523 556 560 Temperature Measurement

The Pt100 temperature probe on the Testo 500 series refrigeration instruments has been carefully designed to work under a wide range of applications while providing the fastest temperature response in the industry. The platinum-based 4-wire construction of the sensor has exceptionally low mass and is shielded from electrical interference from ghost voltages or static electricity common in refrigeration systems. When plugged into the instrument, an O-ring provides a liquid-tight seal that protects the instrument from water damage in the elements.

By design, the ceramic sensor only requires a slight pressure to achieve positive contact with the refrigerant line to get an accurate temperature response. Because the temperature probe is constructed of ceramic, a very hard material, the probe has a tendency to rotate easily, even by its own weight. Adjusting the tension of the spring to increase the clamping force is not recommended because it could crack the brittle ceramic sensor. Adjusting the clamping force has little effect on grip since the hardness of the construction materials limits it. If desired, a small piece of 1/8" silicone can be placed around the probe clamp to reduce rotation—especially helpful on smaller systems.

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

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 refrigeration line from 1/4" to 3" in diameter. Air and immersion probes are available to further enhance testing applications.

When measuring temperature, probe position becomes critical for accurate troubleshooting. The Testo 500 series provides real-time measurement of pressure and temperature and will respond to even slight variances in line temperature due to refrigerant flashing in the suction line (slugging).

Oil traveling back to the compressor at the bottom of horizontal runs will also affect temperature measurement, as oil is an excellent insulator. When checking a TXV-equipped system, we recommend mounting the probe with the temperature sensor in line with the thermostatic sensing element of the valve (usually 3–7 o’clock) as close to the element as possible. This provides the most accurate information on TXV operation. When measuring liquid line temperature, the clock position of the probe is not critical.