How to Convert a Refrigeration System from Mineral Oil to POE Oil Using a Refractometer

A practical guide to measuring residual refrigeration oil during a lubricant conversion

Converting an existing refrigeration system from mineral oil or another legacy lubricant to polyolester (POE) oil requires more than simply draining the compressor and adding new oil.

The objective is to remove as much of the original lubricant as practical and verify that the amount remaining is within the requirements established for the particular compressor, lubricant, refrigerant and application.

A refractometer provides a practical way to monitor that process.

The BOSS Refrigeration Oil Refractometer measures refractive index from a small oil sample. By establishing reference readings for the original and replacement lubricants, a technician can use subsequent system samples to monitor the changing oil mixture and estimate the amount of residual original lubricant.

This guide explains the basic method, how to use the refractometer, how to collect representative samples, and how to evaluate the results.

Important: This guide describes a measurement method. It does not replace the retrofit instructions supplied by the compressor, refrigerant or lubricant manufacturer. Always verify that the proposed refrigerant and lubricant combination is approved for the specific equipment and follow the manufacturer’s requirements for allowable residual oil.

Why Change the Refrigeration Oil?

Older refrigeration systems may use mineral oil (MO) or alkylbenzene (AB) lubricants. Many newer refrigerants and compressor systems use different lubricant technologies, including POE.

The lubricant must be compatible with the refrigerant, compressor and system components. POE oils are commonly used with many HFC and HFO refrigerant applications, while mineral oil has traditionally been used with many HCFC systems.

Oil compatibility is not simply a matter of choosing a replacement oil with similar viscosity. The refrigerant/lubricant combination, compressor design, oil return characteristics, seals and other system components all need to be considered.

For example, Danfoss notes that POE is commonly used with HFC refrigerants and that POE is more hygroscopic than mineral oil, making moisture control particularly important during service. (Danfoss Assets)

Before beginning a conversion, consult the equipment and lubricant manufacturers’ documentation.

Why Residual Oil Matters

When the original lubricant is drained from a compressor, some of that oil remains elsewhere in the system.

Oil can remain in:

• Compressor crankcases
• Suction lines
• Discharge lines
• Oil traps
• Evaporators
• Condensers
• Accumulators
• Heat exchangers
• Other low points and areas where oil collects

Simply draining the compressor therefore does not necessarily mean that the system has been converted to the desired lubricant composition.

The acceptable residual level depends on the particular application.

For example, Copeland retrofit guidance states that when converting from mineral-based oil to POE, the maximum permissible residual mineral oil is 5% for the applications covered by that guidance. Copeland also identifies applications using enhanced surface heat exchangers where excessive mineral oil can affect heat transfer and specifies a 1% maximum for those applications. (Copeland)

Do not assume 5% is a universal limit. Always use the requirement specified for the actual compressor, lubricant and application.

How a Refractometer Helps

A refractometer measures the refractive index (RI) of a liquid.

Different refrigeration oils have different refractive-index characteristics. When two oils are mixed, the refractive index of the resulting sample can change as the relative proportions of the oils change.

This makes refractive-index measurement useful for monitoring an oil conversion.

The general process is:

1. Measure the new replacement oil.
2. Measure the original oil.
3. Remove and replace oil according to the applicable retrofit procedure.
4. Operate the system long enough to obtain representative oil circulation and mixing.
5. Take another oil sample.
6. Measure its refractive index.
7. Compare the result with the original and replacement oil reference readings.
8. Repeat the oil-change/sample process as required by the manufacturer’s procedure.

Nu-Calgon describes this same general approach for its refrigeration-oil refractometer: establish readings for the new and old oils, then measure subsequent system samples and use the results to determine residual old oil. (Nucalgon)

What You Need

For a typical refractometer-based oil conversion procedure, you will need:

• BOSS Refrigeration Oil Refractometer
• Appropriate calibration fluid
• Transfer pipettes or droppers
• Clean sample containers
• Clean cloth or suitable lens/prism cleaning material
• The replacement refrigeration oil
• Information identifying the original lubricant
• Manufacturer’s retrofit instructions
• Compressor and system documentation
• Appropriate refrigerant recovery and charging equipment
• Replacement filter-drier(s), when required by the retrofit procedure

BOSS/NISupply also offers 1.4860 nD refractive-index calibration fluid for calibration of compatible refractometers.

