June 17, 2026
9:42 am
Signal Group

5 Signs Your Gas Analyser Is Letting You Down

Gas analysers are expected to run continuously, often in demanding industrial environments, providing accurate and reliable data day after day. Most of the time, they do exactly that. But like any precision instrument, their performance can degrade – and the signs that something is wrong are not always obvious.

The most common measurement problems don’t announce themselves with an alarm or an error code. They show up gradually: a calibration that takes slightly longer than it used to, a reading that doesn’t quite match a reference instrument, filters that seem to need changing more often. By the time the problem is clearly visible, it may have been building for weeks or months – and the measurement data produced during that period may be unreliable.

This guide covers the five signs that most commonly indicate a gas analyser is underperforming. For each one, we explain what it means, what typically causes it, and what to do about it.

Note: This guide is intended for plant engineers, EHS managers, and maintenance leads responsible for gas analysers used in continuous emissions monitoring, VOC measurement, or process control applications.

Sign 1: Frequent Unexpected Calibration Failures

Calibration is a routine part of any gas measurement programme. Scheduled zero and span checks confirm that the instrument is reading correctly and allow any drift to be corrected. When an instrument regularly fails to accept calibration – or when it passes calibration but drifts back out of tolerance quickly – this is a sign that something is wrong.

What it means for compliance

For analysers used in continuous emissions monitoring, calibration records form part of the data quality audit trail. Frequent calibration failures create gaps in that trail and may trigger data disregard requirements. If the instrument is failing calibration checks regularly, the reliability of the measurement data produced between those checks is also in question.

Common causes

Detector degradation — FID detectors, for example, can lose sensitivity over time due to contamination of the jet or collector. NDIR detectors can be affected by source ageing or cell contamination.
Sample system contamination — particulates or liquid carry-over reaching the detector can cause baseline shifts that manifest as calibration failures.
Calibration gas issues — expired or out-of-specification calibration gases produce calibration failures that appear to be instrument problems. Always verify gas certification dates before investigating the analyser.
Reference gas delivery problems — leaks or blockages in the calibration gas delivery path mean the correct concentration never reaches the analyser inlet.

What to do

Start with the calibration gas supply – verify the certification date and check the delivery path for leaks or restrictions. If gases and delivery are confirmed good, inspect the sample system for contamination. If the problem persists, a service check of the detector is recommended.

Signal tip: Always confirm your calibration gas certificate is in date before concluding the instrument has a problem. Expired span gas is a surprisingly common cause of unexplained calibration failures.

Sign 2: Visible Drift Between Scheduled Calibrations

Some degree of instrument drift is normal and expected – it is why calibration schedules exist. What is not normal is drift that is noticeably larger than it used to be, or that occurs faster than the instrument’s specification would predict.

If you can observe the reading changing gradually over a day or a week without a corresponding change in process conditions, or if the correction applied at each calibration is growing, the instrument’s stability is deteriorating.

What it means for data quality

Increased drift means the gap between a freshly calibrated reading and the true value grows more quickly over time. Data collected in the later part of a calibration interval may be further from the truth than it should be. For compliance monitoring with defined data quality requirements, this can directly affect whether your data is considered valid.

Common causes

Detector ageing — all detectors have a finite service life. Gradual performance degradation is normal, but accelerating drift indicates a detector approaching end of life or suffering contamination damage.
Temperature effects — if the analyser housing temperature control has degraded, or if the instrument is exposed to larger temperature variations than before (for example, due to changes in the local environment), this can manifest as increased drift.
Sample contamination — even low levels of contamination that don’t cause acute failures can gradually affect detector performance and increase drift over time.
Component ageing — electronic components, light sources, and other active components have defined service lives. Performance typically degrades gradually before failure.

What to do

Log calibration correction values over time and look for trends. A gradual increase in the correction applied at each calibration is a clear indicator of growing drift. If the trend is established, schedule a service inspection before the drift exceeds permissible limits.

Sign 3: Slow or Sluggish Response Time

Every analyser has a specified T90 response time – the time taken to reach 90% of the correct reading after a step change in concentration. When the actual response time is significantly slower than the specification, or noticeably slower than it used to be, something in the measurement system is impeding the sample.

Why response time matters

In most continuous compliance monitoring applications, a moderately slow response time is not a critical problem – the process concentration changes relatively slowly, and a few extra seconds to reach the reading rarely affects the measured average. However, in process control applications where rapid response is required to trigger actions, or where peak concentrations need to be captured accurately, a degraded response time can have real operational consequences.

More importantly, a significant increase in response time often indicates a problem in the sample system that, if left unaddressed, will eventually cause more serious measurement errors.

Common causes

Filter blockage — a partially blocked filter restricts sample flow, slowing the rate at which fresh sample reaches the detector. This is one of the most common causes of increased response time and one of the easiest to address.
Sample line restriction — any restriction in the sample path — a partially blocked fitting, a kinked heated line, or a condensate trap that hasn’t been drained — will reduce flow and slow response.
Sample pump degradation — a pump that is losing efficiency will deliver reduced flow, increasing both response time and the risk of sample composition changes in transit.
Increased dead volume — additional tubing, fittings, or sample handling components added to the system over time increase the volume of sample that must be displaced before fresh gas reaches the detector.

What to do

Check and replace filters as a first step. If response time doesn’t improve, check the sample pump flow rate against specification. Inspect the sample line for any restrictions, including condensate traps and fittings.

