Firefighting Suppression Foam: How to Measure Foam Concentration and Mixing Ratios with a Digital Refractometer

Firefighting foam is engineered to work at a specific concentration. Too little foam concentrate can reduce the effectiveness of the finished foam solution, while an incorrect proportioning rate can affect foam expansion, drainage, vapor suppression, and overall fire-control performance.

That makes foam concentration testing an important part of commissioning, inspection, maintenance, and troubleshooting for fire suppression systems.

One of the most practical field methods is the digital refractometer. A small sample of foam solution can be tested in seconds, providing a quantitative measurement that is far more useful than relying only on appearance, foam blanket characteristics, or equipment settings.

Modern digital refractometers from MISCO are particularly well suited to this application because they can measure refractive index with high resolution. A properly selected BOSS refractometer can also provide a practical, economical field-testing option for applications where a low-range Brix scale is appropriate.

This guide explains the major types of firefighting foam, common manufacturers and products, how proportioning works, and how a refractometer can be used to verify the actual foam-to-water mixture.

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What Is Firefighting Foam?

Firefighting foam is a mixture of foam concentrate, water, and air. The concentrate is proportioned into water at a specified percentage and then aerated through the discharge equipment to create the finished foam.

The foam blanket can help suppress fire by:

• Separating fuel from oxygen
• Suppressing flammable vapors
• Cooling the fuel surface
• Preventing re-ignition
• Wetting and penetrating porous Class A materials
• Creating a barrier between the fuel and ignition sources

The required concentration depends on the type of foam, fuel, application, proportioning equipment, and manufacturer's listing or approval.

For example, a product identified as 3% foam generally means the finished foam solution contains 3 parts concentrate for every 97 parts water. Chemguard describes its proportioning percentages this way, with a 1% solution consisting of one unit of concentrate and 99 units of water.

A 3% solution therefore is not the same thing as a 3:1 mixture. It means approximately:

3% foam concentrate + 97% water = 100% finished foam solution

That distinction matters when testing a proportioning system.

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Major Types of Firefighting Foam

The firefighting industry has several major categories of foam concentrate. The correct product depends on the fuel and application.

Class A Foam

Class A foam is primarily used on ordinary combustibles such as:

• Wood
• Paper
• Brush
• Vegetation
• Tires
• Coal
• Structures

Class A concentrates are often used at relatively low proportioning rates. For example, Chemguard CLASS A PLUS can be proportioned from 0.1% to 1.0%, depending on the application and equipment. Its published application ranges include approximately 0.1% to 0.3% for some CAFS applications and 0.3% to 1.0% for low- and medium-expansion handlines.

Because the concentration can be low, accurate proportioning becomes especially important.

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AFFF: Aqueous Film Forming Foam

AFFF, or aqueous film forming foam, has historically been one of the best-known Class B firefighting foam technologies.

AFFF has been widely used for flammable-liquid fires involving fuels such as:

• Gasoline
• Jet fuel
• Diesel
• Petroleum products

Airport rescue and firefighting has been one of the major applications for AFFF and related Class B foam products.

MISCO specifically identifies AFFF and FFFP as applications for which its Palm Abbe digital refractometer can be used to measure foam concentration.

However, the firefighting-foam market has been undergoing a significant transition away from fluorinated foam technologies. Regulations, environmental requirements, site policies, and individual system approvals need to be considered before selecting or replacing an AFFF product.

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AR-AFFF: Alcohol-Resistant AFFF

AR-AFFF stands for alcohol-resistant aqueous film forming foam.

These concentrates are designed for applications involving both hydrocarbon fuels and certain polar solvents or alcohol-containing fuels.

One reason AR-AFFF products require particular attention is that some products have two proportioning rates.

For example, Chemguard describes a 1% × 3% AR-AFFF product as using one proportioning rate for hydrocarbon fires and another for polar-solvent fires.

Other AR-AFFF products are specified at rates such as 3% × 3% or 3% × 6%.

The number printed on the container is therefore not merely a product identifier. It tells the fire protection professional how the concentrate is intended to be proportioned for the specified application.

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FFFP and Protein-Based Foam

FFFP, or film-forming fluoroprotein foam, is another Class B foam technology that has been used extensively in fuel-fire applications.

Protein-based foams use protein-derived components to produce a stable foam blanket. Synthetic and protein-based formulations have different physical properties and application characteristics.

National Foam, for example, lists specialty protein foam products alongside its synthetic Class B products.

