NTC 50K TM172

[otrotabi]

I need to read a 50K @25º with a beta value of 3950K sensor with a TM172/TM171

I never really cared about converting resistance values to temperature because we use 10K sensors.

From what I read in internet, this is the relationship between temperatures and beta values:

And LN function is available, so at least theoretically I could read temperature configuring the input as NTC 10k, find resistance from there and then convert it to the real temperature knowing that my sensor is actually different.

It´s been a while since I last touched a logarithm so it might be a challenge. I looked at values in our usual temperature range and for example at 10º, the 50K sensor would read about 100K, so the PLC would be reading -5º, what I mean is that it seems logical.

So, assuming I could solve the math, what I am missing here ? Is it possible ? Is there a FB already developed for this ? 

[FedericoM]

Hi,

can you share the ohm/°T conversion table?

I will check if it is a probe I've already converted

[otrotabi]

Please check the attached file.

[FedericoM]

Unfortunately I have the NTC 50K 25C B3977...if you can send me this table in an excel format, I could create a FB in a couple of weeks.

[otrotabi]

I am not in a rush, thank you. For your reference in case you need to do it yourself in the future, opening the file with Word and copy+paste the rows to excel does the trick.

Out of curiosity, is it possible to measure resistance with the M172 ? It is not included as an option.

[Bastian_Schmitz]

Hi,

7 and 8 is for resistor measurement

7 is Ohm x100 hekto ohm

8 is Ohm x 10 deka ohm

[otrotabi]

Thanks. 

I will give it a try with the formula and see if I can solve it.

[FedericoM · Accepted solution]

Hello,

try this Function Block from the attached library:

 

[otrotabi]

Thank you ! I will try it and let you know how it goes. Lots of other conversions from what I can see, I really appreciate it ! Merry Christmas 🎄

[Hoang]

Hi expert.

i want write function by myseft.

Can you unlock your library of type 20K honeywell ?

[FedericoM]

Hello,

can you tell me which is the issue with the current conversion?

[Hoang]

I just want to see the temperature conversion function you are writing.
I have researched and written the temperature conversion function, I just want to confirm that my method is correct

[FedericoM]

I'm using 2 methods depending on the probe:

- best polinomial ratio

- interpolation through a table

both requires external tools.

I'm not using the formula above because it is not really accurate.

Sorry, but I'm not allowed to share the source in this case (in any case it is not helpful without the external tools).

[Hoang]

OK, thank you.

[Hoang]

The above formula is only to calculate the beta coefficient of a thermistor.

I'll put the solution here for someone to find.
It is necessary to use the Stein-Hart function to calculate the coefficients A, B, C through the 3-point sample of temperature and corresponding resistance.
These three sample points are taken from the table of thermistor values for the corresponding type.