The viscosity of firefighting foams are measured if they are thick in nature – typically alcohol resistant (AR) foams. This physical property can be critical to ensuring the foam has maintained its alcohol resistance properties. It can also offer insight into potential problems, such as dilution or polymer separation.

What is Viscosity?

Viscosity is the measure of internal friction. Simply put, it measures how much resistance an object would have when traveling through a sample. The more viscous a sample, the more resistance there is to an object traveling through it. This friction applies to how the foam will move through your proportioning equipment. The more viscous the sample is, the slower it will move through the system. This gives rise to the need for listed equipment and flow rates for each firefighting foam.

At Dyne, the viscosity is measured by a device that measures the amount of force applied to a spinning object placed into the firefighting foam. What typically causes the viscosity in foam is the use of a polymer ingredient to make the foam alcohol resistant. You can think of this polymer ingredient as a thick and sometimes sticky gel. The polymer ingredient is designed to fall out of solution when placed on a polar fuel (such as an alcohol) and form a protective layer between the flammable liquid and the foam. Without this barrier, the polar fuels will dissolve away the foam too quickly.

What does an “Out of Spec” or “Fail” Viscosity mean on a Dyne Test Report?

Deviations from the manufacturer’s recommendations of viscosity for their specific product can be indicators of performance problems. If the viscosity is lower than expected, this could mean that there is less polymer than expected in the foam which is most likely due to either water dilution or the polymer separating out of the foam. Polymer separation can occur due to storage conditions, such as the freezing and thawing of non-freeze protected foams or from mixing of incompatible foams which leads to unwanted chemical reactions. Having less polymer than intended could compromise the foams ability to protect against polar fuels. For this very reason, foams with viscosities that are less than half of the manufacturer’s minimum recommendation are failed at Dyne due to the concern about their effectiveness if used on polar fuels.

Viscosities higher than expected can be due to normal aging of the foam or it can be a sign of extreme storage conditions. Extreme heat can especially be harmful to the polymer ingredient and cause the foam to gel up in some cases. Always follow the manufacturer’s storage recommendations to maximize the shelf life of your foam.

Keep in mind that NFPA states the foam on your site should be used through listed equipment at the indicated rate. This is important in ensuring the foam is proportioning at the correct percent. Part of the reason for this equipment and flow listing is due to the fact that different foams have different viscosities. A thicker foam will most likely flow through the equipment much slower and require a larger diameter proportioning orifice. If your foam has gelled up considerably and the proportioning testing showed that the finished solution is dilute, it could be that the foam’s thickness is slowing its passage through the equipment.

Finally, as with any foam “Out of Spec” issue, it is important to ensure the product on the report matches what is on-site. Different products have different formulations which leads to different physical properties. What could look like an “Out of Spec” result for viscosity could actually be due to the wrong product being indicated on the sampling form.

For an explanation on all physical properties and performance properties tested on firefighting foam samples, refer to Foam Testing Explanation LBTR-4054. If you have any questions, our chemists are always available to answer them. Call Dyne at (800) 632-2304 or email,, for assistance with understanding your test results.

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