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Fluorine-Free Foam and SFFF

The industry has increasingly moved toward fluorine-free foam, often identified as FFF, F3, SFFF, or AR-SFFF depending on the formulation.

These products are designed without intentionally added fluorinated surfactants and are now available from several major manufacturers.

Examples include:

• National Foam Universal Green
• National Foam Muni F3 Green Plus
• Solberg Evolution
• Angus Fire Respondol
• Chemguard fluorine-free concentrates
• Ansul fluorine-free products

National Foam's Muni F3 Green Plus 3%, for example, is an alcohol-resistant synthetic fluorine-free foam concentrate designed for Class B municipal firefighting and specified at a 3% solution for hydrocarbon and polar-solvent applications.

GreenScreen's current certified firefighting-foam listings also include products from National Foam, Solberg, Angus Fire, and other manufacturers.

The exact product, proportioning percentage, equipment compatibility, and approval requirements should always be verified against the current manufacturer's documentation.

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Popular Firefighting Foam Manufacturers

The firefighting foam market includes a number of established manufacturers and product families.

Some of the names encountered by fire departments, industrial facilities, airports, marine operators, petroleum facilities, and fire protection contractors include:

Chemguard

Chemguard manufactures Class A, Class B, high-expansion, non-fluorinated and specialty foam concentrates. Its portfolio covers municipal, industrial, marine, mining, petroleum, petrochemical, aviation, and transportation applications.

National Foam

National Foam produces Class A and Class B foam concentrates, including fluorine-free products such as Universal Green and Muni F3 Green Plus.

Angus Fire

Angus Fire is an established international manufacturer of firefighting foam concentrates and foam equipment, with products covering Class A, Class B, protein, synthetic, and fluorine-free technologies.

Solberg

Solberg produces Class A and Class B foam concentrates, including fluorine-free formulations such as SOLBERG Evolution.

Ansul

Ansul is another widely encountered name in fire suppression equipment and foam systems. Its products include Class B foam concentrates and proportioning equipment.

The specific foam installed in a system matters far more than the brand name alone. The manufacturer's specified proportioning rate and approved testing procedure should always control the measurement.

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Why Foam Concentration Testing Matters

A proportioning system can appear to be operating normally while delivering the wrong concentration.

The system could be affected by:

• Incorrect proportioner settings
• Equipment wear
• Pump performance
• Blocked strainers
• Changes in concentrate viscosity
• Incorrect water flow
• Concentrate storage conditions
• Metering problems
• Equipment calibration
• Changes to piping or discharge equipment

A foam solution that looks correct does not necessarily contain the correct percentage of concentrate.

This is where refractometry becomes valuable.

NFPA technical material identifies a digital refractometer as one method for verifying the concentration of foam agent solution. Other methods include conductivity measurement and calibrated flow measurement.

NFPA material also describes determining foam percentage with a refractometer by measuring the refractive index of a foam solution sample.

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How a Refractometer Measures Firefighting Foam

A refractometer measures the way light travels through a liquid.

When foam concentrate is added to water, the optical properties of the resulting solution change. The refractometer detects that change and reports it as a refractive index or, depending on the instrument and calibration, a concentration scale such as Brix.

For firefighting foam, refractive index is particularly useful because many foam solutions have relatively small differences from the refractive index of water.

MISCO notes that some firefighting foam solutions can have refractive-index readings in the neighborhood of 1.3330 to 1.3338 nD, which means a high-resolution instrument is desirable for the application.

This is one reason a general-purpose low-cost Brix tester isn't automatically equivalent to a professional foam-testing refractometer.

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The MISCO Palm Abbe PA202 for Firefighting Foam

For professional firefighting foam testing, the MISCO Palm Abbe PA202 is particularly well suited.

The PA202 provides:

• 0 to 85% Brix range
• 1.3330 to 1.5000 refractive-index range
• 0.1% Brix resolution
• 0.0001 nD resolution
• Approximately ±0.0001 nD precision
• Automatic temperature compensation
• Stainless-steel sample well
• Sapphire prism
• Fast digital measurement

MISCO specifically identifies the PA202 as an ideal instrument for measuring firefighting foam concentration and mixture ratios.

For a fire protection contractor, fire department, airport, industrial facility, foam system inspector, or testing laboratory, the refractive-index scale is the important feature.

The Brix number should not automatically be interpreted as an actual percentage of firefighting foam.

