Process Instrumentation · PV · Flow · Pressure · Temperature · Level · Quality
Every sensor in your plant measures one of five process variables. Know all five : their definitions, units, instruments and why they matter : in one place.
Process variables (PVs) are the physical quantities that an instrumentation and control system measures, monitors and controls in a process plant. The five primary process variables are Flow, Pressure, Temperature, Level and Quality (analytical). This guide covers all five with definitions, measurement units, typical instruments, and a unit converter.
The Five Process Variables at a Glance
Where Are Process Variables Measured in a Plant?
Figure 1: In a typical process loop, all five process variables are measured simultaneously. Flow transmitters (FT) on inlet/outlet pipes, pressure transmitters (PT) on the vessel, level transmitter (LT) inside the vessel, temperature transmitter (TT) on the vessel wall, and analyser transmitter (AT) on the outlet line. All transmit 4-20 mA to the DCS for monitoring and control.
Process Variable 1: Flow : Definition, Units and Instruments
Flow is the movement of fluid from one point to another. It can be expressed as volumetric flow (volume per unit time: m³/h, L/min, GPM) or mass flow (mass per unit time: kg/h, t/h, lb/min). Mass flow is preferred in process industries because it is independent of fluid temperature and pressure : 100 kg/min is always 100 kg/min regardless of conditions. Volumetric flow varies with density changes.
Flow measurement is the single most important process variable in terms of number of instruments installed. Process plants use flow measurement for material balance, product transfer, custody transfer, feed control, and reaction control.
m³/s, m³/h, m³/min
kg/s, kg/h, t/h (mass flow)
L/min, L/s
GPM (gallons per minute)
SCFM (standard cubic feet per minute)
lb/min, lb/h
Orifice plate (DP), Venturi, Vortex, Coriolis, Turbine, Magnetic, Ultrasonic, Variable area (rotameter)
Water: 0.1 to 10,000 m³/h depending on pipe size
Gas: 1 to 100,000 Nm³/h
Process Variable 2: Pressure : Definition, Units and Instruments
Pressure is defined as force per unit area: P = F / A. In process plants, pressure measurement is critical for equipment protection (safety relief valves), flow calculations (DP flow meters), level measurement (hydrostatic level), reaction control, and heat exchanger performance. Pressure is measured in several ways:
- Gauge pressure (Pg): Measured relative to atmospheric pressure. 0 gauge = atmospheric.
- Absolute pressure (Pabs): Measured relative to perfect vacuum. Pabs = Pg + Patm.
- Differential pressure (DP): Difference between two pressure points. Used in DP transmitters for flow and level.
- Vacuum pressure: Pressure below atmospheric, measured as negative gauge.
Pa (Pascal), kPa, MPa, bar, mbar
1 bar = 100 kPa = 100,000 Pa
PSI (pounds per square inch)
inH2O, mmHg, atm, kg/cm²
1 PSI = 6.895 kPa
Bourdon gauge, diaphragm transmitter, capacitance transmitter, piezoelectric sensor, manometer, pressure switch
Process plants: vacuum to 700 bar
Most process: 0 to 100 bar
Utility air: 6 to 10 bar gauge
Process Variable 3: Temperature : Definition, Units and Instruments
Temperature is one of the most measured process variables in any plant. It affects reaction rates, product quality, viscosity, density and the accuracy of almost every other measurement. Temperature sensors include thermocouples (which generate a voltage based on the Seebeck effect) and RTDs (resistance temperature detectors, which change resistance with temperature : see our guide on RTD resistance to temperature conversion).
