• Typical unamplified analog load cells have a sensitivity rating with a unit of mV/V. This rating is specified on a load cell data sheet often in the 1mV/V – 3mV/V range. Most quality load cells will come with a factory calibration sheet which specifies the actual sensitivity rating for each individual load cell produced. This sensitivity value can be used to convert the load cell’s output to scientific units of weight or force.

Convert Load Cell Voltage to Pounds or Kilograms or Newtons

• As mentioned above, each load cell should come with a calibration certificate denoting the sensitivity for each individual cell. This value can be used to convert the load cell’s output signal to usable units of weight or force. To calculate the raw output voltage of the cell relative to the full capacity of the cell, use the following equation:

$V_{out, max}=S V_e$
where

$V_{out, max}=\text{output voltage from load cell when loaded to 100\% of rated capacity (mV)}$ $S = \text{sensitivity (mV/V)}$ $V_e = \text{excitation voltage (V)}$
In other words, the above result will be the voltage output from the cell when fully loaded relative to the full rated capacity.

Typically, load cells are used with amplifiers to transform the small output voltage to a easily measurable voltage (while conditioning the signal with filters, etc.). If we add an amplifier to the mix, we have:

$V_{out, max}=S V_e A$
where

$V_{out, max}=\text{output voltage from load cell when loaded to 100\% of rated capacity (mV)}$ $S = \text{sensitivity (mV/V)}$ $V_e = \text{excitation voltage (V)}$ $A = \text{amplifier gain (V/V)}$
If we were to convert this output voltage to force or weight, we would use the ratio of the actual output voltage to the maximum output voltage which is equal to the ratio of the actual load to the maximum rated load:

$\displaystyle \frac{V_{out}}{V_{out, max}}=\frac{L}{L_{tot}}$
or

$\displaystyle \frac{V_{out}}{S V_e A}=\frac{L}{L_{tot}}$
Rearranging, we can solve for L:

$\displaystyle L=\frac{L_{tot}V_{out}}{S V_e A}$
where

$L = \text{load on cell (Kg, Lb, N, etc.)}$ $L_{tot} = \text{total rated capacity of the load cell (Kg, Lb, N, etc.)}$ $V_{out}=\text{output voltage from load cell when loaded (mV)}$ $S = \text{sensitivity (mV/V)}$ $V_e = \text{excitation voltage (V)}$ $A = \text{amplifier gain (V/V)}$ Note: Load will be in the same units as the rated capacity.

If the load cell is not factory calibrated, it must be field calibrated to determine the sensitivity. To do so, record the no-load output value, then load the cell with a known load and record the output. Subtract the zero output voltage and use the resulting voltage as the calibration factor:

$\displaystyle S = \frac{L_{tot}(V_l-V_0)}{V_e L_{cal}A}$
where

$S = \text{load cell sensitivity (mV/V)}$ $L_{tot} = \text{the total rated capacity of the cell (Kg, Lb, N, etc.)}$ $V_l = \text{loaded output voltage (mV)}$ $V_0 = \text{unloaded output voltage (mV)}$ $V_e = \text{excitation voltage (V)}$ $L_{cal} = \text{known load (Kg, Lb, N, etc.)}$ $A = \text{amplifier gain (V/V)}$ Note: The units for the known load and total rated capacity should match and thus cancel out.