Step 1: Identify the Existing Oil

Before changing anything, determine what lubricant is currently in the system.

Do not rely solely on the refrigerant designation.

Record:

• Compressor manufacturer
• Compressor model
• Existing refrigerant
• Existing lubricant type
• Existing lubricant brand and viscosity, if known
• Proposed refrigerant
• Proposed lubricant
• Proposed lubricant viscosity
• Manufacturer’s allowable residual-oil requirement

If the existing lubricant cannot be positively identified, obtain the best available information from the equipment manufacturer or service records before proceeding.

Step 2: Identify the Replacement Oil

The new lubricant should be selected according to the compressor and refrigerant manufacturer’s requirements.

Record the exact lubricant:

Manufacturer: ______________________

Product: ___________________________

Viscosity: __________________________

Lot or batch number, if applicable: __________________

This matters because refractive-index values are not universal for an entire lubricant category.

Two different POE oils may not have identical refractive-index readings.

Likewise, two mineral oils can have different readings.

For that reason, the technician should establish a reference reading using the actual replacement oil whenever practical.

Step 3: Calibrate the BOSS Refractometer

Calibration should be performed before beginning the measurements and periodically thereafter.

For best results, allow the refractometer and calibration fluid to reach the same temperature before calibration.

The BOSS Refrigeration Oil Refractometer has a refractive-index scale of approximately 1.435–1.520 nD, with 0.001 nD divisions.

Calibration procedure

1. Clean the prism thoroughly.
2. Place several drops of the appropriate calibration fluid on the prism.
3. Close the daylight plate.
4. Allow the sample and instrument to reach a stable temperature.
5. Look through the eyepiece toward a suitable light source.
6. Adjust the focus until the scale and boundary line are clear.
7. Adjust the calibration mechanism until the boundary aligns with the certified refractive-index value of the calibration fluid.
8. Remove the calibration fluid.
9. Clean the prism and daylight plate.

For the BOSS calibration procedure, use calibration fluid with a known certified refractive-index value appropriate for the instrument.

BOSS/NISupply offers 1.4860 nD Refractive Index Calibration Fluid for compatible refractometers.

Do not substitute an unknown liquid for a calibration standard.

Step 4: Measure the New Oil

Before the conversion begins, measure a sample of the new replacement oil.

This establishes the target-oil reference.

Place a small amount of the replacement oil on the clean prism and take the reading according to the instrument instructions.

Record the result.

New Oil Reference

Oil manufacturer: __________________________

Oil: ______________________________________

RI reading: _________________________________

Temperature: _______________________________

Date: ______________________________________

This reading becomes one of the two reference points used to evaluate subsequent system samples.

Step 5: Measure the Existing Oil

Before the first oil change, obtain a representative sample of the original lubricant.

This is the second critical reference point.

Ideally, the sample should represent the oil circulating in the system rather than an isolated quantity that has been sitting in a particular location.

Record:

Original Oil Reference

Oil manufacturer: __________________________

Oil: ______________________________________

RI reading: _________________________________

Temperature: _______________________________

Date: ______________________________________

You now have two reference values:

New oil RI: __________________

Original oil RI: ______________

The difference between those values is what allows subsequent system samples to be evaluated.

Step 6: Perform the First Oil Change

Follow the compressor, refrigerant and lubricant manufacturer’s procedure for removing and replacing the lubricant.

Do not use the refractometer as a substitute for the manufacturer’s retrofit procedure.

Depending on the equipment and conversion, the procedure may involve:

• Recovering refrigerant
• Removing compressor oil
• Replacing oil
• Replacing filter-driers
• Restarting the system
• Allowing oil to circulate
• Sampling the system oil
• Measuring residual original lubricant
• Repeating the oil-change process when necessary

Copeland and other manufacturers provide equipment-specific retrofit procedures. Those procedures should take precedence over a generalized procedure such as this one. (Copeland)

Step 7: Allow the Oil to Circulate

After an oil change, the replacement oil needs an opportunity to mix with the remaining original oil.

