Hi,

In my continuing quest to make a DT800 a thermocouple in disguise, I need to check for the possibility that one of my input thermocouple has been fried or broken.

I see a post in another forum about using a 2-wire resistance check. When I have a thermocouple (type-K) into channel 7, read as 7TK, I get a sensible reading. However, if I try and read its resistance, I am returned either -9000000000.0 Ohm or 9000000000.0 Ohm.

I get the same reading for a channel no matter if a thermocouple is attached or not. Ideas?

Cheers,
Chris.

Hi Chris,

Just checked it out on a DT800 here and it does work. Did you read the thermocouple as a two wire resistance?

BEGIN
RA1S
1TK
1R(2W)
END

Cheers,
Roger

Eek, no.
Thanks!

I just double checked and page 173 of the DT800 User's manual omits mention of the required 2W channel option. I guess it could be inferred from the 4 and 3-wire inputs, perhaps.

Chris

Hi Chris,

The default is a 3 wire measurement.

Some times we just get too close to a problem and a fresh set of eyes is just what we need.

Cheers,
Roger

Indeed. Thanks again for the help.

When I was running some tests today I noticed this type of behavior (1 second schedule):

1R 34.0 Ohm
1*R 28.4 Ohm
2R 33.1 Ohm
2*R 27.3 Ohm
1TN 385.1 degC
1*TN 542.7 degC
2TN 561.0 degC
2*TN 52.9 degC
11TK 17.6 degC

1R 34.0 Ohm
1*R 28.7 Ohm
2R 33.0 Ohm
2*R 27.3 Ohm
1TN 99999.9 degC
1*TN 99999.9 degC
2TN 99999.9 degC
2*TN 99999.9 degC
11TK 99999.9 degC

We were heating the type-N MIMS at the time with a gas torch to test the program, and noticed all temperatures being reported oddly. On the next report cycle things return to normal.

This happens frequently enough with our DT800s that some people here refuse to use them, choosing DT500 instead as they never show this behavior. I realize it is probably hard to comment, but do you have any comments?

Cheers,
Chris

Hi Chris,

What you are seeing is an over range error. I can't comment but I can offer some tips to help diagnose what is happening.

Try reading the thermocouples as a voltage as well. This will give you the raw thermocouple voltage and you will quickly see why you are getting an over range reading.

Cheers,
Roger

Hi Roger,

Yep, I gathered it was over range, and I will log voltages in future but I'm guessing I'm just going to see really high voltages. It is just weird that all TC inputs can go to that state every now and then - these things tend to happen when one TC is heated rapidly, and we wouldn't care if it was just that channel that was affected, but it tends to happen to all logged channels simultaneously.

Anyway, I have a bigger problem that all that today. I've discovered that when I connect the Ao port to the gas controller I am interfacing to, some noise is injected back into the DT800 resulting in the thermocouple inputs bouncing all over the place (ie. up to 3 degC swings being reported), rather than the 0.1degC variation that is normally seen on a TC input. This only happens when the gas controller is powered on.

We are powering both the controller and the DT800 from the same power board (we think), but I was wondering if perhaps there is a better way of using common ground paths - the manual is a bit vague regarding the Ao port. I'm not sure if I should connect the earth of the controller to the Ch GND port, or the GD GND or SR or ...?

I don't want to induce a large earth loop current and fry either or both the gas controller or DT800. Alternatively, do I need to use some sort of noise filter external to the Ao port (but that's getting a bit beyond my electronics capabilities!).

Cheers,
Chris

Good afternoon Chris,

Looks like you need some isolation between the dataTaker and the furnace controller to filter out the noise. I recommend to talk to an instrumentation supplier to see what they have off the shelf.

Cheers,
Roger