Brix is traditionally a sugar-concentration scale. It can be useful as a comparative measurement for many industrial liquids, but a foam system should be evaluated against a calibration relationship established for the actual foam concentrate and water being used.

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The BOSS Digital Brix Refractometer

For applications where a low-range Brix measurement is appropriate, a BOSS digital refractometer can provide an economical field-testing alternative.

A 0–18% Brix instrument is particularly useful when the expected optical response of the solution falls within that range.

The advantage of a low-range instrument is that the measurement scale is concentrated around the lower concentrations encountered in many water-based industrial mixtures.

However, there is an important distinction:

A 0–18% Brix reading does not mean that the instrument is directly reading "3% firefighting foam."

The instrument is reporting an optical measurement on the Brix scale. The relationship between that reading and actual foam concentration must be established for the specific foam concentrate and water combination.

For critical fire protection testing, the MISCO PA202 with refractive-index measurement is the more technically appropriate choice, particularly when the foam solution produces only a very small change in refractive index.

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How to Calibrate a Foam Refractometer

The most reliable approach is to establish a calibration curve using the actual foam concentrate and the actual water used by the system.

This matters because different foam concentrates have different chemical compositions.

Two different products mixed at 3% can produce different refractive-index readings.

NFPA technical documentation describes creating a calibration curve using known foam solution samples. The described approach uses known concentrations around the nominal system concentration so the refractometer reading can be correlated with actual foam percentage.

For example, a technician testing a nominal 3% system could prepare known samples around the target concentration using the actual concentrate and water.

The resulting readings can then be compared with the field sample.

This is much more reliable than assuming:

3% foam = 3.0 Brix

That assumption is generally incorrect.

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Field Procedure for Testing Foam Concentration

A practical field procedure looks like this:

1. Identify the foam concentrate

Record the exact manufacturer, product name, concentration designation, and lot information when available.

2. Determine the required proportioning rate

Consult the manufacturer's current technical data sheet and the system design documentation.

A product marked 1%, 3%, 3% × 3%, 1% × 3%, or 3% × 6% can have very different requirements.

3. Obtain the foam solution sample

Collect a representative sample from the designated test location.

Avoid contamination from dirt, oil, residual cleaning chemicals, or other liquids.

4. Prepare the refractometer

Clean the prism/sample well and calibrate according to the manufacturer's instructions.

For the MISCO Palm Abbe, water calibration is used as the starting point.

5. Place the sample on the prism

A few drops are generally sufficient for a digital handheld refractometer.

6. Take the reading

Allow the instrument to stabilize and record the refractive index.

The MISCO PA202 provides a reading in seconds and automatically compensates for temperature within its specified operating range.

7. Compare the result with the established calibration

Use the calibration curve or approved manufacturer's testing procedure to determine the actual foam concentration.

8. Document the result

Record the foam product, sample location, water source, refractometer identification, temperature, refractive-index reading, calculated concentration, and test date.

This creates a useful record for future testing.

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Why Water Quality Matters

Firefighting foam is not being mixed into pure laboratory water in most field applications.

The water can contain dissolved minerals, salts, treatment chemicals, and other substances that influence its physical properties.

That is another reason to establish the calibration relationship using the same water source and foam concentrate whenever practical.

MISCO specifically points out that conductivity measurements can be affected by water hardness and water quality, while refractometry provides a useful field method for foam concentration testing.

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Refractometer vs. Conductivity Meter

Both techniques can be used for foam solution testing.

A conductivity meter measures the electrical conductivity of the solution.

A refractometer measures its optical properties.

The choice depends on the foam chemistry, testing procedure, equipment, and manufacturer's recommendations.

For low-concentration foam solutions where the refractive-index difference is small, instrument resolution matters.

That makes the MISCO Palm Abbe PA202's 0.0001 nD resolution and precision particularly useful for professional foam testing.

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Foam Proportioning Is More Than a Number

A correct concentration is only one part of a functioning foam system.

The complete system also depends on:

• Correct water flow
• Correct concentrate flow
• Proper proportioning equipment
• Correct nozzle or foam generator
• Operating pressure
• Foam expansion
• Drainage characteristics
• Fuel type
• Application rate
• Equipment condition

Chemguard, for example, specifies that foam expansion can vary according to the discharge device even when the proportioning rate remains the same.

Therefore, a refractometer should be considered a concentration verification tool, not a substitute for a complete foam system test.