Kelvin (K) : SI base unit
Celsius (°C) : most common in process plants
0°C = 273.15 K
Fahrenheit (°F)
°F = (°C × 9/5) + 32
0°C = 32°F, 100°C = 212°F
RTD (PT100, PT1000), Thermocouple (Type K, J, T, E, N), Thermistor, Infrared (pyrometer), Bimetal thermometer, Temperature switch
PT100 RTD: -200 to +850°C
Type K thermocouple: -200 to +1260°C
Cryogenic: down to -270°C
Process Variable 4: Level : Definition, Units and Instruments
Level measurement monitors the amount of material stored in a tank or vessel. Incorrect level control leads to tank overflow (causing safety and environmental incidents), tank running empty (causing pump cavitation and process interruption), and incorrect inventory figures. Level can be measured continuously (giving % or height reading) or as a point level (high/low alarm switch). For two-liquid systems, interface level measurement detects the boundary between the two phases.
Metres (m), millimetres (mm)
Percentage (%) of span
Volume in m³ or litres
Feet (ft), inches (in)
% of tank height
Gallons (volume)
DP transmitter (hydrostatic), Guided wave radar (GWR), Free-space radar, Ultrasonic, Float and displacer, Capacitance, Laser
Small tanks: 0 to 5 m
Storage tanks: 0 to 25 m
Atmospheric and pressurised vessels
Process Variable 5: Quality (Analytical) : Definition, Units and Instruments
Quality variables describe what the fluid is, not just how much or how fast it flows. They include pH (acidity/alkalinity), dissolved oxygen, conductivity, turbidity, density, viscosity, gas concentrations (O2, CO2, H2S), calorific value, moisture content and many others. Analytical measurements are often the most complex to maintain and calibrate because the sensor is in direct contact with the process fluid and must be kept clean, calibrated against certified standards, and replaced regularly. See our guide on pH sensor working principle for the most common analytical measurement.
pH (0 to 14 scale)
% vol (gas concentration)
ppm or ppb (trace concentrations)
NTU / FNU (turbidity)
mS/cm (conductivity)
kg/m³ (density)
cP or mPa.s (viscosity)
% O2, % CO2
mg/L (dissolved oxygen)
Brix (sugar concentration)
pH analyser, Conductivity sensor, Dissolved O2 sensor, Turbidity analyser, Online GC (gas chromatograph), NIR analyser, Density meter (Coriolis)
Product quality, environmental compliance, corrosion control, safety (H2S, O2 limits), reaction efficiency and regulatory discharge limits
Process Variables: Complete Units of Measurement Reference
Process Variable Unit Converter
Convert between common units for all five process variables. All answers use the 4-20 mA transmitter standard output for display in DCS/SCADA systems.
Quick FAQs: Process Variables
- 4-20 mA Current Loop: How Process Variables Are Transmitted to DCS
- DP Transmitter: Measuring Flow, Pressure and Level with One Instrument
- pH Sensor Working Principle: The Most Important Quality Process Variable
- Coriolis Flow Meter: Measuring Mass Flow and Density Simultaneously
- RTD Resistance to Temperature Calculator: The Standard Temperature Instrument
External References
- ISA-5.1: Instrumentation Symbols and Identification
- RealPars: What Is a Process Variable in Industrial Automation?
- AutomationForum: Process Variables Reference Article
What we learn today
- The five process variables are Flow (m³/h, kg/h), Pressure (kPa, bar, PSI), Temperature (°C, °F, K), Level (m, mm, %) and Quality (pH, %, ppm, NTU). Every instrument in a process plant measures one or more of these five. All transmit their measurement as a 4-20 mA signal to the DCS.
- Flow is volumetric (volume per unit time) or mass flow (mass per unit time). Mass flow is preferred because it does not change with temperature or pressure. Pressure exists in four forms: gauge (relative to atmosphere), absolute (relative to vacuum), differential (difference between two points) and vacuum. Temperature uses three scales: °C (process), °F (imperial), K (scientific/absolute). Level is measured continuously (m or %) or as point level (high/low switch).
- Quality variables (the fifth PV) describe what the fluid is rather than how it moves or where it is. pH, dissolved oxygen, conductivity, turbidity, gas concentration and density all fall under quality measurement. These require analytical instruments with more frequent calibration and maintenance than standard FPTL instruments.