Black Rubber Vs. Silicone Rubber

Black Rubber Vs. Silicone Rubber

Have you noticed that some assemblies have the option of black rubber, sometimes Buna or EPDM or a silicone rubber?  Do you wonder which one to use?  If there is silicone rubber in the assembly is it okay to replace it with black rubber or vice versa?  Is there an advantage to one or the other?

This article is mainly about the Watts 957 and Ames Colt 400.  But the pros and cons of both black and silicone apply to any brand or model that uses either rubber.  But I have had several people recently ask about which rubbers should they use in the assembly and how do you know which rubbers to get before you get into the assembly?

No there is not a way to tell from the markings on the outside of the assembly what kind of rubbers you will find inside.  Generally the 2 1/2″ – 6″ Watts 957 / Colt 400  come standard with silicone, the RPDA with EPDM and the 8″ and 10″ are silicone according to Watts Technical.

Pros and Cons

  • EPDM is more resistant to dings and close over small imperfections better than silicone.
  • EPDM is more likely to swell causing the O-rings to help seal better.
  • EPDM is more affected by chloramines which water purveyors use in the water to disinfect the pipes during winter seasons.  The chloramines eats at and/or pits the EPDM until the sealing is lost and the differential drops to zero or you will see a trend that the differential dropping month by month (as reported to me by Watts Technical.)
  • Silicone is resistant to chloramines
  • Silicone is better for higher temperatures
  • Silicone doesn’t swell (this could be a pro or a con)

Which Should I Choose?

If you know water conditions the choice can be easy but often this is not the case.  If you are putting a new assembly it will already come with the silicone in it.  If you need to do a repair you may be able to tell when you examine the old disc, or if it has not been doing well with the rubber that is in it and would suggest trying the other type of rubber.

On other types of brands and models you in most cases do not have a choice of rubbers.  It is what it is.  But it may help you to determine causes of failures or low readings and help you to determine corrective actions.  In the case of the black rubbers it may explain why the O-rings for example will not go back in when you take an assembly apart to inspect and clean.  As you become familiar with the water in your area you will know if you should have on hand replacement parts if you feel it necessary to remove parts and inspect them.

Hopefully this offers some additional understanding regarding issues with rubbers that may look okay upon initial inspection but just do not seem to hold as they should.




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