Gas pressure thermometer
A gas pressure thermometer or gas pressure spring thermometer or gas spring thermometer is the industrial implementation of the gas thermometer as a mechanical dial thermometer. The gas-filled, closed measuring system of a gas pressure thermometer has a constant volume and the temperature-related pressure change of the filling gas is displayed using a manometric measuring method.
construction
The active measuring part of the gas pressure thermometer consists of a cylindrical thermometer vessel. The gas enters a helical Bourdon tube via a capillary tube with the smallest possible internal diameter and is displayed as with the Bourdon tube pressure gauge . In contrast to the capillary line, the Bourdon tube has a volume that cannot be neglected. In contrast to the corresponding pressure gauge, the Bourdon tube is equipped with a bimetal compensation spring so that a possibly high temperature in the housing does not affect the measurement. Depending on the application, the capillary line can be extended up to 25 m so that the display can be housed in a separate location from the measurement location, for example in switch panels. Inert gases , mostly nitrogen, but also helium or argon , are used as filling gases . The filling gas is under a more or less high pre-pressure depending on the measuring range. Due to the toxicological harmlessness of the filling gases, the gas pressure thermometer has ousted the liquid-filled mercury dial thermometer from the market.
Measuring range and error limits
The lowest temperature that can be measured with a gas pressure thermometer depends on the gas used and is just above its critical point (nitrogen at −147 ° C and helium at −268 ° C). The upper limit of the measuring range depends on the materials and joining methods used . In the case of welded measuring systems with measuring parts made of stainless steel, it is approx. 700 ° C.
The error limits for gas pressure thermometers are standardized to DIN EN 13190 for dial thermometers, and are divided into accuracy classes 1 and 2. Depending on the accuracy class and measuring range, the error limits are between 1 and 3% of the scope of the measuring range.
literature
- Prof. Dr. P. Profos [Hrsg.]: Handbook of industrial measurement technology , Oldenbourg, 2002, ISBN 3486225928