Sign 4: Increasing Consumable Usage

Gas analysers use consumables at a broadly predictable rate. Filters have expected replacement intervals. FID detectors consume hydrogen fuel at a known rate. When these consumables are being used faster than expected – or faster than they used to be – it is a sign that something in the system has changed.

Filters

Filters protect the analyser from particulate contamination. If filters are blocking faster than before, the particulate loading in the sample gas has increased. This could indicate a process change upstream, a degraded probe filter allowing more material through, or a change in operating conditions. The root cause should be identified rather than simply increasing the filter replacement frequency – the same contamination that is blocking filters quickly is also reaching downstream components and may be causing damage.

FID fuel (hydrogen)

FID analysers consume hydrogen fuel as part of their measurement principle. Fuel consumption should be stable and predictable. A sudden increase in fuel consumption often indicates a leak in the fuel system – which is both a safety concern and a measurement quality issue. Unexplained changes in fuel consumption should be investigated promptly.

Air filter change intervals

If air filter change intervals are shortening, this often indicates elevated background air contamination reaching the instrument. High background hydrocarbon levels in the incoming air supply can cause significant drift and noise problems – the instrument is being exposed to contamination through its reference air path as well as its sample path. In these situations, a simple consumable carbon trap may become saturated too quickly to be effective. A catalytic air purifier is a more appropriate solution, providing a reliable clean air reference without the rapid saturation issue associated with carbon-based filters in contaminated environments.

What to do

Don’t simply replace consumables at an accelerated rate without investigating why consumption has increased. The consumable is telling you something about the condition of the upstream sample system or process. Identify and address the root cause.

Signal tip: Keep a simple log of consumable replacement dates. A pattern of accelerating replacement frequency is often the earliest visible indicator of a developing problem elsewhere in the system.

Sign 5: Poor Correlation with Other Equivalent Instruments

Comparisons between a fixed installed analyser and another instrument arise in a number of situations: portable cross-checks during routine quality assurance, comparison with a rental instrument during commissioning, or correlation testing against a reference method during QAL2. In each case, a small difference between the two instruments is normal and expected – they are measuring under slightly different conditions, with different calibration references, and potentially at different points in the process. A large, persistent difference is not normal and warrants investigation.

Interpreting the difference

Before concluding the fixed analyser has a problem, consider the following:

Is the reference instrument itself calibrated and in good condition? An out-of-calibration or damaged reference is a common source of apparent discrepancy.
Are the two instruments measuring at exactly the same point in the process, under the same conditions? Small differences in sampling location can produce genuine concentration differences.
Is the comparison being made under stable process conditions? Rapid fluctuations make direct comparisons difficult.

If these factors are eliminated and a significant difference remains, the fixed analyser should be investigated.

Common causes of persistent discrepancy

Sample system losses — analyte being lost in the sample conditioning system before reaching the detector. This is particularly common where heated sample lines are operating below their specified temperature, or where condensate management is inadequate.
Detector sensitivity loss — a detector that has degraded in sensitivity will underread relative to a correctly performing reference instrument.
Calibration gas discrepancy — if the fixed analyser’s span calibration gas is out of specification or has degraded, the calibration will be wrong and all readings will be offset accordingly.
Probe or extraction issues — a partially blocked probe or incorrectly positioned extraction point may produce a sample that is not representative of the measurement location.
Reference instrument problems — portable instruments in particular can be less inherently stable than a continuous fixed analyser, and because they are transported between sites they are subject to knocks and damage that may not be immediately obvious but can introduce significant errors into their readings. This can make it appear that the static analyser is at fault when the problem lies with the reference. Where the discrepancy cannot be resolved between two instruments, a third instrument is sometimes required to determine which is reading correctly.

Signal tip: Signal maintains a fleet of rental analysers for exactly these situations – providing a verified reference instrument to resolve disputes between two instruments without waiting for an external testing organisation. Contact us to discuss rental availability.

FID fuel gas contamination (FID analysers) — due to the way hydrogen is manufactured and stored, it is common to find low levels of hydrocarbon contamination in the fuel gas supply. For FID analysers, this background contamination contributes directly to the flame signal and elevates the reading. If two FID analysers are using separate hydrogen supplies with different contamination levels, they may read very differently from one another — not because either instrument is faulty, but because their fuel sources are not equivalent.

Signal tip: If fuel gas contamination is suspected, swap out the fuel carbon filter for a new one and observe the readings immediately after re-ignition. In cases of high contamination, the new filter will saturate quickly – watch for a reading change in the first few minutes after ignition. If the reading drops after the filter change, the previous fuel supply was contaminated. Using both analysers from the same fuel cylinder during a cross-check eliminates this variable entirely.

What to do

Confirm the reference instrument is calibrated and perform the comparison under stable process conditions. If a significant difference persists, check the sample system temperature and integrity first, then the calibration gas supply, then the detector. Where the discrepancy cannot be resolved between two instruments, introducing a third instrument is sometimes the only way to determine which is at fault. A service inspection of the fixed analyser is recommended if the root cause is not identified quickly.

When to Act

Any of the five signs described in this guide warrants investigation. A single sign may have a straightforward explanation – an expired calibration gas, a blocked filter, a fuel carbon filter that simply needs replacing. But a combination of signs, or a single sign that persists after obvious causes are eliminated, indicates a more fundamental issue with the instrument or sample system.

The key principle is not to normalise degraded performance. If the instrument required weekly calibration correction a year ago and now needs it daily, that is not simply ‘the way it is’ – it is a sign of deterioration that will continue if not addressed.