Needing to get total scfm flow for the day

Hi all, I am new to using a plc, other than taking 1 class in college on it. But I am needing to get a total scfm flow for the day from our ABPLC. We are using a micrologix 1769L32E processor. I have a 4-20 ma signal coming in showing a SCFM readout using ME. Any help would be greatly appricatied.

Thanks

Is this for class or is it for real?

If for class you would want to use a pretty short time interval over which to take an average reading.

IF for real then what do you estimate the daily (8,16,24 hour) total CF to be? What is your tolerance? How much variation is there? THese are needed to determine how often you take an average reading. Readings every second will be more accutate than those over sevaral minutes more accurate than average hourly readings.

You may wish to consider using a data logger which may be more appropriate if high precision and or high cost are considerations.

Dan Bentler

Is this for class or is it for real?

If for class you would want to use a pretty short time interval over which to take an average reading.

IF for real then what do you estimate the daily (8,16,24 hour) total CF to be? What is your tolerance? How much variation is there? THese are needed to determine how often you take an average reading. Readings every second will be more accutate than those over sevaral minutes more accurate than average hourly readings.

You may wish to consider using a data logger which may be more appropriate if high precision and or high cost are considerations.


Dan Bentler

Not for class

Thanks for the response Dan. No, this is not for class this is for our new plc system in our plant. I am wanting a 24 hour average. But it will have to be from the plc system as money is tight, so I got to use what we got.

thanks

But I am needing to get a total scfm flow for the day from our ABPLC.

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I am wanting a 24 hour average.

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SCFM normally means Standard Cubic Feet per Minute (Ft3/Minute). Getting a "total" for that doesn't have much meaning. Do you mean that you want to calculate the total Cubic Feet of flow for each 24-hour period? Do that as Dan said, pick a time period, read the SCFM at the expiration of each time period, multiply the reading by the time period in fractions of minutes, add that Cubic Feet of volume to a total volume, then repeat for each time period until 24 hours have passed.

Now to get an average for the PAST 24 hours, divide the total calculated above by some time period. The hourly average for the past 24 hours would be Total Cubic Feet / 24 = Average SCF per hour. The average per minute would be Total Cubic Feet / (24 x 60) = Average SCFM.

You can compute a "running average", or the average SCFM in the last 24 hour period, with oldest reading dropped off and newest reading added. Each minute you read the SCFM and add to a total, subtract off the oldest reading (or to avoid having to store 1440 values, just subtract the current average), then divide the total by 1440 minutes, and display running average SCFM.

Another way to compute a "running average", is to calculate the average SCFM in the last 24 hour period UP UNTIL NOW. You pick a 24-hour start time, then each minute read the SCFM and add to a total, divide total by elapsed minutes, and display running average SCFM. At end of period, the total is reset to zero and a new period starts.

Which way you pick to do the average depends on what is most useful.

The question I forgot to ask is
OK once you get this 24 hour value (whether average flow or total volume)
IS
what are you going to do with it??

Dan Bentler

Suggestion: Add a feature to test for sensor down

Unfortunately things break. You may wish to consider adding a feature to your code to check for no signal and a display of this information to the user. I could imagine the sensor failing or the power supply that drives it giving up the ghost.

Just my $.02's worth...

Thanks

Thanks guys for all the advice. Remember I am new at this though. I liked your last part about counting up. How though would I divide the total by elapsed minutes? That is probably a dumb question I am sure. Oh and this info is for kyepa. We have to keep track of total fuel use for each furnace running along with other units.

Thanks again I really appreciate the help.

How though would I divide the total by elapsed minutes?

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RSLogix has basic math functions, along with many others. Look under the RSLogix Help Menu, SLC Instrtuction Help. I would use the DIV (Divide) instruction to divide the total flow (stored in a memory location N7:xx) by the Timer Accumulated Value (ACC word). For example if you use N7:10 for the total and Timer T4:0 for the elapsed time, you would use the DIV instruction to divide N7:10 (Source A) by T4:0.ACC (Source B) .

DIV [Divide]
Rockwell Software 2000
Use with processorsAll SLC and MicroLogix processors Example of Instruction(Parameters shown are examples only, your data will vary.)

Description
When rung conditions are true, this output instruction divides Source A by Source B and stores the result in the destination and the math register. The value stored in the destination is rounded. The value stored in the math register consists of the unrounded quotient (placed in the most significant word) and the remainder (placed in the least significant word).

Source A and Source B can either be constant values or addresses that contain values, however Source A and Source B cannot both be constants.
Rockwell Software 2000

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Hi there
Go on adding the instant flow after every one minute, then after 24 hrs multiply the Sum total with 1440, you will get the amount of fuel consumed in 24hrs in cubic feet. Multiply/divide this number to get the volume in desired unit. Dont forget to reset the counter after 1440 counts !!