Functionality

How does a bimetal dial thermometer actually work?

Temperature is the most trusted measured value and is one of seven basic parameters of international system of units. Temperature is the most important measured value with nearly all production processes.

Precise temperature determination requires two fixed points and one linear grading. The most common unit, is °C (degrees Celsius). The origin of this unit is in the fixed temperature points of water, choosing freezing and boiling points and dividing it into 100 sections.

Temperature can have manifold effects on different materials. It can cause a change in volume and length to fluid and solid media. Fluid gas also is a subject to pressure change in containers and pressure change through temperature. Temperature can cause change in its electrical resistance to metals and to semiconductors. So there are many possibilities to measure temperature.

In industrial practice, touching measuring methods are established. These include mechanical and electric contact thermometers. Bimetallic thermometers belong to mechanical contact thermometers, where bimetal coil or helix do expand, depending on the temperature.

They primarily are used as dial thermometers and are robust and reliable in matters of construction. The bimetal thermometer consists out of one dip tube, in which one shrink-wraped bimetal coil or helix is placed. At this helix a unit display is connected with a scale and an indicator. The rotation of the bimetal is transmitted directly over a shaft to the indicator. The coil or helix consist of two connected metal strips, allowing different temperature expansion. The metal strips are spirally wrapped. Depending on winding length, diameter and material a reconciliation is coordinated so that a rotation of 270° can be reached in the nominal temperature range. Through different combinations of bimetals, indication ranges can be chosen between -100 °C and +600 °C.