RSLogix 500 Flow Meter Totalizer Signet 2551

bearsgone

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Hello

I have ML 1400 with Signet 2551 flow meter. I am totalizing flow every day, however there is significant discrepancy about 2 thousand gallons a day PLC is under.

Meter is set up correctly: K factor and range 0 -70 GPM.

On PLC side:

FE103(Flow Meter) I have scaled for PID 60 Hz and 450 ms. It never shows as zero always 6 or 7. Is not it suppose to be zero when there is no flow?

I am off by 2-3k gallons every day. Average day is about 25k gallons, so it is significant.
F8:19 comes from another subroutine but it is also a flow meter scaled in a similar manner but different model, so called blind signet.

I was given spec of SCH 80 PVC pipe 2". According to the manual K factor is 206.69 which is used on signet panel itself for present flow and total flow. I am reading of signet panel into PLC. For 20 mA GPM manual shows 145.15. Sensitivity is 0. Even if I set SCP as: -819 to 16383, scale 0 - 145.15 gpm on PLC I am still under.


Not sure where the problem in my code or PLC scan time or the way I am going about scaling?



Any pointers would be appreciated
 
Just trying to understand your set up.
Do you have the 2551 set for Freq out or S3L?
Do you have the 2551 connected to a 8900?
What card is in the 8900? ex. 3-8900.401-xx
If you have the right card you can just read 4-20mA out
considering that you have analog input on you ml1400.
Lastly the 8900 has a lifetime flow totalizer along with a reset-able totalizer.
 
RE: MATT116's questions

I am using exp card 1762-IF4 Ser A Rev B for ML 1200

Do you have the 2551 set for Freq out or S3L?
Freq

Do you have the 2551 connected to a 8900?
Yes

What card is in the 8900? ex. 3-8900.401-xx
Do not know, how do I find out?

If you have the right card you can just read 4-20mA out
I believe that is what I am reading is 4-20mA. We connected amp meter and getting mA.

considering that you have analog input on you ml1400.
Correct, but ML's AIs are all digital, they wont read 4-20mA.

Lastly the 8900 has a lifetime flow totalizer along with a reset-able totalizer.
Correct, but I do not know how I can read the totalizer into PLC. If I could do that, it would be the ultimate solution


Thank you for your help
 
You have to pull the card out of the 8900 and there will b a sticker with the part number on it. You will have to look up the number to c if it has analog. If it does you may be able to assign the flow total to the output and read it in the plc
 
I honestly believe that the problem is in the code. Any suggestion where I am be going wrong?
There will be some discrepancy but I am off more then 10% which leads me to believe there is an issue with how I go about adding flow to a total. Maybe there is more efficient way
 
There might be other things, but I noticed your totalizer logic can be made much better. Keep in mind any timer in PLC/SLC is not accurate due to scan time issue. ie. A 10 second delay is only an approximation.

The "best" you can do in SLC is to look at the last scan time OR if the pulse == a unit of flow, total the pulse.

If all you can get is flow rate, totalize this way:

TOTAL = FLOW RATE * Time Elased since last scan

TOTAL = (GAL/MIN) * (MIN/60,000 ms) * S:35 + PREVIOUS TOTAL

where S:35 is scan time in milliseconds.
 
Last edited:
Yes, your logic looks overly complicated, but that's besides the point. The real problem that you are having is in the use of timers to perform totalization. You will never be accurate with a timer unless your PLC scan time is extremely fast. If accuracy is extremely important, which it appears to be, then you simply must put your totalizer logic within an STI subroutine as that is extremely accurate.

Although, the above logic by harryting using scan rates also appears very solid. I have never tested that method, but I have tested the STI method and it works flawlessly.
 
Thank you for the tip but I do not think formula is right

TOTAL = (GAL/MIN) * (MIN/60,000 ms) * S:35 + PREVIOUS TOTAL

say 10gpm * 60000 * S:35 + prev = becomes a big number fast

10gpm | 60000 * S:35 + pre is closer but is not right

am I missing something?
 
it's multiple by Minute over 60,000 ms, which is divide by 60,000. Just keep track of the unit; Minute and ms cancel out and the total is in Gallons.
 
I have IF4 card set to PID

Are you actually using this data for PID? Raw/Proportional would be a more accurate. The Scaling for PID is 0 - 16383 for 4 - 20 mA. -819 for less than 4 mA, and 17407 for 21 mA (RA Knowledgebase 19719).

Is it possible that you are exceeding the 70 gpm you are scaling to (Per your attached PDF)? If you exceed 70 gpm, it will still be totalized as 70 gpm, and the excess will be lost.

I agree with other statements that the code seems way too complicated just to run a simple totalizer.

Stu.....
 

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