PROCESS INSTRUMENTATION
Process Instrumentation Basics: Pressure, Level, Flow, Temperature and the 4-20 mA Loop
The half of the control loop that is not code. How the four core measurements work, why 4-20 mA is still everywhere, how to scale a signal correctly, and how to tell a bad reading from a real one.
Process instrumentation is the set of field devices that measure pressure, level, flow and temperature and send those values to a PLC or DCS, usually as a 4-20 mA signal. The skills are how each sensor works, how the loop is wired, how to scale the signal and how to spot a bad reading.
In this lesson by Instrumentation Tools, part of the 4-20 mA module in our free instrumentation course, you get the history of the current loop, why it is used and its advantages and disadvantages. The rest of this guide covers what sits either side of that loop.
The measurements a plant depends on
Pressure. Most transmitters use a piezoresistive or capacitive cell behind a diaphragm. The first question is always which reference: gauge pressure is relative to atmosphere, absolute is relative to vacuum, and differential is the difference between two ports. A differential transmitter is also the basis of many level and flow measurements.
Level. A DP transmitter infers level from the hydrostatic pressure of the liquid column, so its reading depends on the liquid's density and shifts if the product changes. Radar and ultrasonic transmitters measure the time for a signal to return from the surface; radar copes better with vapour and foam. Floats, vibrating forks and conductive probes are used as point-level switches for high and low alarms.
Flow. Electromagnetic meters are accurate and have no moving parts, but need a conductive liquid, so they suit water but not oil. Coriolis meters measure mass flow and density directly. Vortex meters suit steam and gases. Orifice plates with DP transmitters are simple and widespread, but flow is proportional to the square root of the differential pressure, which must be handled in the transmitter or the controller.
Temperature. RTDs, usually Pt100 (100 ohms at 0 °C), are accurate and stable for moderate temperatures. Thermocouples cover much higher temperatures and produce a small voltage that needs cold-junction compensation. Both are often wired to a head-mounted transmitter that sends 4-20 mA.
Analysis. pH, conductivity, dissolved oxygen, turbidity and chlorine analysers are common in water, food and chemical plants. They drift more than physical measurements and need a calibration routine.
The 4-20 mA loop
A current signal is used because the same current flows through every part of a series loop, so long cables and connection resistance do not change the reading as long as the supply has enough voltage. The 4 mA live zero means 0 mA can only be a fault: a broken wire, a blown fuse or a dead transmitter. Our 4-20 mA current loop explained guide covers 2-wire and 4-wire wiring, the loop voltage budget and NAMUR fault levels.
Transmitters come in three wiring types. A two-wire transmitter is powered by the loop current itself. A three-wire transmitter has a separate supply and a shared return. A four-wire transmitter has a separate supply and an isolated output. Wiring a two-wire device to an input card that expects a powered signal, or the reverse, is a classic commissioning fault.
Many transmitters signal faults by driving the current outside the normal range. The NAMUR NE 43 recommendation, widely adopted, keeps readings between 3.8 and 20.5 mA and uses 3.6 mA or below, or 21 mA or above, to signal a failure. Smart transmitters also carry HART, a digital signal superimposed on the analog current for diagnostics and configuration.
Scaling the signal in the PLC
The formula is a straight line: value = LRV + (mA - 4) / 16 x (URV - LRV). For a 0 to 10 bar transmitter, 12 mA is 5 bar. In the PLC you work with raw counts rather than milliamps. On a Siemens S7-1200 or S7-1500, a 4-20 mA input's nominal range is 0 to 27648 counts, and NORM_X followed by SCALE_X converts it to engineering units. Other brands use different raw ranges, so always check the input card's manual. Our free engineering calculators include a 4-20 mA scaling tool for checking your numbers; the calculators themselves are on the resources page.
Two habits separate a reliable program from a fragile one. First, treat readings outside the valid range as faults, not values: a transmitter at 2 mA is not reporting a very low level. Second, give every alarm a deadband so it does not chatter when the value sits near the limit.
