Help with TDH calc.

[L&D's pool]

Hey guys, I am again left scratching my head. Not uncommon. I am trying to generate a TDH in feet based off the readings from my pressure and vacuum gauges. But the number I am getting is no where near the performance curve (to figure out my flow rate) so I must have done something wrong. I am using the calculation: CALCULATE TDH - Multiple the Vacuum gauge reading x 1.13. Multiply the Pressure gauge reading x 2.31. Add the two products together to determine TDH for your system.
It is possible I am using the wrong numbers on my gauges as there are two set of numbers (one reading is bar and the other psi) so perhaps that is where I am messing up? Here are some pictures to show the readings I have at 1500 RPM. I plan on doing all of the RPM curves listed for my pump but if the first time I did this I got an odd number I am wondering where I have gone wrong. I used basically 8 and 8 for the vacuum and pressure gauge readings. That brings me to roughly 27. So on my gauges do I use the black number reading or red? The pressure gauge is the one with the black housing and the vacuum is silver. Thank you for the input!!!!
-Luke

 

[Jimrahbe]

Luke,

Let's see what Mark has to say... Calling @mas985

But... I have to ask, why do you care what the flow is???

Thanks,

Jim R.

 

[HermanTX]

CALCULATE TDH - Multiple the Vacuum gauge reading x 1.13. Multiply the Pressure gauge reading x 2.31. Add the two products together to determine TDH for your system.

Pressure gauge should be read in PSI and Vacuum gauge should be read in Hg

 

[PoolStored]

8.2PSI = ~17inHG
17 x 1.13 = 19.21
8.2 x 2.31 = 18.94

19.21 + 18.94 = 38.15

 

[JamesW]

This is in the wrong spot.

It is showing pressure and not vacuum.

 

[JamesW]

What pump is this?

The red is Bar.

The black above 0 is psi.

The black below 0 is inches of mercury.

The vacuum gauge goes in the front drain port.

 

[JamesW]

 

[L&D's pool]

Wow! Thank you all for the replies! I knew I was doing something wrong. I am hoping to get what flow rates I have at what RPM the pump is running so I can program it for optimal efficiency turning the pool over. I finally upgraded to a VS pump as my electric bill is insane. If I can get proper turn over and use less RPMs in the process that's awesome.
So the Vacuum gauge is hooked directly to the pump vacuum port. Is there another spot on the pump it should be? I am definitely confused there.
With a TDH calculation of 38.15 I am so far away from the 1500 RPM curve that I am not sure what to think? lol I guess as suggested with the first reply, maybe I shouldn't get too wrapped up in this as I have been running the pump for a month and I have no algae problems and I am using much less electricity that I was before with my 1.5hp single speed! I should be happy right? I guess I just like it when things make sense. Ugh.

 

[JamesW]

Can you show the whole pump?

What pump do you have?

 

[L&D's pool]

Thank you James! This is a Dohenys 3hp VS pump. I figured I would give it a try as it was $650. It seems to be running awesome and I really like it so far. Maybe if it craps out in 2 years I will wish I went with Hayward or Pentair but so far so good!

 

[JamesW]

So the Vacuum gauge is hooked directly to the pump vacuum port.

The vacuum port is the front drain port.

 

[L&D's pool]

I can take a pic of the whole pump when I get home.

 

[ajw22]

optimal efficiency turning the pool over.

Pools do not need a certain amount of turnovers. That is a myth and has no impact on water chemistry or sanitation.

You get optimal efficiency by not worrying about turnover and running your pump to adequately filter and skim your water.

Turnover of Pool Water - Further Reading

www.troublefreepool.com

 

[mas985]

Is this on the filter? If so, you will need to do some corrections for the head loss between the pump and filter plus the height of the filter relative to the pump.

Also, gauges tend to get more inaccurate at lower pressures so I would do all your measurements at full speed and then scale flow rate to lower speeds.

The spreadsheets in my signature can help with all of this or just post the following information:

Height of the filter gauge with respects to the pump and to the pool water level.

Plumbing size between the filter and pump. Post a picture as well.

But don't do this for turnover. It is meaningless.

 

[JamesW]

The front is vacuum and the side is pressure.

Your gauge is in the side pressure port.

 

[Jimrahbe]

You get optimal efficiency by not worrying about turnover and running your pump to adequately filter and skim your water.

@ajw22

+1 to that quote Allen...

Thanks,

Jim R.

 

[L&D's pool]

Wow....great info! I will double check my Vacuum gauge location when I get home.
mas985, yes that pic is on the sidemount multiport valve to the sand filter. It sounds like I don't need to worry about turn over but I am left to wonder what "adequately filter and skim your water" is? It seems like I have good flow out of the two jets at speeds above 1700 rpm. 2200 and it's really pushing out the jets and sucking at the intakes like a.....oh nevermind. Water is clean, chemistry is set....maybe I should just leave it alone. lol I wish my system was 2 inch pipe all around but it is mostly 1.5. I really can't change a lot of it because this pool is built into the side of a hill so the pump/filter is several feet under the waterline. The pipes run under concrete and underground in most places. I have no clue how old much of the system is as it came with the house.
You know James....that makes a lot of sense my vacuum gauge is on the pressure port on the pump because it is essentially reading the same numbers as the pressure gauge on the filter.

 

[ajw22]

Water is clean, chemistry is set....maybe I should just leave it alone. lol

You got it! That is all it takes.

I wish my system was 2 inch pipe all around but it is mostly 1.5. I really can't change a lot of it because this pool is built into the side of a hill so the pump/filter is several feet under the waterline. The pipes run under concrete and underground in most places.

It would not make a big difference with the low and slow speed operation of modern VS pumps.

It made more of a difference in the days of high HP single speed pumps always running at max RPM and HP.

 

[L&D's pool]

Thank you guys!

 

[mas985]

Wow....great info! I will double check my Vacuum gauge location when I get home.
mas985, yes that pic is on the sidemount multiport valve to the sand filter. It sounds like I don't need to worry about turn over but I am left to wonder what "adequately filter and skim your water" is?

Make sure water flows over the skimmer weir door rather than around the edges.

It seems like I have good flow out of the two jets at speeds above 1700 rpm. 2200 and it's really pushing out the jets and sucking at the intakes like a.....oh nevermind.

Generally speaking, that usually means the flow rate is much more than what really need. First I would lower RPM until water no longer goes over the top of the weir door. Then bump up the RPM by about 10% to account for the filter getting dirty.

Water is clean, chemistry is set....maybe I should just leave it alone. lol I wish my system was 2 inch pipe all around but it is mostly 1.5. I really can't change a lot of it because this pool is built into the side of a hill so the pump/filter is several feet under the waterline. The pipes run under concrete and underground in most places. I have no clue how old much of the system is as it came with the house.
You know James....that makes a lot of sense my vacuum gauge is on the pressure port on the pump because it is essentially reading the same numbers as the pressure gauge on the filter.