Lowest Analog Value (RS Logix 5000 v 20.04)

I would like to record the lowest hourly analog value and write it to a separate tag.

It's for a temperature transmitter on a sludge dryer. Is there an easy way to grab the lowest value?

Do you want the lowest value in the previous hour, or the lowest value in each one-hour period? i.e. do you want to be able to say "the lowest value in the 60 minutes prior to right now was xxx", or do you want to be able to say "the lowest value between 1am and 2am was xxx, between 2am and 3am was xxx..."?

Two very different approaches.

ASF, thank you for the reply.

I would like to be able to say: "the lowest value between 1am and 2am was xxx, between 2am and 3am was xxx..."?

Wonderware will pick up the tag I write in the PLC and I will export this to a Hach Wims report. The Hach Wims report will be a 24 report showing the lowest recorded temperature for each hour. This will allow us to prove that we make the required sludge temperature set point.

If you can use Function Blocks, there is a MINC and MAXC. Minimum and Maximum capture.

OG

First write the max possible value to the recording tag at the start of the hour, then do a compare if the process value is less than the recorded value and if it is write the lower process value to the recorded tag. At the end of the hour the recorded tag will have the lowest recorded value. If you only want the last 24 hours then have 24 tags, one for each hour and check the current hour to write to the proper tag.

If you want a log of values over a longer time then each hour increment a counter and use the accumulator as a pointer to the current tag to use.

This ought to do the trick. If you need to store multiple hours, you'd need to set up a FIFO register or array, but if your Wonderware is doing all of the historical data logging, then you have no need for that.

Cool! Thank you!