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Platinum Metals Rev., 1970, 14, (3), 85

Measurement of Lunar Heat Flow

A New Platinum Resistance Thermometer

  • F.J.S.

One object of the Apollo programme of lunar exploration is to study lunar heat flow by measurements with temperature probes at depths up to three metres into the Moon’s surface. The thermometers for this work must have stability better than 2 × 10−3 deg K/year together with the ability to withstand the shocks of launching into space and landing on the Moon. Workers at Rosemount Engineering Co and Arthur D. Little Inc have developed a rugged differential platinum resistance thermometer for the task (Rev. sci. Instrum., 1970, 41, (4), 541–544).

The design provides a ±2 deg K differential over the range 200 to 270 K. Isolation from the effects of mechanical and thermal strains is achieved by attaching the platinum wire in the form of a helix to its supporting platinum mandrel by glass insulation of a type which has a similar thermal expansion coefficient. Only 10 per cent of each wire loop is embedded in the glass so that the wire is free to expand or contract while being rigidly supported. To complete the thermometer both wire and mandrel are encased in a platinum tube, the ends of which are sealed hermetically with pure gold solder after the connecting leads of platinum have been passed out through one end.

Each thermometer unit has been calibrated at 42 points (nine differential temperatures and five absolute temperatures between 200 and 250 K). A stability testing programme over one year indicated an average drift of only +0.45×10−3 deg K/year.

Each element has an ice-point resistance of 500±1 ohm. This figure was a compromise between concerns of wire purity, individual coil rigidity, sensitivity, self-heating, and volume.