FC105 Hi Limit value determination

[manmeetvirdi]

Hi there
I got confused so this question:

I have got thermocouple connected to the AI.
Iam using FC105 to scale the values into Engineering value "Centigrade"
Doubt is: FC105 asks for Hi limit and Low limit, so how can I determine the Hi Limit value??
Hi Limit = Actual high value expected in process
Hi Limit = Actual high value which thermocouple can register
Hi Limit = Set it by trial & error method

Thnaks for the time,
regards
Manmeet

 

[cjd1965]

hi

The high figure should be the process value at maximum counts (say 20mA or 10V at the input)



Cheers

Edit: just read thermocouple.. what does your thermocouple module manual say?

 

[Lancie1]

For thermocouples, normally the input must be scaled to high and low limits for the type of thermocouple that you are using.

Hi Limit = Actual high value which thermocouple can register

 

[L D[AR2P#0.0]]

Here's a link to a Siemens document concerning analogue inputs and thermocouples:

http://support.automation.siemens.com/WW/skm/frameset.asp?url=%2FWW%2Fllisapi%2Edll%2F18972936%3Ffunc%3Dll%26objId%3D18972936%26objaction%3Dcsopen%26siteid%3Dcseus%26skm%3D1%26lang%3Den%26query%3Ds7%2Banalogue%2Binput%2Bmodule%26ehbid%3Dhtml%255F76%252Fehb%252F18972936%252Ehtm%26nodeid%3D&Query=s7+analogue+input+module&SearchArea=hb&id=18972936&F11Marker=true&siteid=cseus&query2=&modelled=&lang=en

 

[darrenj]

you can put a thermocouple on a normal Analog in? the Thermo couple puts out Millivolts...not 0 to 10 or 4 to 20??

In AB we use a different module..and tell it what kind of thermocouple we are using (logix platform anyway)

Curious..

Darren

 

[manmeetvirdi]

Hi
This particular Iam talking about it bit Abnormal (6ES7 331 7KF02-0AB0)
It can take voltage +/- 10V,5V,2.5V,1,500mv,250mV,80mV....
I tcan take current input from both 2 and 4 wire transducers
4..20mA,0..20mA,+/-20mA.....
Pt100, Ni 100
And thermocouples of type K,L,J,E
Meanwhile going through suggestions(Lancie,Cjd) and manual (LD) to remove confusion...
Thanks buddies..

 

[rohan tandon]

It appears that the range is determined by the type of thermocouple selected.

For K type temperature of -270 to 1622 will be represented by -2700 to 16220 decimal.

From the manual...

 

[manmeetvirdi]

Hi

So I need to set the high Limit=1372 (I think its not 1622)
and Low Limit=-270
To get the correct display of temperature in centigrades, right?

 

[rohan tandon]

You were right about that, the range is till 1372 and not 1622...

FC105 assumes the range of integers to be from –27648 and 27648(BIPOLAR) but in the case of RTD and thermocouples the range varies.

For k-type I would try HI = 2764.8 LO = -2764.8 and BIPOLAR = true

another option is to convert the PIW to real and divide by 10 to give the correspoding temperature

 

[manmeetvirdi]

Hi Rohan

Just one more thing:
Suppose I set Hi=2764.8 and lo=-2764.8
So when the temperature is 1622, the output of FC105 will be 2764.8
which is not the right temperature, right??

 

[rohan tandon]

Temperature of 1622 would be read as 16220 decimal, which would be converted by the scale function to read 1622.0

The SCALE function uses the forumla

OUT = [ ((INPUT) – K1)/(K2–K1)) * (HI_LIM–LO_LIM)] + LO_LIM

K1 = - 27648
K2 = 27648
hi_lim = 2764.8
lo_lim =-2764.8

 

[dchartier]

Hello guys;

Temperature of 1622 would be read as 16220 decimal, which would be converted by the scale function to read 1622.0

Personnaly, I'll admit to being lazy, whenever I can get away with it.
IntToReal(16220) /10.0 = 1622.0
Works every time, no scaling....
Manmeet, just study the "Numerical representation of analog values" section of the document Rohan provided (S7-300 Module data pdf), especially the part on RTD inputs. You'll see how easy it can be.
Edit: just saw that the document provided covered just that section. For further reference, download the complete document from Siemens' website.
Use the scale function when you have to adapt two scales, one physical, the other 4-20 mA (for example). In this case it is not necessary.

Hope this helps,
Daniel Chartier

 

[manmeetvirdi]

Hi there
Thanks Rohan and Daniel
Well I got confused with the decimal part of explanation.
Now things are clear down to level of teaching other !!

No scaling required, now I know why.

Thanks
Manmeet.