A sample taken immediately after an oil change may not represent the overall system oil composition.

Nu-Calgon’s published refractometer procedure recommends operating the compressor for 48 hours between lubricant changes to obtain a good mixture and warns that false readings can result when adequate mixing does not occur. (Nucalgon)

However, do not treat 48 hours as a universal requirement for every refrigeration system.

The appropriate operating and sampling procedure depends on the system design and the manufacturer’s retrofit instructions.

The goal is to obtain a representative sample after sufficient oil circulation.

Step 8: Take a System Oil Sample

Once the system has operated according to the applicable retrofit procedure, obtain an oil sample.

Sampling location matters.

If the compressor manufacturer specifies a particular sampling location, use that location.

When multiple compressors are involved, do not automatically assume that one compressor’s sample represents the entire system.

Nu-Calgon’s own retrofit procedure, for example, calls for sampling each compressor when multiple compressors are involved and averaging the results in the specific procedure described. (Nucalgon)

Use clean sampling equipment and avoid contaminating the sample with:

• Cleaning fluids
• Moisture
• Dirt
• Refrigerant-system debris
• Other lubricants
• Residue from previous samples

Step 9: Measure the System Sample

Clean the prism before taking the measurement.

Place a few drops of the oil sample on the prism.

Close the daylight plate and allow the sample to settle.

If bubbles are present, carefully open and close the daylight plate as appropriate to help release trapped air from the sample.

Look through the eyepiece and obtain a sharp boundary line.

Record the RI reading.

System Sample

Sample number: __________________

Date: ____________________________

RI: ______________________________

Temperature: _____________________

Sampling location: _______________

Hours of operation since oil change: __________

Step 10: Estimate Residual Original Oil

The refractometer does not directly display “percent mineral oil.”

Instead, the RI measurement is used with the reference readings established for the original and replacement oils.

The traditional method described in refrigeration-oil refractometer procedures is to plot:

• The RI of the new oil
• The RI of the original oil

and connect the two reference points.

The RI of a subsequent system sample is then compared with that reference relationship to estimate the percentage of original oil remaining.

This produces an estimate of residual lubricant content.

A simplified example

Assume, for illustration only:

New oil: 1.4550 RI

Original oil: 1.4950 RI

A subsequent system sample reads:

1.4750 RI

If the system sample falls halfway between the two reference values, the simplified linear method would indicate approximately 50% residual original oil.

This is an illustration of the calculation method, not a universal rule for every oil combination.

The actual relationship between oils should be established using the actual lubricants involved and the applicable manufacturer procedure.

A Useful Calculation

When the applicable oil pair and procedure support a linear interpolation, the estimated fraction of original oil can be expressed as:

Residual Original Oil (%) =

(System Sample RI − New Oil RI) ÷ (Original Oil RI − New Oil RI) × 100

For example:

New oil RI = 1.4550

Original oil RI = 1.4950

System sample RI = 1.4750

Residual:

(1.4750 − 1.4550) ÷ (1.4950 − 1.4550) × 100

= 0.0200 ÷ 0.0400 × 100

= 50%

Again, this calculation should be treated as a method for interpreting the measurement, not as permission to assume that every lubricant mixture behaves identically.

When manufacturer-specific charts or procedures are available, follow them.

Step 11: Decide Whether Another Oil Change Is Required

Compare the measured residual level with the maximum allowable residual specified for the application.

If the measured residual remains above the manufacturer’s requirement, another oil-change cycle may be necessary.

The process is then repeated:

Drain → replace → circulate → sample → measure → evaluate

Continue according to the applicable retrofit procedure until the required residual level is reached.

Do not perform additional oil changes simply because a generic percentage appears on a chart. The target must come from the equipment/lubricant manufacturer’s requirements.

Why Sampling Time Matters

One of the easiest ways to obtain a misleading result is to take a sample before the oils have adequately mixed.

