Pentair VSF flow rate data Question?

[yoster]

The VF pumps use RPM and Watts to determine the flow rate. These three variables (RPM, GPM, Watts) are unique at each value so are fairly accurate (~10%).

The VSF pump does not have a flow meter. The flow is calculated based on the power the motor draws. It is an approximation at best.

I don't mean to bring up an old thread, but the question I was about to post is similar to this topic.

I want to somewhat challenge/question what was said above. I've noticed, using the Pentair VSF readout, that there is definitely not a clear relationship between GPM, RPM and Watts (in other words, I'm not confident it's using an equation based on RPM and Watts).

For instance, I have found that I can run something like 700 RPM at 35 watts, with a 25GPM output.

On the other hand, if I increase to 1000 RPM, watts go up to 80, however GPM is about 23-24 (likely some small turbulence).

This is a simple example, but the trend continues. I can also play around with my valves to some degree to adjust GPM, but with no real change in watts or RPM. If there wasn't a flow meter, and if watts and RPM weren't changing, there isn't a way it would know that GPM was getting 'more efficient' (flowing more) when I open up additional valves.

This tells me that the pump must have SOME form of flow meter?

What's also interesting is, I've found that, once you go below say 600 RPM, watts go back UP a bit (to about 120 watts) to maintain the same GPM. This tells me that the motor is now working 'harder' to maintain that low RPM vs its 'happy place' at say that 750rpm mark. Because GPM doesn't go up in relation to that 120 watt reading, this further tells me that an equation isn't being used.

Now THAT said, I DO have an iChlor30. Is there a chance that, when using that device, it references what that unit is saying, in place of the 'calculated gpm' referenced above? I'm not convinced this is the case as I'm fairly sure I witnessed the same GPM/Watt dynamic prior to installing the iChlor, but can't say with 100% certainty.

Actually now that I think of it, I don't think the iChlor matters. Just read a thread where someone has the same pump, turning 3000 RPMS, 1300 watts but showing only 40GPM. I run closer to 70GPM at that rpm/wattage. If it really WAS using a calculation, our GPM numbers would match 100%.

So the question remains, how is the Pentair VSF getting its GPM reading, and has anyone compared this to an external flow meter?

 

[mas985]

But yeah it just doesn't add up to me. If it's using a calculation, why do other members who post say, the same X RPM, Y Watts, not ALL get the same GPM? People post here all the time with different numbers and even see those GPM's dramatically improve after changing something in the system (without necessarily a change in RPM or Watts). So something just doesn't add up.

That really hasn't been my experience. Most of the data I have seen matches the models fairly closely. There are sometimes outliers but you can't automatically assume that is a failure of the pump GPM readout. There could be instability in water flow which results in the readouts quickly changing and the observer may not be very diligent in capturing this effect not to mention simply recording the numbers incorrectly. In controlled tests that I have performed and witnessed, you can model the pump power, RPM and GPM with fairly good accuracy for any VS pump. But this is exactly what would be expected from standard pump engineering equations.

 

[yoster]

To clarify I'm not referring to a failed part or questioning the values. Quite the opposite. My question is the source of the GPM information.

Another example. My sister.. identical pool equipment with the exception of the Ichlor30.

If we both manually set to the same RPM and get roughly the same wattage, we are a good 10gpm separate from each other. Another piece of evidence that an equation isn't how they're achieving GPM.. otherwise they'd match. So there's some piece to the puzzle that's missing.

Unless my original theory is indeed true.. it's using the ichlor's flow data if connected. And if it's not.. it falls back to a calculation. I need to play around with her system more to determine if this is true.

Regarding the tuning of various rpms to achieve the same or better GPM (1000rpm for 22gpm vs 700 for 25gpm) - I'll shoot a video of that tomorrow and post it. Again not saying it's broken or that any of these values are untrue.. saying it's smarter than I think we give it credit for, since flow will never be a factor of RPM alone, given head (restrictions, pipe size etc). So yeah.. perhaps it's taking that Ichlor flow data.

I'll make a video and post back tomorrow!

 

[DavidArmenB]

I have also noticed that the sweet spot for my VSF (in terms of decreasing rpm to decrease wattage) is 700rpm at 40w, and that below that rpm, the wattage starts to increase again for some reason. Also, I assume I have more head than you because I don’t get 20gpm until I am at 1000rpm and I am at 70gpm at 3,200rpm. Lastly, I have a properly installed independent flow meter so I have learned that the flow reading on my Intelliflo VSF is, on average, ~10gpm below my independent flow meter.

 

[mas985]

If we both manually set to the same RPM and get roughly the same wattage, we are a good 10gpm separate from each other. Another piece of evidence that an equation isn't how they're achieving GPM.. otherwise they'd match. So there's some piece to the puzzle that's missing.

Roughly is not good enough. GPM changes much much much faster than wattage. A small difference in wattage results in a huge difference in GPM so you cannot be "loose" with the numbers. If they are not EXACTLY the same, you cannot compare.

Also, there are several Intelliflo models. Some have different drives than others which will result in a different calibrations and therefore different RPM,GPM,Watt mappings. So you cannot always directly compare pumps readings directly.

Unless my original theory is indeed true.. it's using the ichlor's flow data if connected. And if it's not.. it falls back to a calculation. I need to play around with her system more to determine if this is true.

