Theory of the spanner psychrometer, 1. The thermocouple

✍ Scribed by A.J. Peck

• Publisher: Elsevier Science
• Year: 1968
• Weight: 749 KB
• Volume: 5
• Category: Article
• ISSN: 0002-1571
• DOI: 10.1016/0002-1571(68)90035-6

✦ Synopsis

The operation of Spanner psyehrometers is considered in the case of constant temperature and water vapour concentration on the walls of the container. These conditions closely model the practical situation of walls fully lined with a wet, freely evaporating material. It is shown that the theory of Richards psychrometers should apply to the Spanner psychrometer after the decay of transient behaviour which lasts for about 1 sec. Maximum sensitivity, and the validity of the theory, requires that heating within the psychrometer chamber (by the thermocouple current which cools the junction) should be negligible. Many experimenters appear to have used "massive" junctions which are not massive enough. A suitable size is a 1 cm 3 block of copper. Peltier heating of the massive junctions results in an apparent improvement of sensitivity which may account for Rawlins' anomolous results. The wetting properties of the thermocouple wire and junction influence the form of the condensed water droplet and thereby affect the junction temperature during evaporation. It may be possible to modify wetting properties in such a way that a virtually constant evaporation temperature results. Measurements of the active junction temperature throughout both condensation and evaporation phases are desirable as they enable water potential determination independent of chamber geometry.