Imagine a system in which the compressor has recently received fresh POE.

A sample taken immediately afterward may contain a disproportionately high amount of the new oil.

That sample could make the conversion appear further along than it really is.

The opposite can also happen if a sample is taken from a location containing a concentration of old oil.

For this reason, representative sampling and adequate oil circulation are just as important as the refractometer reading itself.

The refractometer cannot correct for a poor sample.

Temperature and Refractometer Accuracy

Refractive index changes with temperature.

The BOSS Refrigeration Oil Refractometer incorporates automatic temperature compensation within its specified operating range, but good measurement practice still matters.

For consistent results:

• Keep the instrument and sample within the instrument’s operating range.
• Avoid measuring extremely hot oil.
• Allow samples to reach a stable temperature.
• Avoid large temperature differences between calibration and measurement.
• Calibrate using the manufacturer’s recommended procedure.
• Keep the prism clean.
• Use the same sampling and measurement procedure for each test.

The Nu-Calgon procedure similarly recommends standardizing the instrument with calibration fluid and keeping the calibration fluid and instrument at the same temperature or at 20°C/68°F. (Nucalgon)

Common Causes of Bad Readings

Dirty prism

Even a small amount of oil from the previous sample can influence the next reading.

Solution: Clean the prism after every measurement.

Air bubbles

Bubbles can interfere with the optical boundary.

Solution: Apply the sample carefully and eliminate trapped air before reading.

Poor calibration

An incorrectly calibrated instrument produces systematically incorrect results.

Solution: Calibrate before use and periodically thereafter using a known standard.

Sample contamination

A dirty container, pipette or sampling point can introduce another lubricant into the sample.

Solution: Use clean sampling equipment and containers.

Non-representative sample

A sample from an isolated oil pocket may not represent the overall system.

Solution: Follow the equipment manufacturer’s sampling procedure.

Insufficient circulation

Sampling too soon after an oil change can produce misleading results.

Solution: Follow the applicable manufacturer’s operating/mixing requirements before sampling.

Wrong reference oil

Using a generic “POE” or “mineral oil” RI value instead of measuring the actual lubricant can introduce error.

Solution: Establish a reference reading using the actual oil whenever possible.

Do Not Confuse Oil Conversion With Refrigerant Conversion

Changing the refrigerant and changing the lubricant are related procedures, but they are not necessarily the same thing.

A refrigerant retrofit may require evaluation of:

• Compressor compatibility
• Lubricant compatibility
• Refrigerant pressure
• Pressure-temperature relationships
• Expansion devices
• Filter-driers
• Seals and elastomers
• System controls
• Oil return
• Superheat
• Operating pressures
• Discharge temperature
• Refrigerant charge procedure

Danfoss, for example, notes that retrofit projects can involve compatibility considerations for seals and other components, and that moving from mineral oil or alkylbenzene to POE/PVE can change compatibility considerations. (Danfoss Assets)

The BOSS refractometer addresses one part of that process: measuring refractive index to monitor lubricant composition.

Mineral Oil, Alkylbenzene, POE and PAG

The BOSS refractometer can be used with compatible oils whose refractive indices fall within the instrument’s measurement range.

Common refrigeration lubricant categories include:

Mineral Oil (MO)

Traditional lubricant used with many older refrigeration systems and refrigerants.

Alkylbenzene (AB)

A synthetic hydrocarbon lubricant used in certain refrigeration applications.

Polyolester (POE)

Widely used with many HFC/HFO refrigeration applications.

Polyalkylene Glycol (PAG)

Used in certain refrigeration and air-conditioning applications. Compatibility must be evaluated for the specific equipment and refrigerant.

The refractometer measures refractive index, not oil type.

The technician must therefore establish the relationship between the actual oils being evaluated.

The BOSS Refrigeration Oil Conversion Worksheet

Use this worksheet to keep a record of each conversion.

