---
title: "Hydraulic and pneumatic load cells"
subject: "Instrumentation and Measurement"
chapter: 7
key: "semester_4/instrumentation_and_measurement:hydraulic-pneumatic-load-cells"
source: https://www.bctengineer.com/q/semester_4/instrumentation_and_measurement/hydraulic-pneumatic-load-cells
site: BCT Engineering
---

# Hydraulic and pneumatic load cells

Instrumentation and Measurement · Chapter 7: Common Mechanical Measurement System and Transducers

- Asked in: year not given, 5 marks
- Video: [Hydraulic and pneumatic load cell](https://www.youtube.com/watch?v=BF5XDLkzIaE) — Pravisha candideyes
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/instrumentation_and_measurement/hydraulic-pneumatic-load-cells
- Whole chapter: https://www.bctengineer.com/md/semester_4/instrumentation_and_measurement/chapter-7

## Question

Explain the working of a hydraulic load cell and a pneumatic load cell. State their advantages and limitations compared with strain gauge load cells.

## Short answer

Hydraulic load cell converts force to oil pressure p=F/A; pneumatic load cell balances force by air pressure on a diaphragm. Both need no electrical power and are safe for hazardous areas, but have slower response and lower accuracy than strain-gauge cells, which require excitation but are faster and more accurate.

## Answer

Both cells convert force into a fluid pressure, which is measured by a pressure gauge or transducer.

#### Hydraulic load cell
The force acts on a piston that bears on a liquid (oil) held in a chamber and sealed by a rolling diaphragm. The fluid pressure rises:

$$
p = \frac{F}{A}
$$

and the pressure gauge, calibrated in force units, shows the load.

```text
      F
      v
   [=======] <- loading plate
   |piston |
   |-------|  diaphragm seal
   | oil   |---> pressure gauge / transducer
   |_______|
```

- Almost frictionless because of the diaphragm.
- Needs no electric power, so it is suitable for hazardous areas.
- Useful for heavy loads (tank weighing, bridge testing, truck scales) and remote reading.
- Temperature changes affect the oil volume and pressure.

#### Pneumatic load cell
The force is balanced by air pressure acting on a diaphragm. The force pushes the diaphragm (flapper) toward a nozzle, which restricts the air bleed, so the pressure under the diaphragm rises until the balancing force $pA = F$ is reached. This back pressure is read on a gauge and is proportional to the force.

```text
   Air supply --[restrictor]--+--> gauge
                              |
                      nozzle  |
                     ======== flapper moved by F
                        F v
                   [diaphragm]
```

- Intrinsically safe, clean and inexpensive.
- Relatively insensitive to temperature, unlike hydraulic cells; no liquid to leak.
- Typical for food and pharmaceutical work and for weighing small loads (up to about 20 kN) with good accuracy.
- Limited by response lag (compressible air, tubing volume) and needs a clean, dry air supply.

#### Comparison with strain gauge cells

| Point | Hydraulic/pneumatic | Strain gauge |
|---|---|---|
| Power | Not required | Electrical excitation |
| Hazardous area | Safe | Needs approved design |
| Signal | Pressure, remote reading | Electrical, easy for data logging |
| Response | Slow | Fast |
| Accuracy | About 0.25-0.5% | About 0.03-0.1% |
| Overload | Robust | Can be damaged |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does a hydraulic load cell indicate force?
   A. Force on a piston raises the oil pressure p = F/A, read on a gauge
   B. Force changes the oil colour
   C. Force changes oil conductivity
   D. Force stretches a wire in the oil
   Answer: A. The piston presses the oil, and the pressure rise p = F/A is read on a gauge calibrated in force. (Key point: Hydraulic principle)

2. How does a pneumatic load cell work?
   A. Force is balanced by air pressure on a diaphragm, detected by a flapper-nozzle, and the back pressure is read
   B. Force compresses oil
   C. Force generates charge in air
   D. Force turns a turbine
   Answer: A. Air pressure rises until pA = F, and the back pressure is the measure of force. (Key point: Pneumatic principle)

3. Compared with strain gauge cells, hydraulic and pneumatic cells:
   A. Are faster and more accurate
   B. Need no electric power and are safe in hazardous areas, but are slower and less accurate
   C. Need electrical excitation
   D. Are easily damaged by overload
   Answer: B. They suit hazardous areas without power, but response is slow and accuracy is about 0.25-0.5% against 0.03-0.1%. (Key point: Comparison)

4. Which is true about temperature effects?
   A. Pneumatic cells are insensitive to temperature, unlike hydraulic cells where oil volume changes
   B. Hydraulic cells are insensitive to temperature
   C. Both are strongly affected equally
   D. Neither is affected
   Answer: A. Oil volume and pressure change with temperature in hydraulic cells; pneumatic cells are largely insensitive. (Key point: Temperature)

## Related in Instrumentation and Measurement

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