How to find the RTD PT100 resistance and temperature?

[internetuser2k11]

Hello!

With the circuit mentioned here I am getting a value of 2.82V @ 850 degree C. Can somebody tell me how to find the RTD PT100 resistance and temperature from Vo?

What is the equation for rtd resistance and temperature? I don't want to use look up tables.

 

[sky_123]

Re: Help with RTD PT100 Circuit

It is about +4 ohms difference per degree C, with zero degrees C being 1000 ohms. So, 1 degree C would be 4 ohms.
However, most manufacturers have their own tables (since there is a difference slightly between different manufacturers).
So, for ultimate accuracy, lookup tables need to be used.

EDIT: sorry, that was for PT1000 - I need glasses clearly!
For PT100, read as 0.4 ohms and 100 ohms respectively.

 

[internetuser2k11]

Re: Help with RTD PT100 Circuit

I know that RTD resistance increase by 0.385 Ohms / degree C. What I want to know in this
circuit i, how to get rtd temperature and resistance from o/p voltage Vo. I'm getting 2.82V @ 850 degree C and 0.72V @ 0 degree C. the function is not linear.

 

[sky_123]

Re: Help with RTD PT100 Circuit

It sounds about right.. not very familiar with this cct, but looks like an instrumentation amp set to a gain of 6.24??, and a constant current source of 1mA.
So, at 0 degrees, you'd get about (100ohms*0.001A)*6.24 = 0.624V and you're getting 0.72V
At 850 degrees you'd get ((0.4ohms*850degrees)+100ohms)*0.001A*6.24 = 2.7456V and you're getting 2.82V

Your values are a bit higher, maybe component tolerances mean the gain isn't quite 6.24 or something.. maybe it's an offset. What op-amps are you using? Are your results real or a simulation?

 

[kalbun]

Re: Help with RTD PT100 Circuit

Have a look at Wikipedia for some background: https://en.wikipedia.org/wiki/Resistance_thermometer

As you can see, there is a simple formula that gives resistance as a function of temperature. Unfortunately they normally work in a limited range of temperature, and if you need to measure 850°C (or did you mean 85?) you are well beyond that range.

For a more physical description of PT100, you may have a look to the Italian Wikipedia (https://it.wikipedia.org/wiki/Termoresistenza) with an online service like google translate or bing translator.

Hope this helps.

 

[FvM]

Re: Help with RTD PT100 Circuit

It sounds about right.. not very familiar with this cct, but looks like an instrumentation amp set to a gain of 6.24??, and a constant current source of 1mA.

The gain is 7.24 (1+ 20/3.205), not 6.24. The output voltages assumed by the original poster are exactly corresponding to nominal Pt100 resistance values. Resistance at 850 °C is specified as 390.481 ohm.

Can somebody tell me how to find the RTD PT100 resistance and temperature from Vo?

According to the circuit function, that has been basically analyzed correctly by sky_123, Rpt100 = Vo/7.24*1000. Output voltage to resistance is of course a linear function. Resistance to temperature is not exactly linear according to the Pt100 characteristic, but a assuming a linear relation may be sufficient for many cases, particularly in a smaller temperature range. What you most likely mean is that the linear function has a constant term b, Rpt100 = a*t + b. You can easily draw a straight line connecting the 0 an 850 °C points, it will involve about 30 degree deviation in the middle of the range. For an exact measurement, you should refer to a polynomial or a table.

A precise table calculation method has been suggested in an Analog Devices application note.
https://www.analog.com/static/imported-files/application_notes/AN709_0.pdf
https://www.analog.com/static/imported-files/application_notes/AN-709_files.zip

 

[internetuser2k11]

Re: Help with RTD PT100 Circuit

It sounds about right.. not very familiar with this cct, but looks like an instrumentation amp set to a gain of 6.24??, and a constant current source of 1mA.
So, at 0 degrees, you'd get about (100ohms*0.001A)*6.24 = 0.624V and you're getting 0.72V
At 850 degrees you'd get ((0.4ohms*850degrees)+100ohms)*0.001A*6.24 = 2.7456V and you're getting 2.82V

Your values are a bit higher, maybe component tolerances mean the gain isn't quite 6.24 or something.. maybe it's an offset. What op-amps are you using? Are your results real or a simulation?

No, with resistance is taken as 0.389 Ohms / degree C. I'm simulating in proteus. I've plotted the graph for every 50 degree C. It is not linear. I want to find the value of temperature from Vo.

