FX3U (le3u) thermo probe?

xman

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Join Date
Nov 2020
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Republika Srpska
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4
Hi,
I`m new here and also in "plc world", I`m owner of FX3U clone PLC https://www.lollette.com/le3u-24mt-6ad2da-plc-controller.htm with HMI OP320, because I`m still newbie I have one stupid problem, how to add thermo probe to this PLC and how to get temperature from thermo probe? Best option? Easy option?

Most of thermo sensors are PT100 and they are not with linear resistance change on temperature change and I don`t know how to calculate temperature from analog inputs...

Look like one option is to buy this transmitter:

https://www.aliexpress.com/item/32867455650.html?spm=a2g0s.9042311.0.0.14d04c4dmY1Bpq

Other option is to use lm35 but most of them are in transistor housing and I need to use it in wet conditions...

Thanks.
 
PT100 probes come in many forms and some come with a built in converter or the housing is designed so you remove the connector block & fit the converter the convertor is normally the same shape & size of the terminal block, these are IP67-8 so no problem in wet areas. You could also wire from the probe and mount the convertor in the panel, these units are optimised and will produce a linear response over most temperature ranges you will probably use but may depend on how much you pay for them.
A tenth din spec probe is accurate to +- 0.03 Degrees C and the converters are often 0.2% and compensate for the slight non-linearity. To be honest, you will probably find the accuracy of the PLC A/D less accurate than those specs.
The formula for converting it from raw value is:
(((Scaled Max-Scaled Min)/(Max Raw Input-Min Raw Input))*(Raw Input-Min Raw Input))+Scaled Min = Scaled Value
If it is a true clone of the FX then you will use the TO/FROM instructions to set up the card and read the raw value. however, for the others there are some instructions to read analogue data like ANRD or RD3A instructions.
The standard analogues on these units are 12 bit so the resolution is not even up to that of a PT100 so you have no need to worry about the very slight curve in a PT100. typically the resolution you can get will be no more than +- 0.26 Deg C
 
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Thank you for answers!

But, to use this formula
(((Scaled Max-Scaled Min)/(Max Raw Input-Min Raw Input))*(Raw Input-Min Raw Input))+Scaled Min = Scaled Value
What I need to know, in my case Scaled Max, Scaled Min, Max Raw Input, Min Raw Input, Raw Input what mean? How to get this values?

Thanks.
 
Here are a couple of bits of code, I do not know what IDE you are using is it GXDeveloper or GXWorks or some other.

The inputs are the raw value from the A/D converter (actual measured value in the case of FX probably 0-4000).
IN Min is the min value from the analog card (Probably 0 in this case but some will be for example 4000 for min 20000 for max)
The In Max is what 20ma will give you (4000)
So these are either fixed values or maybe adjustable from an HMI.
The Out min & out Max are the values expected for example 0.0 to 150.0 Deg C or maybe 0.0 to 10.0 Bar

ST Scaling Fyunction.png Scale with In_Out Scale.png
 
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Thank you for all info.
Look like I also must know thermo sensor working range but on most of them don`t exist model number and other info, look like I must buy new sensor with all info...
 
Not really, most PT100 sensors are -50-800 but it will not matter, if you use a converter you order them for the range you want, most are 0-150 Dec c so 4-20ma will equate to 0-150 degrees C. or 0-100 A quick & dirty way to check calibration is to stick the probe into water with ice for 0 Deg and a boiling kettle for 100 Deg C, There is a difference depending how high you are from sea level but it is probably too small to worry about
 
I'm not familiar with that clone but on FX I use FX A/D expansion cards these are configurable to 0-10v 0-20 ma or 4-20ma I have a hunch that your in-built ones are 0-20ma so if you use a 4-20ma converter then the In Min would not be 0 but probably 800 with a In Max of 4000, only a guess.
 
What I need to know, in my case Scaled Max, Scaled Min, Max Raw Input, Min Raw Input, Raw Input what mean? How to get this values?


Those should be in the user manual or installation manual for the sensor unit.


A similar question has detailed explanation in this thread, but anyone who does not understand this "in their bones*", then I suggest you get someone to check your work.



* "in your bones" is an American/English idiom, meaning that the understanding of how to scale is something you know how to do almost without thinking about it.
 
Ronnie is correct, however, this is a clone and don't know if it is compatible, but if so then it is worth doing, it makes it much easier and will scale the PT100 without any programming apart from using the TO/FROM functions to get the data into a variable.
 

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