How To Measure Total Dynamic Head With Gauges

WRITTEN BY:  Inyo Pools

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When pool water flows through the recirculation system, resistance to that flow is created in the pipes, the valves, the fittings, the pool filter and basically anything in the flow path. That resistance is called Total Dynamic Head, TDH, and it has a large impact on the determining the size of your pool pump and filter. Your pool's TDH was estimated when your pool are first installed. This guide shows you how to accurately measure the TDH of an existing pool system to verify that your pool pump and filter are correctly configured.

Things You'll Need

Step by Step

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Step 1

You will need to purchase two gauges: a pressure gauge to measure pressure in pounds per square inch (PSI), and a vacuum gauge to measure suction in inches of mercury (inches Hg). Both should have the standard 1/4" threaded end.

Step 2

These two gauges will be inserted into the two drain plug port at the base of your pump. The PSI gauge will be inserted on the pressure side of the pump usually under the impeller. The Vacuum gauge will be inserted on the suction side of the pump usually below the Strainer Basket.

Step 3

Before inserting these gauges be sure to TURN OFF ALL POWER TO THE POOL PUMP at the breaker box.

Step 4

RELIEVE THE PRESSURE in the system. To relieve pressure, screw the relief valve on your filter counter-clockwise. The reading on the pressure valve should drop to zero.

Step 5

REMOVE PUMP - Since the drain ports are too close to the ground to insert the gauges, you will have to take the pump out of the circulation line. This pump was installed with unions on either end so extraction is fairly simple. If your pump does not have unions, see if you can lift it slightly to get enough clearance for turning the gauges. Otherwise, you will have to cut the motor out and replace it later with couplings or add unions for the next time.

Step 6

REMOVE THE SIDE PLUG AND THREAD THE PSI GAUGE on the pressure side of the pump under the impeller. Gently tighten with an open end wrench.

Step 7

REMOVE THE FRONT PLUG AND THREAD THE VACUUM GAUGE on the suction side of the pump under the basket. Gently tighten with an open end wrench.

Step 8

REPLACE PUMP - Reinsert the pump. If you have unions, wrap 3 layers of Teflon tape around the threads before screwing them together.

Step 9

FILL THE STRAINER BASKET with water for a couple of minutes to reprime the pool pump and water lines.

Step 10

CLOSE THE COVER ON THE STRAINER BASKET. Use hand pressure to screw the cover tight.

Step 11

TURN THE POWER ON to the pool pump.

Step 12

CLOSE THE RELIEF VALVE - close the relief valve on the filter when water starts to spray out.

Step 13

READ GAUGES - Record the PSI and In Hg from the two gauges.

Step 14

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. Our readings were 15 and 23 respectively So (15 x 1.13) + (23 x 2.31) = 17 + 53 or 70 TDH.

Step 15

DETERMINE SYSTEM FLOW - With your measured TDH and your pump make and model you can now determine your system water flow in Gallons per Minute (GPM). Our pump is a Hayward 1 HP Super II EE pump. The chart at the left is an abbreviated version of the Performance data for this pump. For a 1 HP Super II EE pump and a TDH of 70, our pool system water flow is 60 GPM.

Step 16

DETERMINE IF GPM IS WITHIN ALLOWABLE LIMITS - The maximum recommended water velocity in swimming pool lines is eight feet per second for main suction lines (before the pump) and 10 feet per second for return lines (after the pump). See local regulations for your area. The table at the left shows the maximum flow in GPM based on pipe size and water velocity. The pipes for this pool are 1 1/2" so the maximim allowable flow on the suction side is 51 GPM; for the discharge side, 63 GPM. Our measured flow of 60 GPM is somewhat higher than the recommended suction side GPM but is within the recommended value for the return side. So this 1 HP pump is on the edge of being too big for this pool. However, if we considered installing the next smaller size motor, 3/4 HP, for a 38 GPM flow rate, it might not provide sufficient flow for the heater and suction cleaner. The better answer would have been to design this pool with 2" piping which is the way most pools are being built today.

Comments

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(1 to 11 of 11)

Cosme  Posted: 07/08/2017 13:15 PM 

Excellent article and easy to perform.
Trick:
To not dismantle the pump I had used suitable nipple.
Thanks!


Inyopools  Posted: 12/02/2016 10:16 AM  Inyo Product Specialist

Todd - You need to take the PSI reading at the pump port.


Inyopools  Posted: 12/02/2016 10:14 AM  Inyo Product Specialist

Dynamic Head - Like you say, I don't think this is a problem for the amount of time you will have the gauge attached.


Todd  Posted: 11/29/2016 12:03 PM 

For PSI, can I use the reading off the filter gauge or do I need to connect a separate gauge at the pump?


Anonymous  Posted: 11/29/2016 11:39 AM 

Hi. Thanks for this information. It is very useful. For the vacuum gauge, is there one that you recommend? I've been looking and can't seem to find one that will work for pool applications. I actually bought a SPAN 5ULR0 vacuum water/air gauge. However, on the box it states "WARNING: Do not use this gauge for applications involving strong oxidizing agents including (but not limited to) chlorine, nitric acid and hydrogen peroxide. These materials can combine explosively with the liquid filling which can cause property damage and personal injury. Misuse of product may cause explosion and personal injury. Read ANSI-B40-1 and apparatus installation/operation instructions before using." Um....should I be worried? I mean, it will not be permanently connected and will only be used for several minutes to record the gauge readings. Thanks in advance!


Inyopools  Posted: 07/05/2016 12:45 PM  Inyo Product Specialist

Teflon tape - You are correct. Teflon tape should not be used for sealing connectors that have O-rings. I've pulled this out of other guides but missed this one. Thanks for your observation.


Anonymous  Posted: 07/05/2016 11:49 AM 

In step 8 the teflon tape is being wrapped around the threads of a union. The union should seal by a gasket/O-ring and not by those threads; that could cause a suction leak. Sorry for nitpicking.


Inyopools  Posted: 04/29/2016 11:41 AM  Inyo Product Specialist

eb1 - I would use water pressure gauges since we are measuring water pressure.


eb1  Posted: 04/28/2016 10:57 AM 

I will attempt to measure my pool TDH using the gauges method that you have described. I will purchase the two required pressure/vacuum gauges but some manufacturers clearly spec their gauge unit as being water pressure only or air pressure only gauge. Which type should I purchase for your TDH measurement method?

shall I used air pressure gauges or water pressure gauges for


Inyopools  Posted: 03/07/2014 10:36 AM  Inyo Product Specialist

dell - Yes. This is a good idea. You should be able to find adapters at a hardware store to attach a short hose to the gauges and pump drain ports.


dell  Posted: 03/01/2014 14:44 PM 

since pumps are not always installed with unions, can a short hose be installed with a threaded by barb fitting on each gauge for easy testing?