Another smart guy Electrical / Electronics problem (???) This one...????

Okay, so I'm testing out a thermocouple today. Wait, let me back up; earlier today I wanted to measure a temperature with one of my multimeters which has a temperature function. I attached the temp probe and got some really wonky readings. Something's wrong. My readings were all over the place. Some, just null.

I have two meters which do temps. My first thought was it was the meter I was using (which is new). So I tried the same probe in an older meter I have. Same weird stuff, but less weird than the first readings. Okay, so now I suspect it's the temperature probe, and not the meter. Time to test the temp probe.

The probe is a Type K temperature probe. The basic principle is two different metals (of wire) brought together at a sensor will create a voltage. In other words, they will convert heat to electrical energy. This is how most electronic temp probes work. Okay, so this should be easy enough to check...

I set one of my better meters to DC voltage and clamp the positive and negative leads. I'm reading mV. I've got a value; let's say it's 20mV. Okay, so I put the probe in some hot water and the voltage goes up. Good! Right? I take it out of the hot water and the voltage goes down. Not the probe, right? Well....
.Then I start watching the voltage with the probe just sitting on my bench The voltage goes all the way to almost zero. How can this be? There's ambient temperature in the room, so there must be voltage...right? Then I grab the probe cable and the mV readings jump up to about 76mV or so (not touching the probe, just the sleeve of the cable). Could be lots of things, so I put the probe and cable back down, watching more closely this time. The probe will suddenly jump up to about 70-80mV for no apparent reason, then when left alone will go all the way back down to near zero. WTH?? Move the probe with a insulated rod, the voltage jumps up. At first I'm thinking a short in the probe cable, but that's not it because a short would only reduce the reaction between the two metals. What's going on?

I checked this with three different meters and got similar reactions, but with different values. Now, on my most accurate meter I get a temp reading of zero. On my other meter I get a reading of 72 F. Well, the room is about 72 F, so maybe it's not the probe...BUT, when I unplug the probe from that meter I get a reading of...72 F! Now I'm really scratching my head! So I shut all the meters off and went to go do something else (more productive). I thought maybe the one meter was holding a residual temp. Nope! Turned that meter back on about an hour later and got the same reading, 72 F (without the probe plugged in).

My Klein meter still registers zero, like there's a fault. Checked the resistance and capacitance across the probe, with nothing weird.

Any ideas? Is it the probe or do I have two bad meters??

BTW...sorry for all these silly brain quizzes, but I just can't stand the politics anymore, so I'm trying to post something different.

Otherwise, I think we'd all be brain-dead.

Call a professional.

Your welcome.

a reply to: Flyingclaydisk

just curious on the one meter that read 72 and held it. does it have a memory function? was it on?

just another thought, did you check/change the batteries, a low battery can make a meter read funky.

a reply to: Flyingclaydisk

Cheers to that!

Sorry, don’t know sheet about electrical. I’ve got an amazing mechanic for that. Shows up at your house ready to go. $80 an hour and incredibly efficient. An electrical genius. I’ve got lots of electrical problems with my limos. He’s a lucky charm. PM me, I’m sure he’d give you advice over the phone. Good guy.

BTW, how’s the shoulder?

a reply to: Flyingclaydisk

First thing? Don't use a normal meter probe to get temps, use a thermometer (clamp probes).

When you were reading it, what setting did you have it on? Also, what kind of meters and probe?

a reply to: Flyingclaydisk

Hmm.

Could you try swapping the thermocouple wires? Perhaps the junction is rectifying, like the old 'cats whisker' crystals, and you are seeing an induced voltage?

If the thermocouple is rectifying, it would also probably indicate some sort of impurity in the bimetal junction. Perhaps the junction has become crystalline somehow (but K type are usually aluminum to chrome, so, weird).

a reply to: Flyingclaydisk

Anyone nearby using this sort of junk???;

www.thermoworks.com...

I used to work with thermocouples all the time on extruders. First forget what you think you know about them.

The red wire is negative in all common cable color coding in thermocouples.

A meter or controller will read funny unless it is connected to a good probe. It may go to high scale, low scale, alternate between the two, float between, or show an error code if you are lucky.

All connections must be snug. Do not overlap wires on terminals, especially solid wires. This will cut the wires.

You cannot read the resistance of a thermocouple probe as the voltage a good one makes confuses the circuits of the meter making the reading.

Millivolt and dc volt readings will not help much unless you have another probe to compare it to. You will also have to use the same setting on the same meter to compare the two as the meter will load up the thermocouple to a different level with different settings and give you different readings.

You can do a continuity check as it should read as a continues circuit from plug pin to pin.

Any break in the insulation where you can see the metal wire must be suspect as it can read a temperature there instead of at the end of the probe. Anywhere the wires touch will read the temperature there and the meter will read the closest one to it.

If you suspect a broken probe tip, make another one from that one. Cut the end off, strip about ½ inch back on both wires and twist them together. This is actually how some are made at the factory and it will work.

