wiring a load cell to thermocouple module ?

Do someone ever try to do a low cost device like that ?

We have an old 1975 machine using a load cell and i would like to data log the result with a small S7-200 plc

The probe itself gives a 2mv/v so for 10v i would have -20/+20mV signal for the full range.

A themocouple module 6ES7 231 7PD22 as a -80/+80mV scale...

Do someone see any problem to connect the probe to this module? i will only use 25% of the module range but if the precision if enough i bet it would works ?

Any comments ?

A load cell is typically a linear device, while NO thermocouple is linear. So, though you wouldn't likely harm the device, you would get no meaningful numbers out of the thermocouple module.

OZEE said:

A load cell is typically a linear device, while NO thermocouple is linear. So, though you wouldn't likely harm the device, you would get no meaningful numbers out of the thermocouple module.

Click to expand...

I think he is discussing setting his thermocouple module to a linear input range (many T/C modules have this capability).

As far as precision goes, you would have to find out the resolution of the input and calculate what the weight change would need to be to move one count. If that weight is small enough for your purposes, I think it should work.

Brian

yes it will work, however watch the grounding.
better is to use a converter, or a xtr105 ic.
just connect it and see the input is very high in resistance so no problem.
and the output is not a straight line, but you must make a table or graph anyway.

Presumably, running the analog input card in 'millivolt' mode will disable the thermocouple cold junction compensation, but it would be prudent to check and make sure that can be done. Otherwise your load readings will vary with ambient temperature.

It might be a jumper change.

The 231 7PD module has dip switch configuration and a setting availaible is for -80/80mV so i'm expecting to be a linear reading....
Their is no info about te cold junction cancell but i expect it to be...I will look if i try that option.

It is a 15 bits for -27xxx to 27xxx so cutting that resolution in 4 wouldn't be too bad for my purpose

In the other hand, i found a cheap load cell amplifier board giving an output of 0-4.5 volts that can be read by a 0-10v input with 50% resolution

Please understand here that it is a personal project with personal low budjet....I wouldn't do that for an industrial customer...!
The project it's an old dynamometer that read power with analog needles! Their so many adjusting pot in the 1975 cards that anything else has to be better than that. It mesure speed and use a loading pump. Torque is read from the pump movement with the load cell.
If i can get the readings in a plc, i will have all i need to calculate and log everything.

Some thermocouple modules have one side grounded. If grounded, your 10V Excitation supply must have a floating output.

A simpler answer would be to use the Load Cell Module for the S7-200 it is built specifically for load cells. It is called the SIWAREX MS Weighing Module

Part# 7MH49300AA01

There is also a software package that allows you to configure the Load cell module. It is called SIWAREX MS Configuration Package

Part # 7MH49300AK01

The Software CD has all the manuals you will need.

keithkyll said:

Some thermocouple modules have one side grounded. If grounded, your 10V Excitation supply must have a floating output.

Click to expand...

yes it is 1 to be sure to check as the cell reading is differential.
Thanks

Clay B. said:

A simpler answer would be to use the Load Cell Module for the S7-200 it is built specifically for load cells. It is called the SIWAREX MS Weighing Module

Part# 7MH49300AA01

There is also a software package that allows you to configure the Load cell module. It is called SIWAREX MS Configuration Package

Part # 7MH49300AK01

The Software CD has all the manuals you will need.

Click to expand...

The clue is to find a cheap solution! this one is the best for industrial users but too expensive for me....These modules are just availaible brand new and not cheap.

Should work well if you can switch the input from t/c to mV 20 mV will give you plenty of span.

Roy Matson said:

Should work well if you can switch the input from t/c to mV 20 mV will give you plenty of span.

Click to expand...

unfortunately just a -80 + 80 mv is availaible

I retained the cheap amplifier giving a 0-5v instead...

thanks for all answers