Lightobject.com Support Forum

I recently bought three PT100 probes and three JDL612 PID controllers. I also bought one of the waterproof probes that consist of a plastic tube protecting the sensor. (I am making sous vide controllers for holiday presents.) After setting them all up, I was trying to set the input offset (PSb) for each one and was surprised at the variance in the readings. I used a human temperature taking thermometer to calibrate all the probes, figuring the temperature reading within 0.2 degrees was close enough for calibration purposes.

I have a personal sous vide controller and probe from a previous build, resulting in a sample size of five. I use a water bath of about 100F to calibrate the probes. The offsets in degrees F:

-2.7 (water proof probe)
6.6
1.0
51.1
0.2 (my original probe)

Clearly there is something wrong with the probe that requires a 51.1 offset. I checked the wiring and the red wire does go to #8 terminal on the PID. I have not checked the impedance for fear of damaging the probe. Tested in another controller showed a similar erroneous reading. Is there another way to confirm there is a problem?

The second reading appears to be suspect as well. I can add an offset but worry the range or accuracy of the probe may be compromised. Is an offset of 6.6 degrees indicative of a faulty probe or can I use it with confidence?

thanks,

Michael
The Sands Mechanical Museum

Probe #4 -- bad probe. You can test its 0 deg C value (should be very close to 100 ohms) with an ice water slurry and a Digital Multimeter -- get a 10,000 ohm (10k ohm) resistor and measure it, then hook it in series with the probe. The resistance of the series combination of the 10k resistor and the probe should go up by 100 ohms, +/ - 1 or 2 ohms. The 10k resistor will protect the probe from over-current damage.

Say the 10k resistor measures 10256 with the meter -- then when you hook the probe in series, the reading should be 10356.

Probe #2 -- may be inaccurate, but if offset fixes and it is stable, run with it.