Testo® 523 | 556 | 560 Refrigeration System Analyzers: On the Oustide Looking In

Temperature Sensors

The platinum based [platinum film on a ceramic substrate] 4-wire construction, of the spring loaded temperature sensor [Pt-100*] has a very low mass [yielding a fast response] and is not affected by stray voltages that may be present on the refrigeration equipment. Unlike traditional K-type [ http://digitalzeus.wordpress.com/2008/01/01/application-demystifying-the-type-k-thermocouple/ ] thermocouples, the sensor is electrically isolated from the RSA. Isolating the temperature sensor electrically eliminates the possibility of incorrect temperature measurement resulting from stray electrical currents or ghost voltages often present on improperly grounded ACR systems.

Because the resistance of a Pt100 sensor bears an absolute relationship to temperature, unlike a thermocouple whose outout depends on the difference between the hot junction and cold junction or a thermistor that is not linear over a wide temperature range no special compensating circuit needs to be provided in the electronics. In short the Pt100 has a wide operating range, excellent accuracy, good linearity, excellent physical strength, long-term stability, and is the preferred sensor for all industrial processes where accuracy and repeatability are required.

Additionally Pt100 probes may be replaced without recalibration of the RSA instruments. Sensors are available in lengths up to 40 feet. The velcro-elastic strap provides insulation from ambient air along with positive contact to the refrigerant line from 1/4″ to 3″ in diameter. Air and immersion probes are available to further enhance testing applications.

Excerpted from: The ACR Applications Guide: An Essential Reference For The Advanced Technician by Jim Bergmann

*Excerpt from PICO Technologies on Pt100 Sensors:

Platinum resistance thermometers (PRTs) offer excellent accuracy over a wide temperature range (from -200 to +850 °C). Standard Sensors are are available from many manufacturers with various accuracy specifications and numerous packaging options to suit most applications. Unlike thermocouples, it is not necessary to use special cables to connect to the sensor. The principle of operation is to measure the resistance of a platinum element. The most common type (PT100) has a resistance of 100 ohms at 0 °C and 138.4 ohms at 100 °C. There are also PT1000 sensors that have a resistance of 25 ohms and 1000 ohms respectively at 0 °C.

The relationship between temperature and resistance is approximately linear over a small temperature range: for example, if you assume that it is linear over the 0 to 100 °C range, the error at 50 °C is 0.4 °C. For precision measurement, it is necessary to linearise the resistance to give an accurate temperature. The most recent definition of the relationship between resistance and temperature is International Temperature Standard 90 (ITS-90).

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