Service Bulletin – Temperature Thermisters

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Temperature thermisters not not reading the correctly. Digital thermometer reading is more then 3F off from the reading showing on the IDEC touch screen.

Possible loose physical connection between the pipe and the probe, interference from high voltage electrical power lines run to close to the leads, defective sensor or defective IDEC thermistor card.

Take the following steps to correct diagnose and correct the problem:

1. Make sure the thermistor is secure to the pipe and fully insulated. Lack of proper insulation will allow ambient air temp in influence input to the PLC.

2. Using a calibrated digital thermometer take a reading of the physical line temperature as close as possible to the sensor location. At the same time, using an ohm meter make a note of the resistance value at the sensor lead ends. NOTE: When taking your ohm readings, make sure to remove at least one of the two leads from the IDEC thermistor card to assure you are getting the correct ready. Using the 10K-3 Thermistor Output Table (attached to this article) verify that your physical line temperature lines up with the Ohm reading from the chart. If the Ohm value is more than 10% off then replace the sensor.

3. Inspect the thermistor lead from the prob location (where it comes out of the pipe insulation) back to the IDEC card connection. In some cases electrical service companies when installing high voltage electrical (as part of the chiller installation) may wire tie or tape the probe leads to a high voltage wire. This can cause interference in the signal between the probe and the IDEC thermistor card.

4. If the Ohm value is within 10% of the physical line temperature then CALL LEGACY TECHNICAL SUPPORT services as the IDEC thermistor card may need to be field calibrated or replaced.

IMPORTANT: When using a digital thermometer MAKE sure to calibrate this service tool using a 50/50 mix of ice and water. After 3 mins the water ice mixture should read 32F.

It is recommended to keep the chillers control system up to date. At the link provided below, users can download and install the latest PLC/HMI software versions.

All Legacy models with the IDEC PLC / HMI touchscreen.



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