4-20mA convert to Temperature

[computrwkr]

I have a Watlow F4 temperature controller, which is re-transmitting via 4-20mA current 
(what the temperature is in the oven.) I want to capture this current loop and convert 
it to a temperature gauge. I have a current input into my VI coming from a cFP-AI-111 
module on a cFP-1804 device; I've read that I need to scale the input using "slope of a 
line (y=mx+b)" equation and I have included this into my VI plus my assumption that the 
temperature will be in Celsius, I have included a conversion to Fahrenheit.

But my whole issue is the in-between - How do I convert the scaled number (or current 
loop reading) into a temperature? I have used the scaling (numeric) functions (the three 
thermocouple related) without success, none of the required info for the scaled vi-s 
relates to the information I can provide since I have a current loop and no voltage. I
know temperature is usually read in voltage and adding a resistor to the current amd measuring
across this resistor to get voltage is the common thing. Am I out-of-luck here in converting
the current to temperature inside my VI from my cFP-AI-111?

I have attached the VI I have created - any help in figuring out what needs to be added would 
be much appreciated. I have been working on this for a long time and would like to get this resolved.
Let me know if you need any further information.

[Kenny_K]

I use a simialr setup in a program I am running now.  I am using the cFP-AI-111 to get readings from 4-20 mA temperature sensors.

A couple of thingfs that I noticed in your vi.  You need to wire something to the index terminal of the Index Array vi.  Also, your vi will only run once, since you dont have a while loop or for loop.

Your equation seems to be off.  With the information in your comments on the block diagram I get the equation y=18.75x-15.  Also, your equation is based off Farenheit, but you said that you thought your instrument outputted celcius.  This will throw your equation off.  You need to check the user manual to see what the default output is.

Using an equation that relates current to temp, you dont need to changfe the scaled output to temperature, it will be temp already

You are on the right track, so keep pluggin away.

Feel free to ask any more questions.

[computrwkr]

Thank you for your quick response.

I took your suggestions and wired a numeric 0 to the index terminal. I also enclosed everything inside a "for while" loop.

I am a little confused still on the equation piece - you came up with y =18.75x-15. I do not understand how you got -15, Shouldn't this be a positive number or zero. The measurement and automation app allows you to scale your channels but does not allow negative numbers (using this feature as another possibility).

Though I may have confused you a little too by my celsius statement. My instrument outputs current only and I want to use the VI to convert the amperage to a temperature and my assumption is that the VI will output a celsius (or other form) number, which then can be equated to a fahrenheit number.

If I am using an equation that relates current to temp, y=mx+b scales the current to another number but how would I take this scaled number to a temperature number then? Or can you point me into the direction of defining what this equation is? This was the "in-between" question I was asking above.

Thank you again for your assistance,

Ian H.

[Kenny_K]

I attached a simple vi that simulates a fieldpoint (not needed in your prorgram) and then converts the output from the fieldpoint to temperature.

Also, I dont use the scaling function in MAX.  I just read in the raw value from the cFP-AI-111 and convert it to temp as in my program.

If you are scaling the reading in MAX and then trying to convert that number in your vi, you are actually converting the reading to a temp twice, and that could be where you are having problems.

What I normally do to get the equation is place the points in excel and then graph it and use the linear trendline and display the equation.

We may be confusing each other with the C and F stuff.  Normally what most current output temp sensors have is a certain current rating per Kelvin (for the AD590 it is 1uA/K).  This gives you a range of -55 to 150 C, or -67 to 302 F. 

Message Edited by Kenny K on 08-09-2007 02:02 PM

[computrwkr]

Thank you for your response - very helpful.

I was told part of my problem was the decimal point placement needed to be moved for converting from mA to Amps.

Here is a copy of my running VI.

Thanks again, Ian