5196106432

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CERAMIC fNSULATING BEADS

ACTlVE

JUNCTlON^

Fig.5: Basic construction of a simple thermocouple suitable 1or measuring temperaturę In an electric oven or furnace.

mfy ACT1VE JUHCTION BONOEO TOINSIOE OF SHEATH

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METAL

SHEATH

(O)

metal SHEATH

fShC ATH CUT AWAYl

Ib)

Fig.6: The three traditional ways of protecting a thermocouple in highly corrosive or reactive envlronments. Method (a) leaves the junction itself exposed, while (b) and (c) include it Inside the sheath.

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Fig.7: A smali low-cost type K thermocouple in a stainless Steel sheath, madę locally by the firm Richard Foot.

Fig.8: Construction of the minerał insulated, metal sheathed or MIMS type of thermocouple, which can be drawn down to diameters as Iow as 0.5mm. (Courtesy Bell-IRH)

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ACTTVE AJNCTłON (EXPO$EO)

sheath was madę from stainless Steel, inconcl or ceramic matcrials. However morę recently, special alloys like Nicro-sil (Nickcl-Chromium-Silicon) have been used. Thcse offer distinct advan-tages in terms of matching the temperaturę expansion cocfficient of the actual thermocouple metals, and minimising thcrmal stresses. Australian scicntist Dr Noel Burley has again pioneered in this area, and has just announced the devel-opment of an improved sheath alloy called Nicrobell.

Dr Burley is generał manager of R&D at Bell-IRH, of 32 Parramatta Road, Lidcombc NSW 2141. In addition to the devclopment of the type N thermocouple and Nicrobell sheath materiał, he has also been responsible for much of the development of the so-called “MIMS” or minerał insulated, metal sheathed construction shown in Fig.8. This is rapidly becoming the pre-ferred construction for all high temperaturę thermocouples.

A feature of MIMS construction is that the insulation between the actual thermocouple wires and the metal sheath is formed from a materiał such as magnesium oxide powder, which is initially only loosely packed. The mc-chanical and thermal characteristics of the sheath are also matched closely to thosc of the thermocouple wires. The complete asscmbly is then drawn or swaged down to the required finał diameter, rather like drawing wire or optical glass fibrę.

This allows the production of highly stable and rugged thermocouples with diameters as smali as 0.5mm!

For basie temperaturę measurement using thermocouples, very littlc elec-tronics is required. Apart from the actual thermocouple itself, all that is needed is a DC miIlivoltmeter capable of allowing accurate measurements up to about 75mV. Next month I hope to describe a simple and Iow cost high temperaturę thermometer, using a type K thermocouple, and suitable for measuring temperatures up to around 900°C. Just the shot for checking the temperaturę of smali heat treatment fur-naces, as it happens!    ®

92

ELECTRONICS Australia, September 1987