Good morning all

I am working on a system with S7 1214 PLC with 4x13bit Analog input module and a KTP600 HMI connected through profinet. Thre is a sensor to measure tank vacuum pressure. I use the following blocks:

Sensor range: 0-1020mbar, 4-20mA

NormX (setting: min=0, max=27648)

ScaleX (setting: min=0, max=1020).

The real display on the HMI Is showing minus value at minimum pressure.

Following are my questions:

1) The input analog raw value goes to -18 at minimum(?) (Max value is slightly less than 27648)

2) I changed the minimum setting of Scale X block to 1.4 to prevent it from displaying minus value. Now it displays 0.07 at minimum

3) How can I display smaller values like 0.001?

Please help

Hi!

1)

Sounds like you have less than 4mA at your input at minimum pressure. Check the wiring.

BTW, can you get 0 mbar in that tank?

3)

You have a 13 bit ADC, in your case that means 2^13 = 8192 steps on the digital side. Even though this is internally converted to Siemens standard (nominal) range 0 to 27648, you won't be able to "see"  changes in mbar less then 1020/8192, approx 0.1245 mbar. You won't get bellow this precision because of the hardware.  

Dear Sir

Thank yo for your immediate response

You mean if I have to go for a higher resolution Analog module?

Dear Sir

0 mbar is not achievable. The gauge connected to the tank shows 0.7mbar

RajSHJ

Dear Sir

Thank yo for your immediate response

You mean if I have to go for a higher resolution Analog module?

 Hi!

I do not think that you need a higher resolution module, however, that depends on your process control needs. What I wanted to point out is that there is no reason to display small values like 0.001.

RajSHJ

Dear Sir

0 mbar is not achievable. The gauge connected to the tank shows 0.7mbar

At 0.7 mbar your sensor should drive a current a little bit more than 4 mA, but looks like you have less than 4 mA.

Hi

The plant requires a vacuum close to zero. The vacuum pump is capable to create as low as 0.001mbar. The display should show the precise reading of the vacuum. 

At present the the raw input from the sensor is from -18 to +26980 (approx).  For a sensor of 0-1020 mbar / 4-20 mA, 4 mA means 0. The vacuum never goes less than zero. Then how the output from the sensor gives less than 4 mA?

In fact the display shows minus value when I set the min of ScaleX to 0. I get 0.7 when I set it to 1.4

Hi,

I also use a sensor (4-20 ma), but I read inside the notice that the sensor will draw a current lower than 4 ma to tell you default or problems. You will see what I mean in attachement.

Check that for your sensor and also the range. If you are using different sensor, check if you don't mix two of them.

Hop it will help.

Regards,

Jeff

hi,

as I know from working with such sensors, which measure close to 0 mBar, the value of 4-20mA doesn´t move linear to 0-1020mbar. 

I had such sensor, too. There was a formular to calculate this value in my program. I had a valtage measure 0-10V. But it doesn´t matter if it is 0-10V or 4..20mA. If you want to measure close to vaccum the measuring can´t be linear. It should work with a logarithmic formular. The formular in my project was like:

p/mbar = 10^(U-5,5)

So you never get to Zero, but very near to it. I got values to 0.001mbar.

Check the manual of your vacuum sensor if you have to calculate with a special formular too.

hope this helps,

regards