline the necessary steps for determining the required gain for
Nameplate calibration
. Both strain gage and load cell examples are provided.
Calculating the Required Gain
Determining a Strain Gage’s Maximum Output Voltage
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:
V
BR
= (V
EXC
* G * S * B) / 4 [See following important notice.]
Where: V
BR
= Bridge output voltage
V
EXC
= Excitation Voltage
G = Gage Factor
S = Strain in user units (in uStrain)
B = Configuration factor (1 for ¼ bridge, 2 for ½ bridge, 4 for full bridge)
The equation, V
BR
= (V
EXC
* G * S * B) / 4 produces a linear estimate. If you are
using a non-linear strain gage you should refer to strain gage theory for additional
information as needed.
For a 120 ohm strain gage with a gage factor of 2.1 and excitation voltage of 5 V, applying
4000 microstrain would produce an bridge output of 10.5mV for a ¼ bridge configuration.
V
BR
= (5 * 2.1 * 4000x10 -6 ) / 4 = 10.5 mV
Determining a Load Cell’s Maximum Output Voltage
Load cells come with a mV/V specification—for each volt of excitation at maximum load, the load cell
will output a specific millivolt level.
V
LC
= R * V
EXC
Where: V
LC
= Load cell output voltage
R = Load cell spec (mv/V)
V
EXC
= Excitation voltage
Consider a 3000 pound load cell rated at 2.05 mV/V using 10 V of excitation (assume a 350 Ω load cell).
When 3000 pounds is applied, the voltage out of the load cell is 20.5mV.