#### Calculating SCALE values for load cells

Many sensors produce an output in terms of units other than
volts – in this case, an additional scaling factor must be applied to the scale
calculation. The scaling factor can be calculated once some basic
information about the sensor is available.

In this example we will calculate the ANALOG:SCALE
parameter for a typical load cell used to measure tension and compression so
that we record the output in the same units that are used to calibrate the load
cell. The load cell data sheet provides the following information for this
device:

Output
2mV/V,

Excitation
10.0 VDC

The load cell output is specified in terms of volts output
per volt of excitation at full load. In this case, the manufacturer
specifies a 10.0 Volt excitation voltage, so the load cell output will be 20mVat
full load, which, for this load cell, is 50 pounds. We now have enough
information to calculate the sensor calibration factor:

This sensor calibration factor can be using in the basic
ANALOG:SCALE calculation to produce data
values scaled directly in pounds:

Assuming a GEN_SCALE value of 1.00, a 12-bit (internal resolution of 4096) with an input range
of ±10 volts, and a gain of x1, this produces an ANALOG:SCALE value of 12.207 that, at a
quick glance, appears to be correct. However the sensor output is, even at
maximum load, very small and as a result, we have very poor resolution using
this sensor and ADC combination. The smallest change in tension or
compression that can be recorded is one bit of ADC data – which, in this case,
is about 12.2lbs. In order to achieve a reasonable measurement resolution
additional gain is required to amplify the output from the sensor to match the
full ADC measurement range – this will, in turn, affect the ANALOG:SCALE
parameter value.

Many modern ADC sampling devices can be programmed to use
different input ranges by changing the ADC gain. If we use an ADC_gain of
x8 in the above scale calculations, we can improve the measurement resolution to
about 1.5 lbs. This resolution can be further improved by adding an
additional gain stage in between the load cell and the
ADC.