Touch thermometer
The term contact thermometer is understood to mean all those thermometers that come into direct contact with the body whose temperature is to be measured. In contrast to this are the non-contact measurement methods, e.g. B. with pyrometers .
Contact thermometers include expansion thermometers ( liquid and bimetal thermometers ) and all electrical or electronic thermometers that work with temperature sensors. Industrial contact thermometers are used to record the process variable temperature e.g. B. used on ovens, reaction vessels or pipelines.
construction
Their structure is similar regardless of the sensor principle. Industrial contact thermometers consist of the temperature-sensitive sensor (e.g. thermocouple , platinum measuring resistor ), which is built into a tube for protection that is sealed at one end; together this is referred to as the measuring insert ; For pictures see under resistance thermometer . In many cases this has a terminal block at the other end for connecting electrical leads. The structure is standardized in DIN 43735. Measuring inserts can also be installed in a protective tube with a connection head in accordance with DIN 43772. The protective tube and connection head together form the so-called protective fitting. This protects the sensitive measuring insert against mechanical and chemical stress and the connection terminals against dirt and moisture. It enables a measuring insert to be exchanged during operation.
Influences
The temperature of the medium has an effect on the contact thermometer, but conversely the thermometer also has an effect on the temperature of the medium, see feedback deviation , whereby the temperature difference to the environment tends to be too small. As a further effect, a platinum measuring resistor experiences self-heating from the measuring current. Both influences are of little importance in flowing liquid media, but should be considered as sources of error in gaseous media.
In the event of temperature changes, heat transport and the associated delay must be considered. It depends on the medium and its flow rate as well as the structure of the measuring device. The thermal response time after which a measured value has followed a temperature jump by 90% is for a thermocouple measuring insert with a diameter of 6 mm (as a rough guide value according to VDE / VDI guideline 3511: Technical temperature measurements)
- in water 1 ... 1.5 s, when using a protective tube 9 or 11 ⌀ more likely 25 ... 50 s,
- in air 150… 180 s, when using a protective tube 9 or 11 ⌀ more likely 280… 400 s.