unscale range 4---20ma with 0----27648 and with -20----80c temperature

buolis

Member
Join Date
Apr 2018
Location
egypt
Posts
2
i make a program ( plc siemens s7-300 )
with analog output
4-------20ma
0-------27648
-20-----80c temp.
how to do this?
 
I have a couple of analog outputs that output "temperature" or "pressure"... although it may be a waste of processing power and wiring, some people wire gauges to analog outputs of the PLC instead of a serial loop.

One way that it isn't a waste is when the instrument is in a hazardous area and the gauge doesn't have to be. Instead of getting intrinsically safe equipment for the gauge, you can just wire anything as a gauge and have the PLC drive it.
 
One way that it isn't a waste is when the instrument is in a hazardous area and the gauge doesn't have to be. Instead of getting intrinsically safe equipment for the gauge, you can just wire anything as a gauge and have the PLC drive it.
Not true. Instrumentation in a hazardous area has to be rated for the hazardous area and installed as hazardous area equipment.

4-20mA is low power, but that in and of itself does not eliminate the requirement for hazardous area rated instrumentation and installation.
 
One way that it isn't a waste is when the instrument is in a hazardous area and the gauge doesn't have to be. Instead of getting intrinsically safe equipment for the gauge, you can just wire anything as a gauge and have the PLC drive it.

I highlighted the relevant bit on that sentence for you...

Makes sense this way?? If an instrument is not in a hazardous area and the signal isn't routed through a hazardous area, it doesn't matter if the variable displayed is from a hazardous area.

So, in that example, the instrument is in a hazardous area with rated box and likely intrinsically safe circuit with the correct parameters of the loop calculated.

But, perhaps the power supply powering the loop can't supply enough power to satisfy the requirement of having a gauge and instrument in the same loop, or maybe the gauge is to be on a wall of an office which is a safe area and therefore there's no need for it to be Ex rated. This being the case, a separate PLC output can drive that gauge without any regard for hazardous area requirements.
 
Last edited:
I highlighted the relevant bit on that sentence for you...

Makes sense this way?? If an instrument is not in a hazardous area and the signal isn't routed through a hazardous area, it doesn't matter if the variable displayed is from a hazardous area.

So, in that example, the instrument is in a hazardous area with rated box and likely intrinsically safe circuit with the correct parameters of the loop calculated.

But, perhaps the power supply powering the loop can't supply enough power to satisfy the requirement of having a gauge and instrument in the same loop, or maybe the gauge is to be on a wall of an office which is a safe area and therefore there's no need for it to be Ex rated. This being the case, a separate PLC output can drive that gauge without any regard for hazardous area requirements.

You first post didn't tell clearly that you install gauge outside of Ex area
 
Hi buolis,

The easiest way to do this is using STL.

L temp (-20..80 degree temperature as real)
L 20.0
+R (change to 0..100 scale)
L 276.48
*R (change to 0..27638 scale)
RND (change to INT)
T OUTPUT (as INT)
 
You first post didn't tell clearly that you install gauge outside of Ex area

English isn't my native language... so I wrongly assumed that saying
and the gauge doesn't have to be.
would be enough for people to understand it.
[SARCASM]I'll be sure to make a hazardous area drawing next time I try to convey this point across. [/SARCASM]
 

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