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Why Digital Refractometers Are Valuable for Fire Departments and Fire Protection Contractors

A digital refractometer fits easily into a field-testing kit.

Instead of relying on a visual assessment, the technician can obtain a numerical measurement from a small sample.

The MISCO Palm Abbe is particularly convenient because it is handheld, uses a small sample, automatically compensates for temperature, and reports refractive index to four decimal places.

For organizations responsible for multiple foam systems, keeping the refractometer and documented calibration information with the inspection equipment can provide a consistent method for comparing results over time.

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Choosing the Right Firefighting Foam Refractometer

For professional fire protection work, National Industrial Supply recommends looking first at the MISCO Palm Abbe PA202 when refractive-index precision is the primary requirement.

Its 1.3330–1.5000 nD range encompasses the refractive-index region relevant to many firefighting foam solutions, and its 0.0001 nD resolution is well suited to applications where the optical difference between properly proportioned and improperly proportioned solution is small.

For lower-cost field screening and applications where a Brix correlation has already been established, a BOSS low-range digital Brix refractometer can be a practical alternative.

The key is not simply purchasing a refractometer with the correct-looking scale. The instrument must be used with an appropriate calibration method for the specific foam concentrate.

BE SURE TO DOUBLE CHECK WITH THE DIRECTIONS OR REPRESENTATIVE FROM YOUR FIRE SUPPRESSION FOAM SUPPLIER.

 

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Frequently Asked Questions

Can a refractometer measure AFFF concentration?

Yes. Digital refractometers are used to measure firefighting foam solutions including AFFF and FFFP. MISCO specifically identifies its Palm Abbe PA202 as an instrument for this application.

Can a refractometer measure fluorine-free foam?

Yes, provided an appropriate calibration relationship exists for the specific foam concentrate and water combination.

Does 3% foam mean the refractometer should read 3% Brix?

No. The 3% designation describes the proportioning rate of foam concentrate in the finished solution. Brix is an optical concentration scale originally associated with sucrose. A foam-specific calibration is required to relate the refractometer reading to actual foam concentration.

What is the best refractometer for firefighting foam?

For professional testing where refractive-index precision is important, the MISCO Palm Abbe PA202 is a strong choice because it provides a 1.3330–1.5000 nD range with 0.0001 nD resolution and precision.

A BOSS digital low-range Brix refractometer is another option where the application has been correlated to Brix and the expected readings fall within the instrument's range.

How much sample is needed?

Only a few drops are required by handheld digital refractometers such as the MISCO Palm Abbe.

Does the foam have to be aerated before testing?

The concentration test should be performed on the foam-water solution, not simply by judging the appearance of the finished aerated foam blanket. Follow the system's approved sampling and testing procedure.

How often should foam concentration be tested?

Testing frequency depends on the applicable standard, system requirements, manufacturer instructions, AHJ requirements, and the type of foam system. NFPA material describes acceptance and maintenance testing of foam proportioning systems and recognizes refractometer testing as one method of verifying concentration.

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Final Thoughts: Measuring Firefighting Foam with a Refractometer

Firefighting foam systems are engineered around specific proportioning rates. Whether the system uses Class A concentrate, AFFF, AR-AFFF, FFFP, protein foam, or modern fluorine-free foam, the concentration delivered by the proportioning equipment needs to be verified against the requirements for that particular product.

A refractometer provides a fast way to turn a small foam-solution sample into a measurable optical reading.

For demanding professional applications, the MISCO Palm Abbe PA202 stands out because of its dedicated refractive-index scale and 0.0001 nD resolution. For applications where Brix correlation is appropriate, a BOSS digital low-range Brix refractometer provides another practical field-testing option.

The most important principle is simple: do not assume the Brix number equals the foam percentage. Establish the relationship between refractive index or Brix and the actual foam concentrate being used, then use that calibration consistently.

Properly applied, digital refractometry gives fire protection professionals a quick, portable, quantitative method for checking foam proportioning and documenting system performance.

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National Industrial Supply

National Industrial Supply (NI Supply) provides professional refractometers and testing instruments for fire protection, industrial, laboratory, food, beverage, chemical, and process-control applications.

NI Supply offers MISCO Palm Abbe digital refractometers, including the PA202 with Brix and refractive-index scales, along with BOSS refractometers and other specialized testing instruments.

National Industrial Supply
30777 Rancho California Road #891420
Temecula, CA 92589, USA

Sales: 951-308-9269
Email: nisupply@gmail.com
Website: www.nisupply.com