Temp jumps around with JLD-614

dirtyoldm69
Posts: 1
Joined: Fri Sep 18, 2015 5:20 pm

Temp jumps around with JLD-614

Postby dirtyoldm69 » Fri Sep 18, 2015 5:27 pm

Here's a puzzle.

Our JLD-614 and Pt100 RTD are installed in an old stove.
Water has never been near it - it's mostly used to cure epoxy.
It's been used maybe 10 times total over 2 years and has worked fine.
It holds within around 2 degrees F of setpoint.

But the other day when turned on, the temp display fluctuated randomly between
say 70F and 95F. It stabilized after about 3 minutes at around 90F and then
worked OK.
But after an hour or so set at 105 F, we noticed the temp read 114 F.
Something more than the display is not right.

Voltage across the RTD is 27.3 mV and stable even while the display fluctuates.
When it rises to 27.6 mV the display stabilizes.

We tried reversing the 2 blue leads, and a 3.3 Mfd tantalum capacitor across
the RTD leads. No help.
On the bench, the JLD-614 with a 110 ohm resistor reads 79F dead stable.

In this topic
viewtopic.php?f=5&t=1677
the user reports his problem fixed with a waterproof RTD, but logically
he might have simply had a random bad one.
I bought mine around the same time as that post, and you seemed to hint
you'd gotten some bad RTDs.

So... any thoughts ?

richiem
Posts: 270
Joined: Mon Jan 03, 2011 3:47 pm
Contact:

Re: Temp jumps around with JLD-614

Postby richiem » Mon Sep 21, 2015 4:17 pm

The voltmeter used to monitor the voltage across the RTD may have an integration period that is long enough to hide variations in the probe resistance or in the current source in the 614. Hard to guess which device is at fault, and the only way to tell is by substitution. It is least costly to try another RTD, and you will also then have a spare...


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