By Yevhen Kovalchuk · Updated October 14, 2026 · 5 min read
Short answer
Oil analysis shows the condition of the oil and, through it, the condition of the compressor: whether the oil can still protect the machine, whether contaminants such as water, acid or dirt are getting in, and which parts are wearing. Its real value comes from trends across regular samples, which often reveal bearing wear, moisture or overheating long before the compressor shows symptoms.
Oil circulates through every bearing, seal and rotor clearance in a compressor, so it carries a record of what is happening inside. A lab report on a small sample answers three questions: is the oil still fit for service, what is contaminating it, and what is wearing. For screw and reciprocating compressors in industrial refrigeration, it is one of the cheapest and most informative condition-monitoring tools available.
What does an oil analysis report measure?
A standard refrigeration oil test package usually includes these parameters:
| Parameter | Typical method | What a rising (or changing) value suggests |
|---|---|---|
| Viscosity | Kinematic viscosity at 104 °F (40 °C), ASTM D445 | A drop: refrigerant dilution, wrong oil added or shearing. A rise: oxidation, contamination or mixing with a heavier oil |
| Water | Karl Fischer titration (ASTM D6304) | Moisture entering the system through leaks on the low-pressure side, wet maintenance or a failing dryer |
| Acid number (TAN) | Titration (ASTM D974 or D664) | Oil oxidation from high temperatures, or hydrolysis of ester oil in the presence of water |
| Wear metals | Elemental spectroscopy (for example ASTM D5185) | Iron, copper, tin, lead, aluminum or chromium from bearings, rotors, pistons or bushings |
| Particle count | ISO 4406 code | Dirt and wear debris getting past filters; filter bypass or a failing filter element |
| Additives | Elemental spectroscopy | Additive depletion, or a different oil topped up by mistake |
The lab usually adds a short comment and flags each value as normal, marginal or critical. Those flags are based on general limits. The more useful comparison is with the earlier samples from the same machine.
How do you read wear metals?
Wear metals point to the part that is wearing, and the combination tells more than any single element.
- Iron comes from rotors, shafts, cylinder liners, roller bearing races and the casing. A slow, steady rise is normal aging; a sharp jump is a warning.
- Copper, tin and lead are typical of bearing alloys and bushings. Rising copper in an HFC system can also come from copper tubing, a sign of chemical attack rather than mechanical wear.
- Aluminum may come from pistons, some bearing cages or certain housings, depending on the design.
- Chromium can come from piston rings or alloy steel parts.
Spectroscopy only sees very fine particles. Larger fragments from a bearing in the late stages of failure may not show up in the elemental results at all, which is why particle counts and, when needed, ferrography are useful companions.
What does the ISO 4406 particle code mean?
ISO 4406 reports cleanliness as three numbers, for example 18/16/13. They are range codes for the number of particles per milliliter larger than 4, 6 and 14 micrometers. Each step up in a code number roughly doubles the particle count, so a change from 16 to 18 means about four times more particles of that size.
Compressor manufacturers and oil suppliers publish target cleanliness levels for their machines. A code that creeps upward over several samples usually means the filter is no longer doing its job or that a new source of debris has appeared.
How is oil analysis different for ammonia and HFC systems?
The refrigerant changes what you look for.
Ammonia systems typically use mineral, hydrotreated or synthetic (PAO) oils that do not mix with ammonia. The oil is separated and drained rather than returned with the refrigerant, and drained oil is normally not reused. Ammonia can react with some oils to form sludge, and high discharge temperatures speed up oxidation, so viscosity, TAN and the appearance of the sample deserve attention. Water in ammonia systems mostly stays with the refrigerant, so water in the oil is not a full picture of system moisture.
HFC systems mostly use polyolester (POE) oil, which mixes with the refrigerant. POE is hygroscopic: it absorbs moisture from the air quickly. Water and heat break POE down into acids, which attack copper and can lead to copper plating on bearing surfaces. In these systems, water content and acid number are the most important early signals, and a rising acid number is a reason to check the filter-driers and the source of moisture.
In both cases, refrigerant dissolved in the sample can affect viscosity. Labs that specialize in refrigeration oils know how to handle this, so it is worth telling them which refrigerant the system uses.
How should a sample be taken?
A bad sample gives a misleading report. A few habits make the results reliable:
- Sample with the compressor running or just stopped, with the oil at operating temperature.
- Use the same sampling point every time, ideally a dedicated valve upstream of the oil filter.
- Flush the valve and line before filling the bottle, so you do not sample stagnant oil.
- Use clean, dry bottles from the lab and close them immediately, especially with POE oil.
- Record the machine, run hours, hours since the last oil change and any oil added.
Follow the safety procedures for the refrigerant. On ammonia systems, opening an oil drain or sample point means working with a toxic refrigerant, and the plant’s procedures for that task apply.
How often should you sample?
There is no single rule. Many plants sample at a fixed interval, such as quarterly or every set number of run hours, and more often after a major repair, a contamination event or an abnormal result. The compressor manufacturer’s recommendations are the starting point. What matters most is consistency, because one sample is a snapshot and a series of samples is a trend.
Our team in Ukraine maintains MYCOM screw compressors on an ammonia system at a cheese plant. On machines like these, the useful question is never whether one number crossed a limit, but how the iron content, particle count and viscosity are moving together from sample to sample.
If you work with industrial refrigeration in Central Texas and care about this kind of detail, we would like to meet you. See our open positions or partnership options.
How do you act on the results?
A report is only useful if it triggers a clear decision. A sensible approach:
- Compare to the baseline. One sample is a snapshot; a trend across several samples is the real signal. Sample from the same point each time.
- Match the finding to a cause. Elevated moisture or acid points to system contamination, while rising wear metals suggest mechanical wear. Compressor burnout causes and symptoms and common compressor failure causes help connect results to likely mechanisms.
- Cross-check with operation. Compare with the physical signs in five signs a compressor is wearing out: vibration, noise, temperature and amperage.
- Use the right oil. Never mix oil types casually; see compressor oil types: POE, PAG and mineral.
- Record the action. Note what was changed and re-sample afterwards to confirm the problem is resolved.
If results are worrying, ask the compressor manufacturer or a qualified technician to interpret them in the context of your machine.
Frequently asked questions
Can oil analysis predict a compressor failure?
It can reveal many developing problems early, such as bearing wear, moisture ingress and overheating, often weeks or months before a failure. It cannot catch everything: sudden failures from a liquid slug or an electrical fault may give no warning in the oil. It works best together with vibration trending and operating data.
Should I change the oil if the report looks normal?
Oil change intervals come from the compressor manufacturer, and many allow condition-based changes when oil analysis supports them. A normal report is evidence that the oil is still serviceable. A sudden change in water, acid number or viscosity is usually a reason to change the oil and find the cause.
What does a high acid number mean in an HFC system?
It usually means the POE oil is breaking down, most often because of moisture, high temperature or both. Acids attack copper and motor insulation, so the source of moisture should be found and the driers checked. Left alone, the condition tends to get worse.
