Why sampling decides the result
When a specification calls for oxygen below a few parts per million in nitrogen, moisture at a very low dew point, or ISO 8573-1 Class 0 or Class 1 oil content in compressed air, the measurement is only as good as the sample. Impurities at these levels are easily added by a leaking fitting, a dead leg, an elastomer seal or moisture adsorbed on tube walls. Equally, a sample taken from the wrong place can miss contamination that reaches the product.
Verification therefore has two parts: a sampling system that delivers representative gas to the analyser, and an analysis method that is calibrated, traceable and sensitive enough for the limit being checked.
Start from the process use
Define purity by what the gas does, not by a general “high purity” label. Ask:
- What touches the gas: product, patients, food, electronic wafers, a catalyst, an analyser?
- Which impurities actually harm that use: oxygen, moisture, oil, particles, microbes, hydrocarbons, carbon monoxide, carbon dioxide?
- What limit applies, and on what basis: ppm by volume (mol) or by mass, dew point at line pressure or atmospheric pressure, particle size class?
- Which standard governs it: ISO 8573-1 for compressed air, pharmacopoeia monographs for medical and pharmaceutical gases, food-contact rules, or the customer’s own specification?
A common mistake is to specify every impurity at the tightest level. That adds cost and testing time without benefit. Focus on the impurities that matter for the use.
Common impurities and how they are measured
| Impurity | Typical reporting basis | Common methods | Sampling sensitivity |
|---|---|---|---|
| Moisture | Pressure dew point (°C) or ppm(v) | Chilled mirror, capacitive or aluminium oxide sensors, quartz crystal, cavity ring-down | Very sensitive to tube wall adsorption and leaks; long settling times |
| Oxygen | ppm(v) or % | Electrochemical, zirconia, paramagnetic analysers | Small leaks let air in; purge thoroughly |
| Oil (aerosol, liquid, vapour) | mg/m³ at reference conditions (ISO 8573-1) | Oil-vapour analysers (PID), aerosol sampling to ISO 8573-2, vapour to ISO 8573-5 | Elastomers and lubricated fittings contaminate; use clean, oil-free components |
| Particles | Count per m³ by size class | Laser particle counters, ISO 8573-4 method | Isokinetic sampling and short, smooth lines needed |
| Hydrocarbons, CO, CO₂ | ppm(v) | Gas chromatography, infrared analysers | Materials can outgas hydrocarbons |
| Microbial contamination | Colony-forming units per m³ | Impaction on agar, ISO 8573-7 method | Aseptic handling; sterile sample devices |
Where to take the sample
Sample as close as practical to the point where the gas meets the product, or at the point the specification defines. A sample at the compressor or dryer outlet shows what the treatment produces; a sample at the point of use shows what the pipework delivers. Both are useful, but they answer different questions.
- Take samples from the top or side of a horizontal pipe for gas impurities, avoiding the bottom where liquids and debris collect, unless you are looking for them.
- Avoid dead legs and stagnant branches, which accumulate moisture and give false readings.
- Locate sample points after mixing is complete, not immediately downstream of a tee where two streams join.
- Label each point permanently and record its purpose, so repeated samples come from the same place.
Materials, surface finish and fittings
For low-ppm work, the sampling line itself can be the largest source of error. Electropolished stainless steel tubing, typically 316L, gives the least moisture adsorption and fastest response. Plain stainless steel is acceptable for less demanding limits. Copper is used for some medical gas systems. Plastic and rubber tubing let moisture and oxygen diffuse in and can add hydrocarbons, so keep them out of any line used for trace analysis.
Use metal-sealed compression or face-seal fittings, cleaned for oxygen or high-purity service where required. Avoid thread sealant tape and pastes inside the gas path. Valves and regulators should be specified for high-purity service with low internal volume and diaphragm seals rather than packed stems.
Pressure reduction, purging and response time
Most analysers work near atmospheric pressure, so samples from a 7 bar (101.5 psig) air line or a high-pressure gas line need a pressure-reducing regulator. Reduce pressure close to the sample point to keep the line volume at low pressure small, and keep a steady flow through the analyser.
- Purge the line with sample gas long enough to replace its volume many times before trusting a reading.
- Watch the reading stabilise; moisture readings in particular can take hours to settle after a line is first opened to air.
- Use a bypass or “fast loop” to keep fresh gas moving past the analyser connection on long lines.
- Record the flow, pressure and time at each reading so later results are comparable.
Line volume sets response time. A long, wide line with low flow may take many minutes to show a change in purity, which can hide short contamination events.
Leak tightness and air ingress
Air contains about 21% oxygen and, in Indian monsoon conditions, a large amount of moisture. A tiny leak in a sampling line can therefore add more oxygen or water than the gas itself contains. Even when the line is at positive pressure, back-diffusion through a small leak can add contamination at the ppm level. Leak-test every new sampling system, check fittings after any disturbance and use a test with a known clean gas to confirm the line adds nothing measurable.
