problems converting resistance to temprature

[TorbH]

Hi

I am working on a program that converts raw value from a pt-100 sensing element.

The value i get from the element using a Agilent 34970A is 120.788 ohm. vitch is correct.

This has been done in MSexcel before, and the formula we have used is                                

" =(($C$18-SQRT($C$18^2-4*$C$19*((B26/$C$17)-1)))/(2*$C$19))"

Where C17=Sensing element at 0¤ (99,6943)

             C18=0.0039135

             C19=0.0000006035

             B26=Measured resistance (120.788)

Converting this into a formula in a formula node gives me                                            T=((A - (sqrt(A**2 - 4*B*((InRt/R) -1)))/(2*B))); 

A= (C17, 99,965)

B=(C18, -0,0039135)

C=(C19, 0.0000006035)

R=(C17=Sensing element at 0¤) 99,965

InRt=(B26, measured resistance 120,788)

The formula node gives out T=-3187.81301

In excel this value is 56¤ +-

What am I doing wrong?

It appears to me that the formula node does not operate in the same manner as excel.

[GerdW]

Hi Torb,

why don't you use the existing functions delivered by NI?

[TorbH]

Hi, thanks for responding

I am using a Agilent 34970A with a multiplexer card, so the Voltage and current is unknown to me. In theory the ohm value should be enough.

I have had a look at them, but I am unable to use them as they are as far as I can tell.

The 34970A measures the resistance in 4 wire, so basicly all I need to do is convert the result.

[GerdW]

Hi Torb,

What exactly is your problem?

Well, you can even wire all of your constants:

What was your problem?

[TorbH]

thanks for responding,

Well...first of all i was not aware that RTDVoltage could also be the resistance you measure.

"RTDvolts is the voltage you read from the RTD" in the helper made me kindof confused.

On the other hand this converter does not include the "C" constant witch I need in order to make the pt100 correct from -70 and up to 200¤C.

I will also have to make a function that selects a formula based on above or below 0 later on, but one step at a time.

I was trying to make the above 0 formula work first, and i got stuck.

appreciate your input

[GerdW]

Hi Torb,

first of all i was not aware that RTDVoltage could also be the resistance you measure. "RTDvolts is the voltage you read from the RTD" in the helper made me kindof confused.

R = U/I. Ohm's Law!

When you provide a value of 120 to that routine you need to set the Iex value according to this: 120V = 120Ohm * 1A…

Edited:

What exactly is your problem?

[TorbH]

thank for responding

Where does 120V and 1A come from? just curious since i was not able to find it in the maual for the 34970A.

I know Ohm's Law, but with 2 unknown numbers i was kind of stuck...:)

anyway, i need to make this work below 0 as well, so this solution is not ideal for me to use...

[GerdW]

Hi Torb,

---

Where does 120V and 1A come from?

---

This comes from help for RTD conversion VI: it expects an input as voltage and the corresponding Iex value. You measure resistance in Ohm. To use this VI you need to provide a faked Iex value.

How to get that value? Using Ohm's law as written above…

---

anyway, i need to make this work below 0 as well, so this solution is not ideal for me to use...

---

This is your own formula. You said before you need to use that formula.

Now you write you can't use this formula…

I don't get it!

[JÞB]

The simplified Callendar equasion implemented by the Convert RTD.vi (and approximated by the OPs Excel formula - though I would challenge the faulty alpha value) is simply not valid below 0C

That would be the problem.

The solution should present itself from the wiki link.

[TorbH]

hi

my initial plan was to use a formula node, the formula i provided is for the temp above 0.

for calculating below 0 there is a different formula, but i did not get that far.

i like to solve 1 problem prior to making an other one.

Using the converter LV provides does not give me that opportunity, so i am back to the formula node.

the example you gave me is one way to go, but i am not an expert in math, so it was not that logical for me to do it in that manner, since i had the formula already it would be easyer for me to just use that.

I also tried to make the same as shown in you image, but the result was not the same. I am totally blank at what could be the problen, they both look the same to me. Making me think there is some formatting that i have done wrong.