Why measurements decide how a compressor behaves
A modern compressor controller is a decision-maker. It reads signals, compares them with set-points and limits, and then loads, unloads, changes speed, raises an alarm or stops the machine. If the input is wrong, every decision after it is wrong too.
- A pressure sensor reading 0.3 bar low makes the machine run to a higher real pressure, costing energy (about 7% more power per extra bar).
- A temperature probe reading high trips a healthy machine; one reading low can let a machine overheat without protection.
- A failed dryer dew-point sensor can hide wet air reaching production.
- A wrong current signal can mislead energy reports and audits.
So instrument reliability is not a detail. It sits underneath control, protection, energy and maintenance decisions.
Key sensors on a compressor and what they control
| Measurement | Typical device | Used for | Common failure modes |
|---|---|---|---|
| Discharge or network pressure | Pressure transducer (often 4–20 mA) | Load/unload, VSD speed, sequencing | Zero drift, span error, blocked impulse line, pulsation damage |
| Airend discharge temperature | Thermistor, RTD (e.g. PT100/PT1000) or thermocouple | High-temperature alarm and trip | Loose probe, poor thermal contact, cable damage, wrong sensor type |
| Oil separator differential pressure | Differential transducer or switch | Separator replacement indication | Blocked tapping, sticking switch |
| Intake filter restriction | Vacuum switch or indicator | Filter change indication | Ignored indicator, failed switch |
| Motor current and temperature | Current transformers, winding thermistors | Overload protection, energy data | Wrong CT ratio, loose connections |
| Dryer pressure dew point | Dew-point sensor | Air-quality alarm, purge control on desiccant dryers | Drift, contamination, needs periodic calibration |
| Flow | Thermal mass, vortex or differential flow meter | Demand monitoring, audits, billing between departments | Wrong pipe data, poor straight-run, contamination |
| Status inputs | Emergency stop, phase relay, door and drain switches | Permissives and safety | Bypassed contacts, loose wiring |
Common symptoms of sensor and controller problems
- Nuisance trips that do not match what a hand-held thermometer or gauge shows.
- Machine loading and unloading at pressures different from the set-points.
- Two compressors in a sequence fighting each other or both unloading together.
- Pressure display jumping or showing a steady wrong value when the machine is stopped and vented.
- VSD speed hunting up and down without a matching change in demand.
- Dew-point display that never moves, even when the dryer is switched off.
- Alarms that clear on reset and return soon after, often linked to vibration or heat on a connector.
How we verify a compressor’s measurements
- List every sensor that controls, alarms or trips, with its location, range and the decision it drives.
- With the machine stopped and safely vented, check that pressure sensors read zero.
- Compare running pressure against a calibrated reference gauge at the same point.
- Compare temperatures against a calibrated contact probe or a carefully used infrared reading at the same location.
- Check impulse lines and tappings for blockage, oil traps or leaks.
- Inspect wiring, connectors, screens and earthing; look for cables routed next to motor or VSD output cables.
- Check the controller’s scaling and units match the sensor fitted.
- Review set-points, alarm and trip settings against the manufacturer’s values and the plant’s real needs.
- Test protective trips where the manufacturer’s procedure allows it.
- Record readings and settings as a baseline.
Safety trips must never be bypassed to keep a machine running. If a protection device is suspect, fix or replace it; do not jumper it.
Sensor location matters as much as accuracy
A perfectly calibrated sensor in the wrong place still gives a misleading signal.
- Package pressure measured before the dryer and filters will read higher than the network. If the plant needs a minimum pressure at the far end, control from a network sensor or allow for the drop.
- In multi-compressor sequencing, all machines should be controlled from a common header or a single master signal, not each from its own discharge.
- Temperature probes need full insertion and good thermal contact; a probe hanging loose in a pocket reads low.
- Dew-point sensors need a representative sample flow at line pressure, away from dead legs.
- Pressure tappings on pulsating lines, especially near piston compressors, may need snubbers.
Controllers, settings and change control
Many reliability and energy problems come from settings, not hardware. Over the years, pressure bands get widened, trip delays get changed and sequences get modified by different people, and nobody keeps a record.
- Keep a dated record of all set-points, pressure bands, timers and sequence settings.
- Note who changed what and why.
- After controller replacement or firmware updates, re-check every setting and sensor scaling.
- Protect access with passwords where the controller allows it.
- Where compressors are connected to plant networks or remote monitoring, treat that link as part of the control system and protect it.
Calibration and replacement intervals
There is no universal interval; follow the manufacturer and your plant’s quality system. Practical guidance:
- Check key pressure and temperature readings against references at each major service.
- Calibrate dew-point sensors as the sensor maker recommends; many need periodic recalibration or exchange.