System Information

Customer: ______________________________________

System: ________________________________________

Compressor: ____________________________________

Compressor Model: _______________________________

Original Refrigerant: ____________________________

Replacement Refrigerant: _________________________

Original Oil: ___________________________________

Replacement Oil: ________________________________

Manufacturer’s Maximum Residual: ________________

Reference Measurements

Measurement	RI
Replacement oil	______
Original oil	______

Conversion Measurements

Sample	Date	Hours Since Oil Change	RI	Estimated Residual	Action
1
2
3
4
5

When Should You Stop Changing the Oil?

There is no single universal percentage that applies to every refrigeration system.

The correct stopping point depends on the equipment and lubricant manufacturer’s requirements.

For some applications, published guidance uses a 5% maximum residual mineral-oil level. Other applications may have more stringent requirements. Copeland, for example, identifies a 1% maximum for certain systems with enhanced surface heat exchangers. (Copeland)

Therefore:

Do not use the BOSS refractometer alone to decide whether a system is ready for conversion.

Use the measurement together with the applicable compressor, refrigerant and lubricant manufacturer’s requirements.

Best Practices for Reliable Results

1. Calibrate before starting

Start the conversion with a known calibration standard.

2. Measure the actual new oil

Don’t rely on a generic RI value when the actual replacement oil is available.

3. Measure the actual old oil

This establishes the second reference point.

4. Keep records

Write down every RI reading, sampling location and oil-change event.

5. Use clean sampling equipment

Cross-contamination can defeat the purpose of the measurement.

6. Allow adequate mixing

A sample is only useful if it represents the oil mixture in the system.

7. Use the same technique every time

Consistency makes trends easier to interpret.

8. Follow manufacturer requirements

The refractometer is a measurement tool, not a replacement for the equipment manufacturer’s retrofit instructions.

BOSS Refrigeration Oil Refractometer

The BOSS Refrigeration Oil Refractometer is designed to give HVAC/R technicians a practical way to monitor refrigeration lubricant composition during oil conversions.

Features

• 1.435–1.520 nD refractive-index range
• 0.001 nD scale divisions
• Automatic temperature compensation
• Handheld optical design
• No batteries required
• Field calibration
• Small sample requirement
• Designed for refrigeration-oil applications
• Protective carrying case

The instrument can help turn an oil conversion from a purely procedural exercise into a measurable process.

Calibration Fluid

For repeatable measurements, calibration should be part of the technician’s normal workflow.

NISupply offers 1.4860 nD Refractive Index Calibration Fluid for compatible refractometers.

Using a known refractive-index standard allows the technician to verify the instrument before beginning a series of oil measurements.

BOSS Refrigeration Oil Refractometer:
[Link to product]

1.4860 nD Calibration Fluid:
[Link to calibration fluid]

Final Takeaway

A successful refrigeration-oil conversion is about more than removing oil from the compressor.

The system can retain lubricant throughout the refrigeration circuit, and the amount remaining can matter to the performance and compatibility of the converted system.

A refractometer provides a practical measurement method for monitoring that residual lubricant.

The basic approach is straightforward:

Establish the new-oil reference.

Establish the old-oil reference.

Perform the oil change according to the applicable retrofit procedure.

Allow adequate circulation and mixing.

Take a representative sample.

Measure the refractive index.

Evaluate the result against the manufacturer’s requirements.

Repeat when necessary.

Used correctly, the BOSS Refrigeration Oil Refractometer gives the technician objective data to help determine how the conversion is progressing rather than relying solely on assumptions about how much original oil remains.

Important Safety and Technical Disclaimer

This guide is provided for general technical and educational purposes. Refrigeration system retrofit procedures can involve high pressures, refrigerants, electrical hazards, hot components and other hazards.

Only qualified personnel should perform refrigeration service.

Always follow the applicable compressor, equipment, refrigerant and lubricant manufacturer’s instructions, including requirements for refrigerant recovery, evacuation, oil selection, oil quantity, compatibility, allowable residual lubricant, filter-drier replacement, charging and system commissioning.

The BOSS Refrigeration Oil Refractometer is a measurement instrument. A refractive-index measurement does not by itself establish that a refrigeration system is suitable for a particular refrigerant or lubricant, nor does it replace manufacturer-specific retrofit requirements.