by Joan Leedy, Technical Director

Dyne Fire Protection Labs offers testing of foam solutions to determine the concentration of the foam concentrate in the system water. Customers use this service by running a foam system in the field, collecting a sample of the discharge, and sending it to Dyne along with a sample of the foam concentrate and the system water. Using the foam concentrate and system water, Dyne technicians make up known foam solutions and then compare the refractive index of the known solutions to the solution provided by the customer. By using a bench-top analytical refractometer capable of reading refractive indexes accurately to the 5th decimal point, Dyne can provide an accurate result.

Unfortunately, we see many foam solutions that are not in specification. NFPA 11 requires a system proportion the foam “within minus 0 percent to plus 30 percent for the manufacturer’s listed concentrations, or plus 1 percentage point, whatever is less.” In order to pass this specification, a 3% foam system would need to proportion foam between 3.0% and 3.9%. Though a small number of foam concentrate samples fail in our laboratory, a larger amount of foam solutions fail. Let’s take a look at the reasons why a foam solution sample would fail when the proportioner is actually working as designed.

  1. The wrong type of foam is being used in the system – If the system was initially designed to accommodate a standard AFFF foam – which has a relatively low viscosity – but then was switched to an alcohol resistant foam. Alcohol resistant foams are typically much more viscous than standard AFFF foams and can require a larger proportioning orifice.
  2. The system is not run at a flowrate within the listed range – Proportioning systems are designed to proportion within a certain foam solution flowrate. This flowrate range is published with the listing. If the system is run at a lower flow (which is sometimes done to create less solution) the system will not proportion properly.
  3. The system was not run for a sufficient time – The system will require time to reach a steady state. Again, if the tester is trying to minimize the amount of foam generated, the tendency might be to run the system for a short time. If the system has not reached a steady state, the system will not produce the required foam solution concentration.
  4. The foam concentrate sample sent to the lab is not representative of the foam concentrate used to make the foam solution – It is imperative that the foam concentrate sample sent to the lab is representative of the foam concentrate used to make the foam solution. Consider where the foam concentrate sample is being drawn from and if the tank may contain different types or even different lots of foam.
  5. The water sample sent to the lab is not representative of the water used to make the foam solution – As with the foam concentrate, it is also imperative that the water sent to the lab is representative of the water used to make the foam solution. This is not typically a problem unless the system is using sea and or brackish water to generate foam.                

As you can see, there are a lot of things that could cause the concentration to be out of specification even though the system is performing as designed and all of them need to be considered when testing a foam proportioning system. Of course, an additional reason for an out of specification result, is that the proportioner is broken, plugged or that the wrong orifice has been installed.

Note: Many states are restricting the testing of systems that contain foams with fluorinated surfactants (PFAS), so this will eliminate the testing described in this paper. If these restrictions are imposed, verification of the proportioning will be done by using an alternative method, such as the surrogate liquid method or the water equivalency method.

If you have any questions regarding this article, please contact Dyne Fire Protection Labs at or (800) 632-2304.

©Dyne Fire Protection Labs 2021