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.
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 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. 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 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 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: 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.