[foxboro] OSV Settings on AIN Blocks
- From: "Jim Kahlden" <James.Kahlden@xxxxxxxx>
- To: <foxboro@xxxxxxxxxxxxx>
- Date: Fri, 05 May 2006 15:59:18 -0500
Just thought I would pass on what we found when setting the OSV on AIN =
blocks that use an SCI of 3 (4-20ma). Don't use a number greater than 2.5 =
for OSV if you want the to get a BADIO alarm if a transmitter fails above =
20ma. We changed the OSV on some of our blocks to 3 and some to 5 to stop =
BADIO alarms when transmitters drifted below 4 ma and this worked fine. =
The problem came when we had a transmitter fail on the high side and we =
had an OSV of 3 and never got the alarm. After doing some research we =
found out why. The blocks raw count was at max count of 65535, and the =
transmitter was at 22ma. When you do the math you will find that there =
are 3200 counts per milliamp and if you get a value of 22 ma you would =
expect a BADIO. However since we had set the OSV to 3 this is 1536 counts =
and that added to 64000 is 65536 which is one more than the max count that =
we had so no BADIO alarm. This was an indication only feedback of a valve =
which we did not have a process alarm assigned, only the BADIO. We had =
assumed the valve was going to 100% open when actually it was a false =
indication because we did not get the BADIO alarm. =20
Hope this helps someone else from running into the same problem we did if =
you plan to change the default OSV settings of 2.
Jim Kahlden
System Reliability Analyst
Lower Colorado River Authority
Fayette Power Project
6549 Power Plant Road
La Grange, TX 78945
(979) 249-8514 (Ph)
(979) 249-8390 (Fax)
jim.kahlden@xxxxxxxx
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