Help me acquire variables for NTC thermistor equation

Oi to all guys. I need a little help. Till now i have been measuring temperature up to 100 degree centigrade with LM35. Now i have to measure temperature up to 250-300 degrees.

Thermistor is best for it as it is small in size but highly nonlinear. Its equation is given as Steinhart-Hart in which there are 3 variables A,B,C which is usually given in datasheet.

Problem: In my Area there are thermistors without any model except their resistance at 25 degrees. Is there any way to find these 3 variables.

One way is to acquire its graph but how will i do it. I need a constant temperature to measure its value and then increase temperature and that is quite difficult.

Re: NTC thermistor Guide

Hey sajjadkhan. So.. you're also using thermistor. I think that would be hard if you have no equipment that will show you accurate temperature readings with heat being produced. I have used a temperature calibrator which shows temperature and it has holes where the thermistor probe can be inserted. I measured then the resistance on every temperature and placed them in the table. After taking the resistances on its corresponding temperatures, I used then this web application on determining the 3 constants. **broken link removed**

Are you planning also to interface it with a microcontroller?

Re: NTC thermistor Guide

Hey, any specific model of temperature calibrator?

I was thinking of a way around. I have a muti-meter UT70A. it can measure temperature up to 300 degree centigrade. it has two long wires with a thermistor connected at the end. If i tie my thermistor to it and then check it for different temperatures...e.g. i heat up the solder and slightly bring it closer to it and i get a graph? what u say?

Re: NTC thermistor Guide

I'm using this one Tecal 425s Calibrator and it is quite expensive.
**broken link removed**
I'm not sure but I've read the manual of your multimeter and i think it shows that it uses a K type thermocouple that can read a maximum of 200degC. Even if it's thermistor there are lot of thermistor types that the multimeter might used which has different resistance readings at 25degC. Maybe you should first determine what kind of thermistor if it's ntc or ptc (like thermocouple). I think that's possible (although I doubt) because you are making a parallel resistance therefore your thermistor's resistance will be affected by the resistance of the multimeter's sensor so you'll have calculations first.

Re: NTC thermistor Guide

No i am just attaching with it, without any terminal connection. Just because it will be close to it so whatever temperature is at the multimeter thermistor or thermocouple it will be also on my testing thermistor.

Re: NTC thermistor Guide

If that's the case then that should work. Try it.

Hey lloydi12345 how are you?

I have managed to acquire accurate values of thermistor at different temperature and fed them in the calculator as you have given the link. Now it shows 2 models: S-H model and Beta model. Do you know what are these and which model is more accurate? thanks.

Hi sajjad khan,
As you said you will just attach your Thermister probe with Meter probe, thats a good idea just to compare. Then quality of your measuring device will be dependent to the quality of your meter. The specs also say in the temperature measuring tag :
" l Included is a K-Type (nickel chromium ~ nickel silicon) point contact temperature probe which could only measure
temperature below 230ºC. If you want to measure temperature higher than 230ºC, you must use the rod contact
temperature probe. "

You must take this in consideration.

Hey Raza... Yea you are right, my meter give temperature like 150,151 but not like 150.1,150.2 so it is accurate up to one degree.
My Multimeter probe is of thermocouple and problem was that thermocouple response time is much faster then thermistor. I took three points at 25 degree, 100 degree and 250 degree. 25 degree was easy as the temperature at night was falling too slow and thermocouple stayed there for minutes.

hard part was to get resistance of thermistor at 100 and 250 degree centigrade. for that i used a small baking plate, put the plate over thermistor and thermocouple (tied together) and put iron on top (I modified the iron so it run continuously). At 100 degree temperature was still increasing but i put up the iron a little bit so temperature stayed there for a minute. same i did for 250 degree. Though my iron shape is totally changed from bottom, so you need a tough one.



Then i put these values in the following table
**broken link removed**
And now i have error maximum of 1 degree using S-H model. So i think its good.

Can't use surface touched base sensors because i am modifying my laminator so it should not touch the rollers.
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now i am thinking something else, an Idea to implement (if i got the time). Temperature calibrators are very expensive. so

I am going to use a small black metal box with a heater wire inside controlled by a Triac. The temperature of that box then can be controlled as well as maintained by Triac by triggering it at certain angles. What do you guys think?

Hello sajjad,
You do not want sensor to touch the Soldering iron body, no problem. Just get a MICA spacer for transistor, peel off it for very thin layer and place it in between the Iron and sensor. No physical contact with body and even you can get high temperature to heat up.
My suggestion, instead of using a normal Soldering Iron, use a temperature controlled Soldering Station which can give a better controlled temperature up to even 400°C.
Hope it may help you a bit.