Reading P&IDs and tags
Instruments are identified on piping and instrumentation diagrams by tags built on ISA-5.1 conventions. The first letter is the measured variable and the following letters are the functions: PT-101 is a pressure transmitter, LIC-201 a level indicating controller, FV-301 a flow control valve. Being able to follow a loop from the transmitter through the controller to the valve on a P&ID is the first thing an instrumentation interview tests. Our guide on how to read a P&ID walks through tags, bubbles and signal lines.
Calibration, and knowing when to distrust a reading
Calibration compares a transmitter with a traceable reference, usually at five points across the range (0, 25, 50, 75 and 100%), recording results before adjustment (as-found) and after (as-left). In the control system, add simple checks that catch a failing instrument before operators act on it:
- A value frozen for longer than the process allows.
- A rate of change the process cannot physically produce.
- Disagreement between two instruments measuring the same thing.
From measurement to control
Most measurements end up in a control loop: transmitter, PID controller, then a control valve driven through an I/P converter and positioner, or a pump on a VFD. Choosing whether a valve fails open or closed on loss of air or signal is a safety decision, not a detail. In refineries and gas plants the most critical measurements also feed an independent safety system, as described in oil and gas automation. For tuning, see our PID tuning guide with a simulator.
Free process instrumentation courses
Start with Industrial Instrumentation and Process Control if you are new to the field: pressure, level, flow and temperature, 4-20 mA calculations, control valves, P&IDs, HART and calibration. If you already write PLC code, Instrumentation for PLC Engineers follows one measurement from the transmitter through NORM_X and SCALE_X to a valve. Then close the loop with PID Compact and analog processing in TIA Portal or PID Control for PLC Engineers. The process control path puts them in order.
Every course is free in full, with credited video lessons, notes, practice tasks and one final assessment; no card is needed. The optional EDWartens Certificate of Completion is a small one-off fee, US$8.99 for the beginner course and a little more for the intermediate ones. It can be checked at edwartens.com/verification. It is not an ISA or vendor certification and is not accredited. Create a free account to start.
Take the free course
FreeInstrumentation · Beginner · Free
Industrial Instrumentation and Process Control
FreeInstrumentation · Intermediate · Free
Instrumentation for PLC Engineers
FreeProcess control · Intermediate · Free
Siemens TIA Portal PID Compact and Analog Processing
FreeProcess control · Intermediate · Free
PID Control for PLC Engineers
Questions
Why is 4-20 mA used instead of 0-20 mA?
The 4 mA live zero lets the controller tell a real zero reading from a broken wire or dead transmitter, which would read 0 mA. It also leaves enough current to power a two-wire transmitter from the loop itself.
How do I convert 4-20 mA to engineering units?
Subtract 4, divide by 16, multiply by the span and add the lower range value. For a 0 to 10 bar transmitter, 12 mA gives (12 - 4) / 16 x 10 = 5 bar.
What is the difference between an RTD and a thermocouple?
An RTD such as a Pt100 measures temperature by the change in a metal's resistance and is accurate and stable over moderate ranges. A thermocouple produces a small voltage from the junction of two metals, covers much higher temperatures, and needs cold-junction compensation.
What does HART add to a 4-20 mA loop?
HART superimposes a digital signal on the analog current, so you can read diagnostics, change the range and read extra variables from a smart transmitter without disturbing the 4-20 mA measurement.
Do I need hardware to learn instrumentation?
No. The free courses teach the principles with video and worked calculations, and the PLC scaling and PID exercises run in a simulator. A multimeter and a loop calibrator help if you have them, but are not required.
Sources
- NAMUR: current recommendations and worksheets, including NE 43
- ISA: ISA5 committee, ANSI/ISA-5.1 Instrumentation and Control Symbols and Identification
- Instrumentation Tools on YouTube
Written by the EDWartens engineering team for general education. Product names are trademarks of their owners; mentioning them does not imply endorsement. Prices and terms of other providers were checked on the date shown and can change.
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