Variable Flow Problem

Thought this might be a fun problem to post here.

I want to create a pump system that will give me 5 gallons. I am using a flow meter to do this. Essentially I want to measure out exactly 5 gallons from this flow meter. My issue is how to calculate exactly 5 gallons accurately and quickly. I might be thinking about it too much but I feel like its not quite as simple as it seems. I have a few thoughts on how to do this but thought I would toss this out there to see if anyone has any ideas or something they have implemented in the past.

Just trying to keep it simple and effective.

Out of what? 10" water main or a 1/4" poly tube?

Into what? pressurized distribution line or a ten gallon pail sitting on an electronic scale?

I'm being facetious, but we need more detail.

Good question OkiePC.

We are pumping from a 60psi diaphram pump (or should i say pumping at 40-60psi).

The tubing at the moment is 3/8" pvc but I think we plan on bumping that up to 1/2".


We are using clamp on flow meters from Keyence, FD-Q series, I think we plan on getting their newer ones soon as well.

We are pumping into anything between a 32 oz container to a 5 gallon jug. I will have to manually choose the size.

Assuming your flow meter is calibrated to what ever units you want i.e. if it was litres but you wanted gallons then just scale it. or if it was tenths of a gallon then divide it by ten just count the pulses.
If the flow meter is just a non intelligent type like a paddle then the unit probably has a sheet stating how many pulses per what ever "K" Factor. I have used a number of paddle types and the logic is pretty simple.
Before starting the dose, reset the counter or whatever. to 0, run the system and do the maths i.e. if it was 56.4 pulses per litre then convert the pulse count to a float, divide it by 56.4 & hey presto you have your litres.
I have done a number of these for small dosing systems and incorporated a calibration routine, basically you zero the count, dose water into a vessel, weigh the vessel input the actual weight (1 Litre = 1kg). enter the litres measured & calculate the "K" factor and store it.
Not giving us any information on type of flow meter, whether it is pulse or flow (flow is difficult for batch) is it intelligent or is it just pulses given per data sheet. Intelligent ones like E&H give a pulse for a given amount set by you in it's configuration.
Oh.. Just noticed you have posted some details as I was writing this, are you using pulse train or coms etc.

What is the control mechanism? Start and stop the pump? Or are there also valves involved?

In either case, there is probably going to be a "pre-act" offset, because there will still be some liquid moved after you tell it to stop.

I worked on some batch controls similar to what Parky describes. We did a "calibration" into a bucket and then let the operator enter the weighed result into the HMI. That info went into the math for the offset. Ours was totalizing from a 4-20mA input but the priniciple is the same.

If you want the flowmeter display to match up with what your controller says happened, then pulses will be more precise since there won't be analog scaling variations at both ends to worry about.

Here is a link to the flow meter we are using. https://www.keyence.com/products/process/flow/fd-q/specs/

I think i was thinking along the same lines though. I will need to account for the pulse readings regardless to get an accurate reading.

For you curious folks out there, this is going to be used on a sanitizer production line that weve been using to donate santizer.

Just a thought, you have not said what you are going to control this with, also diaphragm pumps are not recommended for flow meters, greater errors.

Ah Sorry of course! I will be using a Click PLC and ya we are going to make a more refined system but I was told to use what we have and do the quick and dirty. Since were donating we dont need to meet weights and measures just quite yet.

Depending what you intend to use, if pulses then for accuracy the more pulses per gallon or what ever the greater the accuracy, you may need a high speed counting system for example if a PLC then one with high speed inputs or a counter card.

I believe you could probably set the flow meter to the number of pulses per gallon or at least 1 pulse per 0.1 gallon so if a gallon only takes 10 seconds to dispense then it's only 1 pulse per second so a standard input card & scan time could read a lot, lot faster than that. If you do not have an HMI then you could calibrate it using fixed K factor, or a retentive memory to store the factor.
And adjust it via programmer.

I would use the pulse output to high speed counter input
with Rockwell the counter modules have programmable outputs you can set up to control the valve.
don't forget the pre act
as mentioned before diaphragm pumps are a poor choice to control flow
if 5 gal is the max you will ever dispense I would look at setting up a 10 gal reservoir
have pump fill it and dispense from it with the flow meter and valve on the output
you will get better results

From that flowmeter page, regarding repeatability:

*4 This specification is valid when the flow velocity distribution is stable. This value does not take into account the effects of pulsation or fluctuations in flow velocity distribution due to facility factors. Convert the F.S. (full scale) listed in the table according to the rated flow range.

Click to expand...

You are using a diaphragm pump, or maybe it has a pulsation dampener?


As someone else said, if assuming the system characteristic does not change much over time (say one calibration per day), I might calibrate a simple timed application: if the calibration determines that 123.456s from pump on to pump off yields 5gal, then you create a rung with a TON running the pump on/off switch, which is triggered by a start bit, which is ORed with the pump_on bit for a seal-in, and canceled by the .DN (done) bit.


Then you are not fiddling around with second-order effects of pulsation, variability of scan time, non-linear events at on/off, the flow meter and scaling. Instead you make the system characteristic your flow meter. One calibration per day, or week, involving a bucket and a weighing device, could be described in a one page procedure, or re-calbration could even be integrated into the PLC:

1. hook up weigh scale output to PLC
2. put empty bucket (that's Bouquet) on scals
3. press calibrate
   • PLC runs pump for 10s, 20s, 30s,
     • with 15s in between each run
   • PLC weighs result at 10s after each off
   • assesses linearity of result
   • predicts and sets timer preset for 5gal, if linearity is acceptable.
   • issues "calibration complete" or "calibration failed" result

I know some (including me) will say this violates KISS, and if a calibration says the second-order effects are small and counting pulses or summing per-scan ([flow measurement] times [scan time]) values is accurate enough, then certainly go that route as a first pass. But if not, and you end up adding offsets and other compensations to that model, then calibrating the system as whole to solve for a timer preset probably is KISS.



OT but somewhat relevant/another drbitboy's dad story:

My dad was the GE Large Steam Turbine representative for a throat tap nozzle calibration at the Cornell University hydraulics lab




(which was very good because its head came from Beebe Lake and so provided stable flow over each calibration run as opposed to pumps at other institutions' labs that were not steady, but was eventually bad because that available head was fixed and limited the maximum flow rate as turbines, and nozzles to be calibrated, got bigger).

Dad shows up and Professor (forgot his name) shows him the multi-sheet calibration procedure. My dad asks "What's this? A flow calibration should be a one-page calculation." Professor says "Well, we have a leaky weigh tank, and we calibrate the leak on each run."


So what I am saying is "calibrate your system as a whole and then running it is simple."

If you count pulses, then you don't even need the PLC: the FD-Q has an integrating mode. How's that for KISS?

Glad post #13 was after previous, the OP already said he is using a flow meter not best guess on a PLC timer. Assume the pump is air operated then if the air supply drops will be a bit of a short 5 gallons. I just looked at the flowmeter & that is correct Only a quick look but I bet it has batch control.

These are all great ideas! I have looked at the pulse option, I think that would be better with a constant flow rate. The kicker i forgot to mention is that I am eventually going to have 4+ of these puppies connected in parallel. And when one shuts off the others are going to get a major boost in flow rate.