What mV/V actually means
A load cell is a strain gauge bridge. Excite it with a voltage and it returns a proportional signal — but a very small one. The rated output states how small:
A 3 mV/V cell at 10 V excitation gives 30 mV at full capacity. That is the entire signal range — millivolts, from a sensor often sitting a long cable run from the amplifier, on a structure that vibrates. Almost everything that goes wrong in weighing traces back to how small that signal is.
Worked example
A vessel on four 2,000 kg cells, 3 mV/V, 10 V excitation, 1,500 kg dead load, 3,000 kg of product:
- Total capacity = 4 × 2,000 = 8,000 kg
- Total load = 1,500 + 3,000 = 4,500 kg → 56% of capacity
- Output = 10 × 3 × (4,500 ÷ 8,000) = 16.875 mV
- Full scale = 30 mV, so the live weight only ever uses part of it
The dead load problem
This is where weighing systems are won and lost. The cells carry the vessel permanently, and that dead load consumes capacity before any product arrives. In the example above, 1,500 kg of the 8,000 kg capacity is gone before you start — and if the vessel were heavier relative to its contents, the usable fraction would shrink further.
The consequence is resolution. The amplifier resolves a fixed number of microvolts, which maps to a fixed number of kilograms. A system where the live weight occupies 20% of the signal range has five times worse effective resolution than one where it occupies the full range — same cells, same amplifier, much worse performance. Oversizing cells "for safety" is the usual cause.
Field notes
- Use 6-wire sense connection on long runs. Cable resistance drops part of the excitation voltage before it reaches the cell, and since output scales with excitation, that becomes a direct span error. The sense pair measures voltage at the cell and lets the amplifier compensate — the same principle as a 4-wire RTD.
- Piping is the enemy. Rigid pipe connections to a weighed vessel carry load that the cells never see, or add force they see falsely. Flexible connections and horizontal pipe runs matter more to accuracy than the cell specification does.
- Never weld on a vessel with cells fitted — welding current through a strain gauge bridge destroys it. Fit dummy blocks first.
- Check the shunt calibration value. Most systems allow a known resistor to be switched across one bridge arm, simulating a specific load. It verifies the whole signal chain without filling the vessel, and the expected reading should be recorded at commissioning as an as-found reference.
- Temperature affects the whole structure, not just the cells. A tank that grows and shrinks against its supports creates apparent weight changes that look like a drifting instrument.
- The transmitter output is an ordinary 4-20 mA signal — check the scaling with the 4-20 mA converter.
Frequently asked questions
How do I calculate load cell output in mV?
Output = excitation voltage × rated output (mV/V) × (applied load ÷ total capacity). A 2 mV/V cell with 10 V excitation gives 20 mV at full capacity and 10 mV at half.
What does 2 mV/V mean on a load cell?
It is the rated output: the cell produces 2 millivolts of signal for every volt of excitation when loaded to its rated capacity. At 10 V excitation that is 20 mV full scale — a very small signal, which is why load cell cabling and amplification matter so much.
How does dead load affect a weighing system?
The vessel, agitator, piping and insulation sit on the cells permanently and consume part of the capacity before any product is added. Usable span is total capacity minus dead load, and a vessel whose dead load is most of the capacity will have poor resolution on the live weight.
How many load cells does a vessel need?
Normally three or four, and the total capacity is the sum of all cells. Three cells self-level on uneven floors; four are common on rectangular structures but need careful shimming. Each cell must be rated for more than its own share plus any eccentric loading.
Provided for reference and education; ignores cell non-linearity, creep and temperature effects. Legal-for-trade weighing has its own approval requirements. See our disclaimer.