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 that are both common in refrigeration systems. When plugged into the instrument, an o-ring provides a liquid tight seal that will protect 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, and 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. (We found out the hard way!) Adjusting the clamping force will have little effect on the grip of the sensor as it is the hardness of the materials used for construction that do not allow it to grip the line. If desired, a small piece of 1/8” silicone can be places around the probe clamp to curtail this problem. We found this especially helpful when working on smaller refrigeration systems.
Because the resistance of a Pt100 sensor bears an absolute relationship to temperature, unlike a thermocouple whose output depends on the difference between the hot junction and cold junction or a thermistor that is not linear over a wide temperature ranges, 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.
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 ¼” to 3” in diameter. Air and immersion probes are available to further enhance your testing applications.
When measuring temperature probe position becomes critical for accurate troubleshooting. The Testo 500 series is designed to provide real-time measurement of pressure and temperature, and will respond to even the slightest 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 the temperature measurement as oil is an excellent insulator. When checking a TXV equipped system, we recommend that the probe is mounted 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 will provide the most accurate information on the TXV operation. When measuring liquid line temperature, the clock position of the probe is not critical.
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