FAQ | Error in temperature sensor reading
Common questions about temperature sensor errors.
QUESTION: My temperature sensor occasionally shows x.x, 185°F, or 85°C. Any suggestions?
ControlByWeb devices are compatible with any one-wire (DS18B20) digital temperature sensor. These include the X-DTS-U, X-DTS-S3C, and X-DTS-S32C sensors. These sensors require a voltage source, a ground, and a data line to transmit the temperature information.
Temperature measurements are made with two 1-wire bus transactions. A measurement command is sent to all of the sensors and the measurement data is subsequently collected from each sensor one by one. If the measurement sequence fails due to noise or insufficient power, the subsequent measurement data will be +85°C (185°F) which is the power-on reset value of the temperature register. Be aware that a defective sensor can corrupt the measurement sequence and spoil the measurement for all of the sensors on the bus.
The DS18B20 digital temperature sensor can optionally work in the 'Parasitic' mode. With this scheme the sensor uses only one data wire and a ground wire. The sensor stores a small amount of energy internally to sustain communications. We have found that with the parasitic mode the sensor has insufficient energy to make temperature conversions and we DO NOT RECOMMEND this type of operation. The sensor response can be sporadic and will report the correct temperature, a temperature of 185°F (85°C), or no temperature at all (x.x).
If you are experiencing a random temperature of 185°F, 85°C, or x.x, please follow these suggestions:
- Disconnect and reconnect the connections to the temperature sensor in question. Solder the connection wires if a faulty connection is suspected. Make sure there is plenty of metal to metal contact.
- Verify that the sensor is receiving +5V across the voltage source wire and the ground wire at the sensor connection. This will help insure that the sensor is not functioning in parasitic mode.
- The 1-Wire bus is "single-ended" and has no intrinsic noise protection. It is susceptible to interference if the cable is routed near power lines, fluorescent fixtures, motors or other noise sources. Keep the cable wiring short and avoid routing it near other electrical equipment.
- If you are still experiencing issues after the above steps, connect the sensor directly into the module itself, leaving out any long lengths of wire. Typical installations can support extending the temperature sensor up to 600 feet total combined cable length in a daisy-chain (linear) topology. It may vary depending on many factors such as type of cable used, electrical noise, etc. Connecting the sensor directly into the unit will eliminate any issues with extending the cable sensor length.
- If you are still experiencing temperature issues after these steps, please contact us for advanced technical support
For more information on these sensors, please refer to the manufacturer documentation: http://datasheets.maximintegrated.com/en/ds/DS18B20.pdf