Calibration, traceability and control samples
- Calibrate analysers with certified reference gases traceable to national or international standards.
- Check zero and span at intervals set by the analyser maker and your quality system.
- Run a known clean or “zero” gas through the full sampling path, not just the analyser, to prove the line adds no contamination.
- Where laboratories test samples, use accredited laboratories for the specific method and keep their reports with the sample record.
Online analysers or grab samples?
| Approach | Best for | Limitations |
|---|---|---|
| Continuous online analyser | Critical duties where a deviation must be seen quickly: oxygen in nitrogen blanketing, dew point after dryers | Needs maintenance, calibration and a well-designed sample system |
| Portable analyser at intervals | Routine checks on several points; audits | Only shows conditions at the time of the test |
| Grab sample to laboratory | Oil, particles, microbes and full composition analysis | Delay before results; container and handling can affect the sample |
For grab samples, use clean containers suited to the gas and impurity, label them with point, date, time, pressure and sampler, and keep a chain-of-custody record from sampling to laboratory report.
When to sample: operating states matter
Purity often changes with operating state. Sample during normal running and also after start-up, after a dryer regeneration change, after filter replacement, after maintenance that opened the system, and at peak demand. For compressed air with heatless desiccant dryers, remember that the dryer uses a significant share of the dried air for purge, and dew point can drift if purge control or valves fail; checking dew point across a full cycle shows this.
How Reddy Compressors helps
We sell (new and used/refurbished), rent and service air compressors, oil-free compressors, dryers and gas compressors, including nitrogen and booster machines, and supply their spares. From our office at Tarnaka, Secunderabad, more than 20 service engineers and technicians serve sites across Telangana and Andhra Pradesh, including the pharmaceutical and food industries around Hyderabad, and we supply equipment and spares across India.
For purity work we check dew point and air quality at the compressor house and points of use as part of our air audits, which follow the ISO 11011 method (we do not issue conformity certificates). We help locate sampling points, find sources of moisture, oil and particle contamination, and fix the cause: dryer faults, failed drains, overdue filter elements, oil carry-over or pipe contamination. Where a laboratory test against ISO 8573 or a pharmacopoeia method is needed, we arrange sampling with an appropriate accredited laboratory and keep the results with your records. Gas compressor duties for oxygen, hydrogen or toxic gases still follow the gas-specific codes and the maker’s design rules.
What to send us
- The gas (compressed air, nitrogen or other) and what it is used for.
- The purity specification and the standard it refers to.
- Your system layout: compressors, dryers, filters, receivers and main points of use.
- Any existing test results, analyser readings or failed batches.
- Make, model and serial number of the compressor, dryer and filters.
- Line pressure (bar and psig) at the sample points.
Frequently asked questions
How do I test compressed air for ISO 8573-1 purity?
Measure each contaminant with the method in the matching part of ISO 8573: particles (part 4), moisture and dew point (part 3), oil aerosol and liquid (part 2), oil vapour (part 5) and microbes (part 7). Sample at the point that matters, purge the line, use calibrated instruments and compare results with the class your process needs. Report results at the reference conditions the standard defines.
Why does my dew point reading keep changing?
Moisture analysers respond slowly, and tube walls absorb and release water. After a sampling line has been opened to humid air, readings can take a long time to settle. Steady changes can also come from a dryer regeneration cycle, failing drains or changing demand. Purge properly, use stainless steel tubing, and log the reading over a full dryer cycle before drawing conclusions.
Can plastic tubing be used for gas sampling?
Not for trace analysis. Plastic and rubber tubing let moisture and oxygen diffuse in from air and can release hydrocarbons, which distorts low-level results. Use clean stainless steel, ideally electropolished 316L, with metal-sealed fittings for ppm-level work. Plastic may be acceptable only for less demanding checks where the maker of the analyser states it is suitable.
How often should high-purity gas be tested?
Frequency depends on risk. Critical uses, such as nitrogen blanketing of sensitive products or air touching pharmaceutical products, often justify continuous dew point or oxygen monitoring plus periodic laboratory tests. Less critical uses may need checks at set intervals and after maintenance. Your quality system, customer requirements and regulatory expectations decide the frequency, and the results should drive any change.
Where should a compressed air sample point be located?
Place it where the air meets the product or where the specification applies, usually at the point of use after final filtration. Take the sample from the top or side of the pipe, avoid dead legs, and keep the sample line short. A second point at the dryer and filter outlet helps you tell whether contamination comes from treatment or from the pipework.
Do oil-free compressors remove the need for air purity testing?
No. An oil-free compressor does not add lubricating oil to the air, but the intake air can still carry hydrocarbons, moisture, particles and microbes, and pipework can add rust, scale and condensate. Dryers and filters are still needed, and testing at the point of use is the only way to confirm that the air meets its purity class.
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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.