I am trying to identify what Thermistor I currently have. I believe it to be of the PTC type. Since the resistance is rising as the temperature rises.

There are plenty of charts out there for NTC type thermistors but not much on the PTC type.

This one is potted inside a cylinder that I believe is aluminum. Two wires come out of one end.

At 67 degrees Fahrenheit I am getting a reading of 36.34 M Ohms.

The unit this is installed into can reach a temperature of around 150 degrees Fahrenheit.

I am attaching a picture of the Thermistor and a picture of it on the Schematic (there are two thermistors on the Schematic they are both identical).


How can I determine what this is so I can buy a replacement?

Thanks.

 

OK, does "36.34 M" ohms means 36.34 meg ohms or 36.34 milli-ohms.

 

MΩ Megaohm.. My fluke displays it as M Ω (see attached picture)

 

Okay I'm an idiot. Apparently I was testing the Thermistor that is malfunctioning.

These might actually be NTC type.

 

This is not in the range of any thermistor I have met.
5 volts into 36 million ohms causes 139 nanoamps of current. This is simply not practical for a real world sensor.

 

So this is actually an NTC type. However I'm not exactly sure which one.

Assuming the "good" thermistor is still good and based on the readings I'm not seeing anything that specs out.

I am getting a reading of 160 k ohms (160000 ohms) at 65 F (18.33 C).

To me it seems like it should be around a 100k NTC Thermistor. But could it be more like a 120k NTC Thermistor?

 

So, before you read 36 meg on the bad part and now you are reading 160K on a good part. Is this correct?

 

So, before you read 36 meg on the bad part and now you are reading 160K on a good part. Is this correct?

Yes this is correct.

 

Does the resistance increase as it gets hotter, or decrease?

 

Does the resistance increase as it gets hotter, or decrease?

Resistance decreases as it gets hotter.

 

Then it is NTC. What exactly are you working on? Have you tried the manufacturer?

 

OK
It looks like your sensor is consistent with a standard 100K NTC thermistor. A Murata NXRT15WF104FA1B040 can be purchased at digikey for about 43 cents. This should do it.

 

Then it is NTC. What exactly are you working on? Have you tried the manufacturer?

It's a heater for a machine. Yes the manufacturer won't sell the components for the assembly and they are not interested in sharing specifications because the part is around 10K USD.

 

OK
It looks like your sensor is consistent with a standard 100K NTC thermistor. A Murata NXRT15WF104FA1B040 can be purchased at digikey for about 43 cents. This should do it.

That is what I was figuring. I just wasn't getting exact measurements based on the charts out there.

If I use the Murata NXRT15WF104FA1B040 would I just Pot it myself in an aluminum cylinder?

Any recommendations on assembly and potting material that works well with Thermistors?

Thanks.

 

Well, I am not going to pot it for you. Look at your failed part. What did they use? Copy it as best as you can. At 43 cents a piece, you can try several ways for not much money.

 

Well, I am not going to pot it for you. Look at your failed part. What did they use? Copy it as best as you can. At 43 cents a piece, you can try several ways for not much money.

Okay.

I will research potting materials tonight.

Just wasn't certain if there was a particular material that worked best to normalize temperature from the thermistor to the cylinder container.


Thank you for all the assistance. I really appreciate it!

 

Thermal conductivity of the potting compound has everything to do with time lag. Do the best you can with something that feels cold to your touch. If it's too slow, the machine will, "hunt" for the balance point.

 

A tip on potting... After you pour the potting material into the casing, you should put the unit in a vacuum chamber to pull out the air bubbles. If you don't remove the air, the thermal resistance will be a lot higher than it could be. This might be a problem -- I am not sure.

 

A tip on potting... After you pour the potting material into the casing, you should put the unit in a vacuum chamber to pull out the air bubbles. If you don't remove the air, the thermal resistance will be a lot higher than it could be. This might be a problem -- I am not sure.

Good point.

I read that this potting material (Polytec TC 437) flowed really well for putting a thermistor into a small space. The aluminum cylinder the current thermistor is in is 1 inch long x .1875 inch diameter.

Doesn't leave a whole lot of room. Unfortunately where the thermistor is placed it's in a machined groove on an aluminum part. So it probably can't exceed 5mm in diameter.

So far these are the materials I am looking into.

Polytec TC 437 - by Polytec PT - http://www.intertronics.co.uk/products/thermally_conductive_adhesives_01.htm

Duralco 4703 - by Cotronics Corp - http://www.cotronics.com/vo/cotr/ea_ultratemp.htm

832TC - by MG Chemicals - http://www.mgchemicals.com/products...capsulating/thermally-conductive-epoxy-832tc/

My interest in MG Chemicals is that it's readily available through many electronics distributors. Duralco and especially Polytec appear to be superior. The article on the Polytec usage is here: http://www.newmaterials.com/News_De...thermocouple_cavities_12929.asp#axzz3TYpHndvP


Any thoughts to weigh in with?

 

Polytec 301 is the obvious winner on that page because it HAS a thermal rating and the number is, "3".
(Higher number is good.)

Would you like to convert BTU inches per hour, per square foot, per Fahrenheit into Watts per meter per Kelvin?