How can I measure a force with a bridge sensitivity that exceeds the configurable 10mV/V limit of Acromag’s 851T load cell transmitter?

Acromag limits the 851T transmitters’s sensitivity to 10mV/V for superior precision in common load cell applications with lower values. However, higher sensitivities can be accommodated because the 851T has an adjustable excitation and allows the input scale to be over-ranged up to 125%.

See full example application below.

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Example: Load cell application with high sensitivity.

The method for calculating the full scale output from a Bridge is:

[Sensitivity (mV/V) x Excitation Voltage (V)] x [Actual Full Scale Load / Maximum Capacity of Bridge]

For our application, this becomes:

22.2910 mV/V x 5.000 V x 37.5 psi / 75 psi = 55.7275 mV

So, measuring a 37.5 psi load, the output from the Bridge will be 55.7275 mV.

Now in our configuration software, we limit the Sensitivity to 10 mV for greater precision at the more common lower values. With this limit, the 851T expects a 50mV bridge signal at full scale load:

10 mV/V x 5.000 V = 50 mV

Figure 1 below shows how to configure the transmitter for use with the load cell.

Figure 1: The configuration software’s Bridge/Load Cell Setup page

We label the Sensitivity as, "Gauge Rated Output". After the Sensitivity and Excitation Voltage are entered, we click on the "Calc Range" button which then displays the full scale mV signal. It is represented as a bipolar signal which we adjust on the Configuration page.

However, the transmitter doesn’t really care about the Sensitivity rating. It only measures the bridge mV signal. And in this case, the signal from the bridge reaches 50 mV when the applied load is 33.65 psi.

22.2910 mV/V x 5.000 V x (33.65 psi / 75 psi) = 50 mV

So the 851T would normally produce a full scale 10V output at only 33.65 psi. Then, as the psi increases from 33.65 to 37.5, the output would remain about 10 VDC.

To compensate for this situation, the configuration software includes an "Over-Range" function. The software allows the input scale to be over-ranged by up to 125% of the "Calc Range" value to correspond to the full scale output (10V).

Figure 2 below shows how to scale the input values.

Figure 2: The configuration software’s Transmitter Setup page

The scale is represented as the percentage of full scale where an Input of ±50 mV = ±100 % and 0-10V Output would represent 0-100%. So for an Input that is 0% (0 mV) of the ±50mV input range, the Output would be 0% (0 V) and for an Input that is a 111.45% of the input range (55.7275 mV / 50 mV = 1.11455), the Output would be 10 V.