---------- Post added at 07:10 ---------- Previous post was at 06:56 ----------

In Rpt100 = a*t + b What is the value of a, b, and t. Without knowing t, I cannot calculate r, and without knowing r, I cannot calculate t. What is the polynomial equation for calculating rtd temperature and resistance.

I got this equation Rt = Ro [1 + At + Bt^2] for 0-850 degree C range. But I don't know t to calculate Rt.

---------- Post added at 07:18 ---------- Previous post was at 07:10 ----------

I am using LMP2011 MF and V+ = 5V

 

[FvM]

Re: Help with RTD PT100 Circuit

Sorry, your post doesn't make sense. With a the assumption of a linear Pt100 t.c., the output will be linear. Don't know if it's a problem of Proteus or your mathematics.

I got this equation Rt = Ro [1 + At + Bt^2] for 0-850 degree C range. But I don't know t to calculate Rt.

There should be A and B numbers given together with this equation. What's your problem in calculation then? Did you try a pocket calculator?

 

[internetuser2k11]

Re: Help with RTD PT100 Circuit

OK. I finished the project. I wanted to get the value of rtd pt100's resistance and rtd pt100's temperature from Vo of the Instrumentation amplifier.

 

[FvM]

Re: Help with RTD PT100 Circuit

I wanted to get the value of rtd pt100's resistance and rtd pt100's temperature from Vo of the Instrumentation amplifier.

Did you look into the AD application note?

 

[internetuser2k11]

Re: Help with RTD PT100 Circuit

Dp you mean PIC mcu datasheets ADC section?

 

[FvM]

Re: Help with RTD PT100 Circuit

I meant the application note linked in post #6.

 

[internetuser2k11]

Re: Help with RTD PT100 Circuit

I've checked those documents.

In this https://www.analog.com/static/imported-files/application_notes/AN709_0.pdf document there is an equation for T. You can check it for yourself. It is giving wrong values

In RTD Table for 500 Degree C, resistance = 280.98. So, you can put r = 280.98 and check fro yourself what the value comes for T. Ro = 100.

Please check it and tell me.

---------- Post added at 20:06 ---------- Previous post was at 20:02 ----------

I've checked those documents.

In this https://www.analog.com/static/imported-files/application_notes/AN709_0.pdf document there is an equation for T. You can check it for yourself. It is giving wrong values

In RTD Table for 500 Degree C, resistance = r = 280.98. So, you can put r = 280.98 and check for yourself what the value comes for T. Ro = 100.

Please check it and tell me.

 

[internetuser2k11]

Re: Help with RTD PT100 Circuit

Hello! I did like this. All variables are double type.

[syntax = c]

val1 = (100.00 - resistance)/100.00;
val2 = 4.0 * (-5.775e-7) * val1;
val3 = sqrt(15.27480889e-6 - val2);
val4 = -1.155e-6;
temperature = (-3.8093e-3 + val3) / val4;

[/syntax]

But it is giving wrong values.

 

[FvM]

Re: Help with RTD PT100 Circuit

I suggested the application note for it's table caculation, not the "direct method".

The direct calculation works perfectly, but there's a typo in your calculation
temperature = (-3.8093e-3 + val3) / val4;

The most important disadvantage of the direct method is that it can't be applied to negative temperatures. And importng a sqrt() library may nbe inappropriate for small micros.

 

[internetuser2k11]

Re: Help with RTD PT100 Circuit

I fixed the typo. Here is my final project

How to do the negative temperature calculations?

---------- Post added at 15:52 ---------- Previous post was at 15:50 ----------

I fixed the typo. Here is my final project

How to do the negative temperature calculations?

 

[FvM]

Re: Help with RTD PT100 Circuit

As said in the application note, inversion of the more complex polynomial defining the Pt100 standard resistance for negative temperatures isn't feasible.

For a temperature range down to -100 °C, the deviation isn't larger than 0.2 degree, so the calculation may still work for most technical Pt100 applications except cryo cooling. For lower negative temperatures or better accuracy, use a polynomial approximation or piecewise linear approximation (table interpolation), as explained in the application note´.

 

[markocrni]

Re: Help with RTD PT100 Circuit

Can you put your code, please?

 

[internetuser2k12]

Re: Help with RTD PT100 Circuit

Check this. https://www.edaboard.com/threads/252959/ If the code isn't there I will put the code from my old HDD.

 

[emr]

Re: Help with RTD PT100 Circuit

Hi internetuserk12,

Could you please upload your source code if it's not a problem for you.

Regards,