You’ll see the term bi-metal used quite a bit on this website so we thought it would be a good idea to tell you exactly what that means in regards to our snap disc thermostats.

Bimetal Snap Disc

These discs have been stamped out of a strip of bimetal and then heat treated.

For general information about our snap discs, take a look at our catalog.

Our snap disc thermostats are created with bimetal discs in them that react to temperature changes (rise or fall) and ‘snap’ causing the circuit to open or close.

We form our discs from one of three bimetal compositions.  You can see the compositions below.

High Expansion Low Expansion Alloy%
2400 (B1) 22 Ni, 3 Cr, Bal. Fe 36 Ni, Bal. Fe 50/50
2800 (E3) 25 Ni, 8.5 Cr, Bal. Fe 42 Ni, Bal. Fe 50/50
6650 (P6775R) 72 Mn, 18 Cu, 10 Ni 36 Ni, Bal. Fe 55/45

As the temperature changes the metals will flex and pull against each other.  Since the metals expand and contract at different rates, each side can be formed to flex (snap) at a desired temp.

Along with the different bimetal compositions, our bi-metal snap discs come in all shapes and sizes.

Depending on your specifications and quantity, Senasys can build custom bi-metal snap discs to fit inside a thermostat switch. Our standard snap disc thermostat switches use a 1/2″ or 3/4″ disc. The necessary diameter for your project depends on if an application requires a faster response time to a temperature change or needs to be heavy duty (25A@120VAC / 30A@12VDC) we recommend our 3/4″ disc. The larger disc covers a larger area making the temperature response time more rapid.

You may be wondering the difference in 1/2″ and a 3/4″ bi-metal Disc besides just the size. Overall, the ½’’ and ¾’’ switches function the same, but there are a few key differences in how they can be used.

  • ½’’ switches are smaller, so they are good for applications where space is limited.
  • 3/4’’ switches have more surface area on the disc, so they are slightly more accurate in sensing temperature.
  • If an application requires a tighter tolerance, a ¾’’ switch would be the better option.¾’’ switches also have a higher electrical rating. For example our 430 line of ½’’ switches has a rating of 15 amps at 120 VAC, while our 2511 line of ¾’’ switches has a rating of 25 amps at 240

What is Bi-Metal?

Bimetal is two different metals that are fused together. For example, iron on one side and copper on the other. The specific types of metals will vary depending on the temperature that is being detected. The two different metals have different coefficients of thermal expansion. This means that the two metals expand and contract at different rates when exposed to heat. Our switches use a bimetal disc which “snaps” at different temperatures. This disc then pushes on a ceramic pin, which opens the contact in the switch.

15 Amp Auto Reset Thermostat SwitchesBimetal switches aren’t typically used in applications over 450°F. This is because the metal starts to become molten, which causes it to expand differently. Due to the imprecise nature of bimetal, there is a tolerance on the temperatures. Typically this is ± 5°F. If a tighter tolerance is needed, then a digital switch should be used.

How snap discs are created

Now that we’ve covered how our bimetal is composed, it’s time to talk about how the discs are created to ensure they react to the proper temperatures.

All of the discs are designed around temperature rise.  Discs (and switches) are designated F or L to describe what they will do on temperature rise.  F are designed to close on rise and L to open on rise.  You’ll also see measurements, such as 3/4 or 1/2, which reference the diameter of the bimetal disc.

Heat Treating and Forming the Discs

The first step is to take our metal, which comes in a big coil, and stamp it into discs.  These small discs (blanks) are then heat treated and are formed to react to the customer’s desired temperature, as shown below.

Forming Snap Discs


Testing the Snap Discs

After we’ve formed the discs, we test them using oil baths to make sure that they will react at the correct temps. The discs are placed in a bath to determine what temperature they “snap” at. If the disc doesn’t snap, it gets sorted out of the pile. The discs that do snap at the desired temperature get labeled and passed onto assembly.

Snap Disc Oil Bath


For more information about our snap disc thermostats, visit our website or contact us!

Snap Disc Thermostats Purchase Snap Disc Thermostats Contact us about our snap disc thermostats

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The superior controls manufactured by Senasys have a history of solid performance in many industries. From 22.5 mm selectors, to the four quadrant lighted feedback CMC controls, almost any switching need can be, and has been utilized in a variety of applications. Select a group below for more information. When you have found a switch that fits your needs, simply fax or email your specs to us and let our highly trained engineers find the most efficient configuration for your applications.

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Senasys products are designed & manufactured in Eau Claire, Wisconsin, U.S.A.