# The four process variables: pressure, temperature, flow and level

> Almost any continuous process is run by measuring and controlling four physical variables. Knowing them is the quickest way to understand what a plant's instrumentation actually does.

- Source : https://industryhub.site/en/learn/process/grandeurs-du-procede
- Track : Process
- Level : Discovery
- Reading time : 6 min
- Language : English

## In brief

Pressure, temperature, flow and level describe the state of a continuous process and are measured then controlled at all times. A fifth family — analysis (composition, pH…) — completes the picture.

## Four variables run almost everything

In a continuous process — a refinery, a water treatment plant, a boiler — there are not hundreds of different things to watch. Four variables show up everywhere: **pressure, temperature, flow and level**. Nearly every sensor in a plant measures one of them; nearly every valve acts to correct one. Knowing them is already understanding what the instrumentation is for.

## Pressure

Pressure is the force a fluid exerts on the walls that contain it, divided by area. It is measured in bar or pascal — a [pressure converter](/en/tools/process/pressure-conversion) moves between them. It tells you whether a pipe is under strain, whether a reactor is nearing its limit, whether a pump is pushing hard enough. Too much pressure and the equipment risks rupture: that is why relief valves go with it. Pressure is measured with diaphragm transmitters and controlled by acting on a valve or on pump speed.

## Temperature

Temperature drives reaction rates, product quality and the efficiency of heat exchange. It is measured with two main sensor families: thermocouples, rugged and wide-range, and Pt100 resistance probes, more accurate over a narrow range — expressed equally in °C, K or °F through a [temperature converter](/en/tools/process/temperature-conversion). Controlling it means adjusting how much heat is added or removed — steam, burner, cooling water — to hold a setpoint.

## Flow

Flow measures how much material passes per unit of time, in m³/h or kg/h — two unit families separated by density, which is why a [flow converter](/en/tools/process/flow-conversion) is handy. It is the variable of balances: what comes in, what goes out, what is dosed. It is measured with an orifice plate, by Coriolis effect, electromagnetically or by ultrasound, and controlled with a control valve or pump speed.

## Level

Level is the height of liquid or solid in a vessel: tank, drum, column. It protects pumps from running dry, prevents overflows and keeps a buffer between two stages. It is measured by hydrostatic pressure, radar, ultrasound or float, and controlled by adjusting the inlet or outlet flow.

## Sensors and typical ranges

| Variable | Common technology | Indicative range / accuracy |
|---|---|---|
| Temperature | type K thermocouple | −200 to +1370 °C |
| Temperature | Pt100 probe (IEC 60751) | −200 to +850 °C, ±0.1–0.5 °C |
| Pressure | diaphragm transmitter | 0 to 1000 bar, ±0.1% |
| Flow | Coriolis / electromagnetic | ±0.1–0.5% of reading |
| Level | radar / ultrasound | 0 to 30 m |

## The fifth family: analysis

As soon as quality enters the picture, a fifth family of measurements becomes critical: **analysis**. Composition, pH, conductivity, oxygen content, dew point… These measurements are more expensive, slower and harder to maintain, but they alone tell you whether the product meets spec. They are handled by process analysers.

## From measurement to control

Measuring is not enough: each variable usually sits inside a [control loop](/en/learn/automation/boucle-de-regulation) that compares it to a setpoint and corrects continuously through an actuator. The signal linking sensors, controllers and valves is still the 4-20 mA loop, whose failure behaviour is fixed by [NAMUR NE 43](/en/standards/namur-ne-43): a reading **≤ 3.6 mA** or **≥ 21 mA** signals a sensor fault, not a real value. It is this mesh — measure, compare, act, and detect its own faults — that keeps a process holding steady on its own, day and night.