Type 80/81 Temperature Reading on Disconnected Probe


On a type 81, two channel vibration sensor, when the vibration probe (lets say s1) is disconnected, we still get a s1_temperature reading. Do you know why?

I tried to explain to the customer that it was kinda like a thermocouple in that the probe just extended the circuitry, and that the full loop was on the PCB board, but they didnt quite buy my explanation and asked if I could ask the experts.

Hey Kyle,

I believe it has more to do with the parsing script in the Node-Red library. The sensor isn’t actually reporting a valid temperature value but the Node-Red script currently doesn’t handle that well. I would recommend checking the data.probe_1_error parameter which is correctly showing there is an error. If that is the case all readings should be disregarded.

There are a couple ways we could handle this as far as the readings go. First would be to not populate anything in the sensor_data object but if someone writes the application to always check them it could cause crashes since the values would be null.

The alternative is to set the temperature to 0.00, but since that’s technically a valid reading it should never be relied upon to indicate an error.

I believe the correct way to handle it is to check the probe_1_error parameter, but as always I’m open to alternative thoughts.

The temperature sensor is a thermistor, which is connected to an ADC. When the probe is not connected, the ADC is floating. a Floating ADC can pick up any noise. The code will still read the ADC value and try to convert the noise into a temperature value.
As @TravisE_NCD_Technica mentioned, we can put a check in node-red, which will zero out the temp when the probe is not connected.

Other options would be null or NA. We can do the same for the vibration data as well, but this may cause some issues with DB insertion if the db column/property is typecast.

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Agreed. I’m not sure there is a perfect solution here.

I think continuing to pass the ADC floating value is fine. I think it should be upon me to detect, head disconencted, and do with it what I need to.

I had coded it to pass the value anyway, because some value is better than forcing it to 0, and there is the context of the head is disconnected. I also kinda recalled that on the thermocouple, it works the same way, even without a thermocouple connected, it will still read a value, because internally its still connected.

Our reliability folks didnt quite understand and were hoping for the better explanation, which you gave.

Thank you guys!