If you want to make your own probes, get some of the correct extension cable and some plugs. You must keep the colors of the jackets and any connectors the same as marked for the type of thermocouple. In the case of type K, it is yellow. Each type of thermocouple is made from two different metal wires and you cannot get a good reading with mixed types of parts and wires. Do not use any terminal strips in thermocouple circuits not specifically made for the type you are working with; the screws and terminals affect the readings as they are a different metal.

Always match the probe type to the meter. Other types will plug in but not give you correct readings.


I think you have a broken wire in that temprature probe.

a reply to: Flyingclaydisk

Broken wire in probe cable, making intermittent connection?

All,

Great responses so far. I'm going to have to go to bed now, but I will be back around (3am, my normal wake up time) and I will respond then.

Thanks!!!

originally posted by: Flyingclaydisk
Then I start watching the voltage with the probe just sitting on my bench The voltage goes all the way to almost zero. How can this be? There's ambient temperature in the room, so there must be voltage...right?

Wrong.

The probe doesn't measure temperature, it measures temperature difference. If you let the cable sit so the temperature along the entire length of the cable becomes homogeneous or the same, then there is no temperature difference between the ends, so it is not supposed to register a voltage, so you could say the voltage should be about zero in that case.

At first I'm thinking a short in the probe cable, but that's not it because a short would only reduce the reaction between the two metals. What's going on?

Remember we are talking about thermocouple wires here, not regular electrical lead wires. I think once again your assumption here may not be correct and there could be a possibility for leakage and not necessarily a complete short, especially if the insulating material has absorbed any water, from humidity or whatever.

I thought maybe the one meter was holding a residual temp. Nope! Turned that meter back on about an hour later and got the same reading, 72 F (without the probe plugged in).

Could that be the temperature at the meter, and then could plugging in the probe cause that reading to either increase, or decrease, depending on whether the temperature at the thermocouple junction is higher or lower? If the meter is at 72 degrees, and you plug in the thermocouple probe and measure something that's 72 degrees, the voltage generated should be about zero as stated earlier, so that's sort of like not having the thermocouple plugged in. If the measured temperature at the thermocouple probe is higher or lower than 72, then that 72 reading on the meter should change, obviously.

Those first two points are addressed in this explanation of some things that can go wrong with thermocouples, such as your type K, and note the possibility is mentioned of possible "leakage" through the insulation in certain cases, not necessarily a complete short, along with the fact you shouldn't get a voltage if the entire wire is at the same temperature, followed by a suggestion to check the resistance (ohms), which he says should be low if the probe is good. I can't confirm everything he says, but I did work with thermocouples a lot in the past, and he sounds like he knows what he's talking about.

Type K Thermocouple Failure

The thermocouples are drifting.

Drift is caused by chemical change to the thermocouple junction. This is referred to in the literature as "inhomogeneity", meaning the junction is no longer an combination of its two 'pure' alloys, Chromel [90% nickel and 10% chromium] and Alumel [95% nickel, 2% manganese, 2% aluminium and 1% silicon], for type K. The junction is polluted with additional elements or compounds so it is no longer a type K thermocouple.

The theory, as I understand it, is that ions migrate from the sheath to the junction through the mineral insulation (MI). The higher the exposure temperature, the greater the chemical activity, the greater the chemical changes, the faster the pollution occurs with resulting drift.

The commonly used insulation material, mineral insulation (MI) MgO, will absorb and hold water (hygroscopic). The presence of water increases the electrical conductivity through the insulation material, or inversely, reduces the resistivity of the insulation material. Overall lower insulation resistance aids the ability of ion traverse through the MI and the subsequent pollution of the junction.

Green Rot is only one form of chemical pollution, frequently seen on type K elements without mineral insulation, on the heavy gauge elements that are inserted into 1/2" diameter protection tubes. If you carefully grind the end of the sheath away, you might see green discoloration of the junction tip, or you might not, depending on what chemical change your T/C experiences.

If you put your mis-reading thermocouple on a bench and let both ends equilibrate to room temperature so it's isothermal it will not generate an EMF since both ends are at the same temperature. Measure its resistance with an ohm meter and I'll bet it's well above 100 ohms. Under the same conditions, measure the resistance of a 'new' thermocouple. Its resistance will be well under an ohm. The increase in resistance is an indication of a polluted junction. Honeywell UDC and HC-900 temperature controllers measure T/C loop resistance and will alarm on high resistance as an indication that the thermocouple is drifting.

There is no method of correcting for drift because the chemical composition of the polluted junction is an unknown. If the chemical composition is unknown, there is no reference table to refer to in order convert EMF/mV to temperature. The type K table does not apply, as you've seen, because your bastard thermocouple produces an EMF/mV output equivalent to 575 Deg F when it is at 800 Deg F. You need a different reference table, which does not exist, because its chemical composition is an unknown.

The presence of water is a serious problem. Water exacerbates drift.

If you do that resistance check with the thermocouple not generating any voltage, you could try also jiggling the wire with your glass rod or whatever while watching the resistance. If the resistance jumps around you may have a sign of a problem with the wires.

