Reference Temp for DMM6500? - Page 1
Forum section: electronics / beginners

author: fishwhispers
author_rank: Contributor
timestamp: September 01, 2021, 12:54:34 am
content: Hi, apologies if this is a very basic question, but I just bought a new Keithley DMM6500 and I've been trying to figure out how to get an accurate absolute temperature reading using a K-type thermocouple on the front panel. From what I've gathered from the manual and some videos from Keithley, there doesn't seem to be an internal reference junction built into the unit itself, unless you buy the 2001-TCSCAN card (which costs almost as much as the dmm itself.). So without the card, you are supposed to use the "simulated" reference temp. When I powered the unit on for the first time, the simulated ref temp was set to 23.1C, which is the same temp that is on the cal certificate. Using this temperature though, causes the thermocouple to disagree with every thermometer I could find in my house by several degrees. I assume this because my room is not at exactly 23.1C. (Maybe?) I can change the simulated temp to agree with the reading from my Fluke 87v, but that doesn't seem like the right way to do it.So my question is, how are you supposed to get an accurate reading when the reference is just some temperature that you type in? Also, what is the "Simulated Reference Junction" supposed to represent? Is it the temperature at a certain location inside the instrument?Thanks!

author: fishwhispers
author_rank: Contributor
timestamp: September 01, 2021, 02:34:37 am
content: Well I found a reply by someone at Keithley on their forums regarding the simulated reference temp. His recommendation was to use 23C if you just turned the machine on, and 27C or 28C if it has been running for at least an hour. I wish there was a way to add a CJC sensor to the instrument without paying for the rest of the 2001-TCSCAN card. It's a shame to have all those digits available and then use a "pretty good guess" for the reference. Maybe I'm expecting too much out of a thermocouple measurement?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2021, 09:26:13 am
content: The cold junction temperature is the temperature where the TC wires change to copper. With the normal contacts and plugs this would be the banana plug you use for the thermocouple. The best the meter could do is measure the temperature of the connector.The solution possible with no extra hardware would be the room temperature estimated from the internal temperature. This would be the internal temperature minus some offset. This could work reasonable, though it takes quite some time to stabilize after turn on.To get a good reading one would need an external cold junction compensation (or the scan card with the compensation close to the contacts) and a setting to choose external cold junction compensation.

author: fishwhispers
author_rank: Contributor
timestamp: September 01, 2021, 04:22:29 pm
content: I see. That makes sense. Thank you for the explanation.So even if I had the scan card, I probably wouldn’t want to use the front panel anyway because of the location of the CJC sensor. Ok so in the interest of taking a fairly precise temp measurement, fairly low heat, and using the convenience of the front panel connectors, I should probably look into getting a different temp probe. Probably something like a RTD, right?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2021, 04:30:49 pm
content: A RTD, like PT1000 more suitable to the banana connectors. The scan card would only help with the TC wires directly going to the scan card. Any extrension cable would need to be the same TC type or a slightly cheaper similar material extension cable specially for TC of that type.PT100 starts see effects of the connector / cable resistance, so it is not that well suited for 2 wire connection.There are also thermistors that can work well, especially in a limited temperature range.I don't know if the DMM 6500 supports it, but a diode also makes a resonable temperature sensor. Especially when you could change the test current between 2 different settings this could even work without individual calibration of the sensor. Chances are the HW could support this and maybe a script could be used to actually implement it.

author: fishwhispers
author_rank: Contributor
timestamp: September 01, 2021, 05:37:12 pm
content: Ooo that’s a cool idea with diode. I’ll have to try that out. I’ve been looking for a project to learn how to script with the unit.Thanks again for the info! I’ll look into finding a PT1000 that suits my application.