Giedrius_Kai

Hi,

Just recently I've been working with AI module and I was a bit confused. I'd like to clafiry some vague things about it for good :)
It was a 16-bit analog input module and VEGABAR 82 (4..20 mA) sensor with the measuring range -1..+100 bar and smallest measuring range 0.025 bar.
As I understand, its span is 100 or 101 bar, not sure about its settings, but its not that important. Let's say it's 100 bar.
Span / smallest measuring range = 100 / 0.025 = 4000 measuring units
4..20 mA: 20 - 4 = 16 mA
So, each sensor's unit: 16 / 4000 = 0.004 mA

On the PLC side it's:
0..27648 = 27648 units
16 / 27648 = 0.00058 mA (you can find the same value in Siemens specs)

My colleague told me to show just one decimal place in the HMI, because "the sensor is not accurate enough".
I understand that sensor and PLC have two different resolutions and that sensor is not accurate enough, but I think that I could show 2 decimal places and that value would still be correct.

0.004

0.008

0.012

0.016

0.02

0.024

0.028

I don't miss any values in a x.xx range.

Is my theory correct? How can I calculate number of decimal points to represent? How do you calculate how many decimal places to represent for different analog sensors?
Thanks for you replies.

Regards,
Giedrius K.

Giedrius_Kai

I think its accuracy was 0.1 %.
So, 1 bar is equal to 0.16 mA, because 16 mA (mA range) / 100 bar (bar range)
and 0.1 bar is ±0.016 mA, because of the accuracy.
Why is it feigned then? If it were a ±0.16 mA, I would agree with you.