This one I have no clue about. I usually test thermocouples with a meter and heat applied to see if it works properly. The only thermocouples I have tested were on hot water heaters. It has been a long time, I cannot remember what I did, a plumber I knew told me how to test it and it was the thermocouple that was the problem.

originally posted by: Flyingclaydisk
Okay, so I'm testing out a thermocouple today. Wait, let me back up; earlier today I wanted to measure a temperature with one of my multimeters which has a temperature function. I attached the temp probe and got some really wonky readings. Something's wrong. My readings were all over the place. Some, just null.

I have two meters which do temps. My first thought was it was the meter I was using (which is new). So I tried the same probe in an older meter I have. Same weird stuff, but less weird than the first readings. Okay, so now I suspect it's the temperature probe, and not the meter. Time to test the temp probe.

The probe is a Type K temperature probe. The basic principle is two different metals (of wire) brought together at a sensor will create a voltage. In other words, they will convert heat to electrical energy. This is how most electronic temp probes work. Okay, so this should be easy enough to check...

I set one of my better meters to DC voltage and clamp the positive and negative leads. I'm reading mV. I've got a value; let's say it's 20mV. Okay, so I put the probe in some hot water and the voltage goes up. Good! Right? I take it out of the hot water and the voltage goes down. Not the probe, right? Well....
.Then I start watching the voltage with the probe just sitting on my bench The voltage goes all the way to almost zero. How can this be? There's ambient temperature in the room, so there must be voltage...right? Then I grab the probe cable and the mV readings jump up to about 76mV or so (not touching the probe, just the sleeve of the cable). Could be lots of things, so I put the probe and cable back down, watching more closely this time. The probe will suddenly jump up to about 70-80mV for no apparent reason, then when left alone will go all the way back down to near zero. WTH?? Move the probe with a insulated rod, the voltage jumps up. At first I'm thinking a short in the probe cable, but that's not it because a short would only reduce the reaction between the two metals. What's going on?

I checked this with three different meters and got similar reactions, but with different values. Now, on my most accurate meter I get a temp reading of zero. On my other meter I get a reading of 72 F. Well, the room is about 72 F, so maybe it's not the probe...BUT, when I unplug the probe from that meter I get a reading of...72 F! Now I'm really scratching my head! So I shut all the meters off and went to go do something else (more productive). I thought maybe the one meter was holding a residual temp. Nope! Turned that meter back on about an hour later and got the same reading, 72 F (without the probe plugged in).

My Klein meter still registers zero, like there's a fault. Checked the resistance and capacitance across the probe, with nothing weird.

Any ideas? Is it the probe or do I have two bad meters??

A thermocouple is a “couple” Two different conductors with different work functions; that’s what drives the voltage in one direction instead of the other. It sounds to me like the connection between the two conductors is intermittent. Usually, they are just twisted together at the ends. That’s what I would look for.

a reply to: Arbitrageur

The temprature of the wire does not matter. The ones I worked with were over 400 Fahrenheit for most of there length. The two ends are what matters. One is the probe and one is inside the meter for a reference.

You still can’t get an accurate resistance reading when measuring across a small battery with a DMM set to ohms. It confuses the circuits. You have to think of a thermocouple as a weak battery when measureing them.

a reply to: Flyingclaydisk

Well, if it's not an intermittent connection/wire/thermocouple junction, as others have suggested above, and not TWO broken meters ( seems unlikely), might want to consider electromagnetic interference.

Would there be a large transmission tower nearby or is there a magnetic source nearby like a running electric motor or generator ?

The grounding is the most important. The interference I would imagine be more extreme with the 5G?

a reply to: beyondknowledge

You have to think of a thermocouple as a weak battery when measureing them.

That's kind of how I thought it worked too. I also read that different meters have different reference voltages to start with, so there's no set value.

The temperature of the wire does not matter.

Again, my understanding also;temp only matters in the sense that there is a different differential voltage delta as the probe is exposed to different temps.

a reply to: Arbitrageur

Wrong.

The probe doesn't measure temperature, it measures temperature difference. If you let the cable sit so the temperature along the entire length of the cable becomes homogeneous or the same, then there is no temperature difference between the ends, so it is not supposed to register a voltage, so you could say the voltage should be about zero in that case.

Well, that doesn't really make sense, because how then could such a probe ever measure ambient temperature? In all the working probes I've seen the first reading you get after putting the setup together is ambient temp (whether it's 70 F, or minus -30 F.) And, when you get done testing something, the meter returns to displaying ambient temp. If what you're suggesting is correct and there is no differential temp then ambient temperature would always be the same value.

originally posted by: hounddoghowlie
a reply to: Flyingclaydisk

just curious on the one meter that read 72 and held it. does it have a memory function? was it on?

just another thought, did you check/change the batteries, a low battery can make a meter read funky.

Yes it does, but I checked and there was nothing in memory. After that, I cleared all memory just to be sure.