As far as I know, iChlor does not mesure flow rate. It determines if there is flow rate or not. A big difference. Also, the Intelliflo will report flow rate with or without an iChlor. This we know.

since flow will never be a factor of RPM alone, given head (restrictions, pipe size etc).

Actually, it can be. RPM and GPM are directly proportional for a constant plumbing curve. This is a reflection of the pump affinity equations:

• GPM B = GPM A * (RPM B / RPM A)
• Head B = Head A * (RPM B / RPM A) ^ 2
• Hydraulic HP B = Hydraulic HP A * (RPM B / RPM A) ^ 3

And these same equations are a basis for the relationship between GPM, RPM and Watts. Again, well established pump engineering. Nothing new.

 

[mas985]

I have also noticed that the sweet spot for my VSF (in terms of decreasing rpm to decrease wattage) is 700rpm at 40w, and that below that rpm, the wattage starts to increase again for some reason. Also, I assume I have more head than you because I don’t get 20gpm until I am at 1000rpm and I am at 70gpm at 3,200rpm. Lastly, I have a properly installed independent flow meter so I have learned that the flow reading on my Intelliflo VSF is, on average, ~10gpm below my independent flow meter.

What kind of flow meter do you have? Blue Whites are notoriously inaccurate. Also, the difference should be dependent on RPM so what RPM results in a difference of 10 GPM? All flow meters get more inaccurate with lower flow rates too.

 

[yoster]

When I say roughly the same I'm talking a single watt or two. I just can't believe that 1-2 watts can explain a 10gpm offset. I will check again though this weekend just to be 100% sure.

Pumps are 100% identical. Purchased together at the same time and installed within 30 days of each other.

As far as I know, iChlor does not mesure flow rate. It determines if there is flow rate or not. A big difference. Also, the Intelliflo will report flow rate with or without an iChlor. This we know.

Right - the point I was trying to make was that it had two sources to reference, but you raise a good point about the iChlor. Just brought up the part and it says "switch" right in the name, so, so much for that idea.

@DavidArmenB - Glad someone else is experiencing the same thing lol. This gives me a new thought/idea. Curious if rather than an equation, they're modeling based on known/tested data. RPM and Watts still play a part, but isn't a "plug and play" equation, but rather used in reference to a data table.

 

[JamesW]

Note that the gray range is labeled as the Operating Range for Flow Control.

 

[JamesW]

iChlor30

The ichlor uses a simple On/Off Flow switch that has no flow metering capability.

 

[JamesW]

The pump has input power and output power.

The display in watts is probably the input power.

The output power is the three phase voltage and current going to the motor from the variable frequency drive.

The motor might be calculating the GPM from the output power and not the input power.

You might have the same input power, but possibly different output powers depending on how the power is calculated and if the efficiency or power factors are different.

 

[yoster]

This is true - edited post above to reflect that. New idea/thought included.

 

[yoster]

The pump has input power and output power.

The display in watts is probably the input power.

The output power is the three phase voltage and current going to the motor from the variable frequency drive.

The motor might be calculating the GPM from the output power and not the input power.

You might have the same input power, but possibly different output powers depending on how the power is calculated and if the efficiency or power factors are different.

That's an interesting thought. Would not the two rise together though? What would explain a situation where we drop RPM, and the 'read' watts (whatever they show on the screen) increase, but GPM decreases? This is where I think they might be using lookup tables.

 

[mas985]

@DavidArmenB - Glad someone else is experiencing the same thing lol. This gives me a new thought/idea. Curious if rather than an equation, they're modeling based on known/tested data. RPM and Watts still play a part, but isn't a "plug and play" equation, but rather used in reference to a data table.

Yes, more than likely it is a look up table rather than an equation implemented within the pump. I use an equation in my pump spreadsheets as it works just as well and you can easily invert the equation to solve for any of the three parameters.

But in general, inconsistencies in measurements usually comes down to measurement accuracy. If you want to get to the root of the issue, you need to take accurate readings from stable setups and also include the accuracy of the measurement device. Also, with a VSF pump, I would do all measurements in RPM mode rather than GPM mode. In GPM mode, the pump must hunt for the correct RPM to get the GPM setting so it may take some time to settle to the correct value.

 

[JamesW]

There are no sensors like pressure gauges that detect flow by any means so it has to be from the RPM and power usage.

The power factor might be different or the efficiency might be different.

You would have to compare two systems that were exactly identical with the same suction pressure (Vacuum head loss) and the same discharge pressure.

https://www.horizonpoolsupply.com/_uls/resources/IntelliFlo_VF_Manual.pdf

 

[yoster]

Yes I've found GPM mode to be less than efficient. For instance, I can get (according to the device) 25gpm at 700 RPM.. but.. if I TELL it to achieve 25GPM on its own.. it asks for 1000 RPM (and as a result, higher wattage).

That's actually what started me down this rabbit hole. To answer the question, how is it determining this, the fact that lower RPM and lower watts would result in higher GPM in some cases. A straight equation wasn't answering it, which led me to perhaps the iChlor, but now I'm leaning on the idea of tables.

 

[mas985]

Forgot to mention that no two pools are alike so it would be very unusually to get the same wattage for the same RPM on the two pools. The pools would literally have to have exactly the same plumbing setups.

 

[JamesW]

Check the power, gpm, rpm, system pressure and filter pressure at several speeds for both pools and post the numbers.