Temperature TopicsFebruary 12, 2004

1 The Principles

• Bimetallic expansion

• Liquid bulk expansion, mercury or alcohol in glass, total or stem immersion

• Thermoelectric effect

• Resistance thermometers, platinum or semiconductor

• Radiation, non-contact high temperature but not necessarily, ear fever/IR filter for maximumwavelength transmission at black body radiation peak at 313K ≈ 40C.

Q = σεHεC(T 4HA − T 4

CA)

2 A Problem

Design a bimetallic strip temperature sensor to fit, free, to fit into an enclosure (box) of about25mm maximum dimension. It is made of equal thickness of aluminum joined to iron trip. Youmay select the actual thickness and the properties of thermal expansion are given as αI = 11.7×10−6/C, αA = 23.8× 10−6/C. The system is designed for stability at room temperature of 20C.The air-conditioner triggers at 2mm excursion, as does the heating system on a deviation of 1.5C.

(26)ST42r.tex

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