What type pf thermocouple does this meter use?

[treez]

Hello,
The Tenma 72-7725 meter can measure temperature with a thermocouple. Is it for use with a type K thermocouple?
Page 21 of the operating manual gives a measurement range of -40degc to 1000 degc, so presumably it is K type?

Operating manual
https://www.farnell.com/datasheets/316033.pdf?_ga=1.227538269.1978562376.1462294663

Tenma 72-7725 meter
https://uk.farnell.com/tenma/72-772...Categories&searchView=table&iscrfnonsku=false

 

[SLK001]

I don't see that it can measure temperature at all. There is no TEMP setting on the dial and there is no place to plug in a thermocouple.

 

[FvM]

According to user manual, the temperature probe is connected to V and COM jacks. It's said there are range switch C and F positions.

Type K is definitely the right guess.

 

[Warpspeed]

Pages 20 and 21 mention temperature measurement.

It is suggestive of a type K thermocouple, but does not say so.
Its not obvious how the cold junction temperature correction works.
Its very unusual that a proper yellow K type thermocouple plug is not being used, something I have never before seen.

The other much less likely possibility is a thermistor sensor is used.

Try shorting out V and COM jacks.
If its for a thermocouple the meter should display correct ambient temperature. The internal cold junction correction causes that effect.

If it displays maximum temperature or goes over range, its probably meant to use a thermistor.

 

[treez]

Thanks,
Page 21 of the “operating manual” linked in the top post shows the K type thermocouple wires NOT being terminated into proper thermocouple connectors. Instead, as Warpspeed said, each of the two thermocouple wires is simply soldered to general 4mm plugs and then inserted into the COM and V jacks of the meter.
This is surely incorrect?

I mean, the thermocouple connectors have to be made of the same type of metal as the K type thermocouple wires, or else there is a dissimilar metals junction which will mean poor accuracy in the readings.

So why have Tenma not used a correct K type thermocouple socket in the meter?

..then again, the 4mm plugs will likely be at the same temperature as the "reference junction" within the meter, so its probably ok?..if its OK, then why do people almost always use the standard green thermocouple plugs for thermocouple measurements of power deives in electronics?....after all, why not just use the more ubiquitous 4mm plugs like the tenma meter of the top post does?

A proper K type thermocouple connector (in the green Euro colors here..)
https://www.labfacility.com/thermocouple-connectors/iec-thermocouple-connectors.html

 

[FvM]

Connecting thermocouples through banana jacks is no problem as long as there's no significant temperature difference between the TC-wire to connector interface and the internal reference sensor. And yes, you would basically prefer a dedicated TC connector, although it's not said that the instrument's accuracy deserves it.

There's a number of similar cheap meters on the market. You get what you see.

 

[Warpspeed]

A proper K type thermocouple connector (in the green Euro colors here..)

Reading that was rather a shock.
K type are always yellow in Australia.

Then I looked it up, and there are two different world standards for thermocouple plugs, ANSI and IEC.
ANSI type K is yellow.
IEC type K is green.

https://www.omega.com/temperature/pdf/tc_color_codes.pdf

Oh that little anomaly could cause some real dramas for the unsuspecting.

 

[treez]

Connecting thermocouples through banana jacks is no problem as long as there's no significant temperature difference between the TC-wire to connector interface and the internal reference sensor.

..thanks, thats what i thought, but in most electronics test environments, it always would be, so i wonder why everyone clammers so much for the proper green (or yellow!) connectors?

 

[Warpspeed]

..thanks, thats what i thought, but in most electronics test environments, it always would be, so i wonder why everyone clammers so much for the proper green (or yellow!) connectors?

We are trying to measure a very small thermoelectric voltage with microvolt accuracy between the junction of two very specific dissimilar metals.

As soon as we start adding more and more junctions of other dissimilar metals in a series circuit, we are introducing extra junctions and extra introduced voltages of varying magnitude and polarity..

Now the theory goes that this does not matter provided that all of the extra junctions are at identical temperatures That might be a problem if great distances are involved, or in a rapidly changing thermal environment.

So to keep the whole system calibration as intact as possible, we use special connectors that have individual pins made from the same (or similar) metal alloys as the thermocouple wires from the junction. So we get to use special horribly expensive colour coded plugs and sockets to maintain the measurement integrity of the system.

 

[betwixt]

Just a warning: I have a similar meter to the Tenma and it has a yellow thermocouple (K) plug. It does not appear to have true cold junction compensation although I haven't opened it up to look inside. With the thermocouple unplugged it reads room temperature so it must have some other temperature sensor inside although I'm not sure what it is.

It caused me no end of problems using the thermocouple to calibrate another sensor, it didn't have to be particularly accurate as it was only a water heating controller but the results were wildly wrong. The problem was the yellow ANSI plug - they are reversible but marked + and -. On mine the wires to the thermocouple were reversed, an increase in junction temperatue was causing a decrease in reading!

Double check the polarity - in this case the non-standard connectors probably aren't marked anyway.

Brian.

 

[Warpspeed]

Quite often there will be an internal temperature sensor such as an LM35 which is supposed to sense the temperature where the thermocouple wires solder onto the PCB or go into a screw terminal block (yet another junction).
The voltage across this unavoidable internal junction is compensated for by the LM35 either in hardware or sometimes in software.

It all works, provided everything except the actual probe tip is allowed to settle and fully assume correct ambient temperature.

But its also easy to screw things up if we start mixing different coloured plugs and sockets with different thermocouple types, or as Brian points out, connecting the right type of plug to the right thermocouple the wrong way around.

All the plugs I have seen are marked + and - so that at least is a guide you can probably rely on. But the colours are probably more of a trap than a help.

Its not easy, because there many companies supplying thermocouple wire with different colour coded wires, and as we now know, there are two standards for plug top colours. You absolutely must correctly identify the thermocouple wire and plug, and know where (which company) both came from originally.

If you get it right, thermocouples can be very reliable and surprisingly accurate.

 

[treez]

Now the theory goes that this does not matter provided that all of the extra junctions are at identical temperatures That might be a problem if great distances are involved, or in a rapidly changing thermal environment.

..thanks, from this, i reckon that for an electronics bench measurment of an SMPS, the temperature of the junction of the thermocouple wire into the 4mm plug, is going to be pretty much the same temperature as the junction inside the meter, so i reckon these cheaper meters with 4mm plugs soldered to the k type thermocouple wires are fine.?

...for a rough reading on an electronics workbench, it seems a waste of time to get the proper green (or yellow) k type thermocouple connector .(?)

 

[Warpspeed]

its entirely up to you.
The proper engineering approach would be to use the correct hardware.
If its a temperature probe for a ten dollar multimeter, who cares ?