- Flow meters used for audits or energy reporting should be checked for correct pipe data and calibration status.
- Replace sensors that show drift, damage or intermittent behaviour rather than repeatedly offsetting them in the controller.
- Regulated users such as pharmaceutical plants will have their own calibration and documentation requirements; we work to the plant’s procedure.
Wiring, earthing and electrical interference
Many intermittent instrument faults are wiring faults. Low-level signals such as 4–20 mA loops and temperature probes are easily disturbed by noise from VSD output cables, contactors and large motors.
- Route signal cables separately from power and VSD motor cables, crossing at right angles where they must cross.
- Use screened cable where the manufacturer specifies it, with the screen earthed at the correct end only.
- Check connectors and terminals for vibration loosening, corrosion and moisture, especially after monsoon or wash-down.
- Keep the panel sealed against dust; conductive dust on boards causes strange, intermittent behaviour.
- Check the control supply voltage; a weak or noisy control transformer can reset controllers.
Remote monitoring and plant integration
More plants now connect compressors to a building management system, a plant SCADA or a remote monitoring service. This helps with trend logging and early warning, but it adds new dependencies. Signals passed through gateways and protocols can be scaled wrongly, delayed or lost. Before relying on remote data for energy reports or maintenance decisions, compare a few remote values against local readings at the compressor. Make sure the compressor can still run safely on its own controller if the network fails, and that remote write access to set-points is limited to people who need it.
A short local log kept by the operator, noting pressure, temperature and running hours once a shift, remains a valuable cross-check on any remote system. When the two disagree, the difference itself is useful evidence that a sensor, gateway or setting needs attention.
How Reddy Compressors helps
Our service engineers, based at Tarnaka, Secunderabad, check and correct compressor controls and instruments across Telangana and Andhra Pradesh, and supply parts across India. We are an independent multi-brand specialist, working on ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and many more; we are not an authorised dealer. We have tie-ups with all major and reputed brands.
- Diagnosis of nuisance trips, wrong loading pressures and sequencing problems.
- Sensor verification against reference instruments and replacement where needed.
- Pressure transducers, temperature probes, switches and controllers identified from the nameplate and confirmed before quoting.
- Settings review, set-point optimisation and documentation.
- Measurement and logging as part of compressed-air audits.
- Rental compressors from our own fleet if a controller fault stops a critical machine.
- Instrument checks inside service and AMC contracts.
What to send us
- Compressor make, model, serial and nameplate photo.
- Controller type, with photos of the display and any alarm message.
- What you see: trips, wrong pressures, hunting or sequencing issues.
- Your set-points and the pressure the plant really needs.
- Number of compressors and whether they are sequenced.
- Site location and contact person.
Frequently asked questions
Why does my compressor show a high temperature trip when it is not hot?
If an independent thermometer shows normal temperature, the likely causes are a faulty or drifting probe, a loose probe with poor thermal contact, damaged or wet cable, a corroded connector or the wrong sensor type for the controller. Check the reading at the same point with a calibrated instrument, then inspect the probe and wiring. Never bypass the trip; replace the faulty part.
How do I check if a compressor pressure sensor is accurate?
With the machine stopped and safely vented, the sensor should read zero. Running, compare it with a calibrated reference gauge fitted at the same point. Differences show as zero offset or span error. Also check the tapping and impulse line for blockage or oil. If the error is significant or unstable, replace the sensor rather than correcting it in the controller.
Why do my compressors load at different pressures than the set-point?
Common reasons are sensor drift, each machine reading pressure at a different point, pressure drop through dryers and filters, and settings that have been changed over time. In multi-machine systems, control from a common header or sequencer gives consistent behaviour. Verifying each sensor and recording settings usually resolves the issue and can lower system pressure.
How often should compressor sensors be calibrated?
There is no single rule. Follow the compressor and sensor makers’ recommendations and your plant’s quality procedures. As practical guidance, check pressure and temperature readings against references at major services, calibrate dew-point sensors as their maker advises and verify flow meters before they are used for audits. Regulated industries may require documented calibration at fixed intervals.
Can I replace a compressor controller with a different brand?
It is possible but needs care. The new controller must handle the machine’s sensors, protective trips, load and unload logic, starter or VSD interface and any sequencing. Every input must be scaled and tested, and all safety functions proved before the machine returns to service. A like-for-like replacement is usually simpler where available.
What causes a compressor dew-point sensor to give wrong readings?
Dew-point sensors drift with age and can be contaminated by oil, dirt or liquid water. Wrong installation also causes errors: no sample flow, a dead leg or a sample at the wrong pressure. If the reading never changes, or disagrees with a portable reference meter, have the sensor cleaned, recalibrated or exchanged as its maker recommends.
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Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.