Hello,

I apologize if I don’t write English very well, I am doing the best at possible for me, but I don’t speak English very well, google translate helps me a little bit for it.

I'm making a test bench for testing pneumatic panels with some components on it.

I have a S7-1200 PLC, which I plugged 7 pressure sensors from a SM 1231 AI8 module.
4 sensors that measure 0-6 bar with a signal in 4 - 20 mA
3 sensors measure aui 0-10 bar with a signal 4 - 20 mA.

The conversion of these sensors are in the attachments «1.jpg», «2.jpg» and «3.jpg».
On NORM_X, putting 0-27648, the sensor didn’t give me the right values (I compares the values with a portable pressure gauge has been calibrated by the manufacturer).

I had to change the min and max for have a good values. (when my sensor is disconnected, to have 0 bar, and when I plug it into an outlet with pressure 9 bar that shows me well 9 bar (6 bar for 0-6 bar).
I wish to make a procedure, and I followed the example found on this board (see attachment « newmax.gif» and «Calibration for pressure sensor AI 4-20 - Entries - Forum - Industry Support - Siemens.pdf»).
This is OK to have realy 9 bar, but not for have 0 bar when i disconnect the sensor. How can I calibrate by the same way for have 0 bar when I disconnect the sensor?
By doing this calibration system, it would mean that I should calibrate my sensor every time I shut down and I turn on the PLC? It is possible to have a solution for avoid that ?

I put the setting inputs in the attachment «4.jpg ».
Even putting the most of smoothing, the values of the sensor continually move.

Would it be possible to further smooth the value of my sensors? (I have to monitor decrease pressure of 0.012 bar during 60s on certains equipment, and it's not possible if the values continually move with too much deviation).

Thank you very much for all your information and again sorry if I do not write well in English.
Thomas

Lets start with the simple stuff.  You have an 8 input unit but you are only using 7 of them, you need to short out the spare input to cut interference.

unknown user

On NORM_X, putting 0-27648, the sensor didn’t give me the right values (I compares the values with a portable pressure gauge has been calibrated by the manufacturer).

This isn't a surprise, any analogue sensor going to an analogue input will need to be calibrated to show the expected value.  If you are having to make a large change to the values then you are looking for electrical noise and offfset problems.

unknown user

 but not for have 0 bar when i disconnect the sensor

 When you say disconnect the sensor, do you mean electrically or disconnect from where it is measuring pressure.  If you mean electrically then it won't show zero, it only shows zero when you have 4mA flowing, if you disconnect the sensor you have 0mA flowing.  If you mean physically disconnect the sensor from the system where you are measuring pressure, then see my comments below on electrical noise.

unknown user

Even putting the most of smoothing, the values of the sensor continually move.

The value will always move, how much it moves depends on how much noise there is in the signal.  Try turning off the smoothing and see how much change you get.  You need to use isolated pressure sensors and you need to make sure that you have grounded one side of 4-20mA loop voltage supply.  Some may argue about grounding one side of the supply, but it has generally given me better results than leaving it floating.  If you can't use isolated pressure sensors then make sure that the place where you are measuring pressure is linked to your 4-20mA loop power supply 0V.

Time to give you a headache!

The level of change you are looking for should be within the capabilities of the analogue module.  You are trying to measure 0.012bar from a 6 bar sensor using a 12bit input.  0.012 Bar in the 6 bar signal is 1/500 of the range.  A 12 bit converter has 4095 steps between zero and full scale, you loose some steps because you are only using the bit from 4 to 20mA, 4095 / 20 * 16 = 3276 steps from 0 bar to 6 bar.  1/500 of that range is 6.5 steps.  So you should be able to measure a 0.012Bar change with a 6Bar range.