temperature offset with jld and pt100

[grandpayum]

I have this http://www.amazon.com/Waterproof-PT100- ... B0052IGFZ4 thermocouple hooked up to a jld612 for a sous vide rig. The rig works fine but the reported temperature is about 25 degrees below what it should read (e.g. the set temperature is 110.0 but the actual temperature of the water is 135.0 verified by external thermometers). I have the default settings except having it set to display fahrenheit and having run the auto tune feature. I don't believe my thermocouple is bad because the temperature is maintained fine (again confirmed by external thermometers). Is there anything i can do to fix the reported temperature (such as adjust an offset?)

Thanks!

[richiem]

That's a huge error -- at 110 deg F, the probe should have a resistance of about 117 ohms, while at 135 deg F, the probe resistance should be about 122 ohms. At room temp of 70 deg F, the probe resistance should be around 108 ohms. If you have added wires to the probe to get additional cable length, that could be a problem. Then the solution is to adjust the PSb setting to compensate for the extra wire resistance.

First check the PSb setting in the parameter setting menu -- it should be 0. If it isn't, that could be the problem. Also use the IntY setting of Pt10.0, which will give the best resolution.

If those settings are correct, and you haven't extended the probe cable length, then if possible, check the room temperature resistance of the probe using a digital multimeter -- DO NOT use an older analog meter. As mentioned above, the resistance should be near 108 ohms -- test between the red wire and one of the blue wires. Use the highest resistance range that will show the nominal 108 ohms of the probe at room temp -- it will be the 2kohm range on a 3-1/2 digit meter; test quickly because some meters on low ohms can supply a high test current that can damage the probe.

If the resistance is around 108 ohms, plus or minus then please check the connections of the RTD probe to the JLD612 -- red to terminal 8, one blue to terminal 9, the other blue to terminal 10 -- doesn't matter which blue goes where, but it really matters where the red goes. Then if there is still a huge error, it is either because the probe is in the wrong place in the water bath, or because the JLD612 is faulty.

Let us know what you find.

[grandpayum]

That's a huge error -- at 110 deg F, the probe should have a resistance of about 117 ohms, while at 135 deg F, the probe resistance should be about 122 ohms. At room temp of 70 deg F, the probe resistance should be around 108 ohms. If you have added wires to the probe to get additional cable length, that could be a problem. Then the solution is to adjust the PSb setting to compensate for the extra wire resistance.

First check the PSb setting in the parameter setting menu -- it should be 0. If it isn't, that could be the problem. Also use the IntY setting of Pt10.0, which will give the best resolution.

If those settings are correct, and you haven't extended the probe cable length, then if possible, check the room temperature resistance of the probe using a digital multimeter -- DO NOT use an older analog meter. As mentioned above, the resistance should be near 108 ohms -- test between the red wire and one of the blue wires. Use the highest resistance range that will show the nominal 108 ohms of the probe at room temp -- it will be the 2kohm range on a 3-1/2 digit meter; test quickly because some meters on low ohms can supply a high test current that can damage the probe.

If the resistance is around 108 ohms, plus or minus then please check the connections of the RTD probe to the JLD612 -- red to terminal 8, one blue to terminal 9, the other blue to terminal 10 -- doesn't matter which blue goes where, but it really matters where the red goes. Then if there is still a huge error, it is either because the probe is in the wrong place in the water bath, or because the JLD612 is faulty.

Let us know what you find.

Inty is 10.0 and Psb is 000.0. The wires are connected as you describe and I have not lengthened the cable in any way. I have a multimeter but the battery is dead, I'll go pick one up tomorrow and see what it says. As for placement, its located towards the center in a relatively small tank (~1 gallon). Thanks for your help!