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What is a Bi-Metal Snap Disc Anyway?

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.

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.

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