Scale the 4-20 mA input to the analyser range, alarm on out-of-range currents as a fault, and use the analyser's fault and calibration-in-progress signals to hold or flag the value. Set alarms with a deadband to stop chatter, and use Modbus for diagnostics and status. Decide in advance what the process should do when the analyser is faulty or being calibrated.
- Scale a 4-20 mA input and detect a fault
- Plan alarm set points and deadband
- Handle calibration, warm-up and fault states
- Use Modbus data sensibly
Scaling and range
In the control system, configure the input range to match the analyser range exactly. A mismatch gives an offset or scale error that looks like a process change. Label the engineering units, and record the range in the loop documentation: Analyser outputs: 4-20 mA, relays and Modbus explained.
Faults and bad data
- Alarm on input current below about 3.6 mA or above about 21 mA as an instrument fault, not as a process value.
- Read the analyser's fault relay or status register, and flag the value as bad when it is set.
- Apply a timeout for Modbus data: if values stop updating, treat them as stale.
- Decide whether control should hold the last good value, switch to manual or trip, when the analyser fails. For a safety function this belongs to the safety design.
Calibration and warm-up states
During calibration the analyser sees test gas, not the process. A control loop that keeps using that reading will react to nothing. Use the analyser's calibration-in-progress signal, or a timer on the auto-calibration command, to hold the output and the alarms for the duration, then release after a settling delay. Do the same after power-up while sensors stabilise: Auto-Calibration and validation checks for gas analysers.
Alarm set points and deadband
- Set alarm points from the process need, such as a limiting oxygen concentration with a margin: What is limiting oxygen concentration (LOC).
- Add a deadband (hysteresis) of a few percent of range so the alarm does not chatter when the reading hovers at the set point.
- Use a short delay to ignore single-sample spikes, but not so long that it defeats a fast protection function.
- Separate warning and trip levels.
What Modbus adds
Over Modbus you can read the concentration at full resolution, the alarm and fault states, diagnostic values and stored history, and write a command to start a calibration or change a set point. Keep a register map in the project documents, and test each register at commissioning.
Document it
Record the scaling, the fault handling and the calibration logic. Years later, the person troubleshooting an alarm will need it.
- Match the control-system range to the analyser exactly.
- Treat out-of-range current and fault signals as bad data, not process values.
- Hold outputs during calibration and warm-up.
- Add deadband and a short delay to alarms; document the logic.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
What does input current of 2 mA indicate?
A fault or a broken loop, not a process reading, since the live zero is 4 mA.
Why use a deadband on an alarm?
To stop the alarm chattering on and off when the reading hovers at the set point.
What should the control system do during analyser calibration?
Hold the value or flag it, so it does not act on test gas readings.
Key terms
Frequently asked questions
How do I scale a 4-20 mA input?
Set 4 mA to the low end and 20 mA to the high end of the analyser range, in the same units.
What should happen when the analyser fails?
That is a process decision: hold, switch to manual, alarm or trip. Decide it in advance.
Can the control system start an analyser calibration?
Where the analyser supports it by Modbus or a digital input.
How do I avoid nuisance alarms?
Use a deadband and a short delay, and separate warning from trip levels.
