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 22 of 22)

Chris - Pentair Intelliflo 2 VST  Posted: 8/28/2018 

A flow gauge? Ok...I thought the gauges in this article were the recommended way to measure flow? I'd of just purchased the flow gauge instead. Thanks,
 Reply

InyoPools  Posted: 8/27/2018 

Hello Chris - The head changes proportionally to the speed squared. Let's say you reduce the speed by 50%. This means that the head will be 25% of the full speed head. However, we would recommend installing a flow gauge on your system. A flow gauge is easy to install and gives you the most accurate reading.
 Reply

Pentair Intelliflo 2 VST  Posted: 8/27/2018 

I just put in the two gauges as described, but I am still having trouble determining the GPM. I am getting different TDH for various RPMs I set the pump for: The second reading I am posting for VAC and PSI is after the multiplier. RPM: 3450, VAC: 29 (33), PSI: 21 (49), TDH: 81 RPM: 2350, VAC: 16 (18), PSI: 13 (30), TDH: 48 RPM: 1500, VAC 7 (8), PSI: 8 (19), TDH: 26 Q1: Shouldn't TDH be constant across all RPMs? Q2: I'm not clear how to read Pentair's Performance Chart for the Intelliflo 2 VST. For instance, the TDH values for 2350 and 1500 RPMs do not intersect with the 2350 and 1500 performance curve lines. Unfortunately, Pentair doesn't seem to publish an easy chart like that of which you illustrated above from Hayward. Perhaps I am doing something wrong or misunderstanding how I should be reading the chart. Thank you.
 Reply

InyoPools  Posted: 8/16/2018 

Hello Danny - The two drain plugs are located on the lower left side of the pump housing if you are looking at it from the front of the strainer. The plug closest to the front would be used for the vacuum gauge. The one further back would be used for the pressure gauge.
 Reply

InyoPools  Posted: 8/15/2018 

Hello Danny - Yes, a clean filter is ideal if you want the most accurate reading.
 Reply

Danny  Posted: 8/10/2018 

Won't the cleanliness of the filter also affect the different pressures? Seems like any obstruction could skew the readings. Perhaps one value when the filter is clean and another before the filter is ready to be cleaned?
 Reply

Danny  Posted: 8/8/2018 

Where are the ports located on the Hayward EcoStar to test the TDH with gauges? I can see a pair below the strainer, but not sure where the other port is at.
 Reply

Inyopools  Posted: 5/8/2018 

For the gauge method to work well, it's best to run the pump on high speed. The feet of head will change with the speed. The head changes proportionally to the speed squared. Let's say you reduce the speed by 50%. This means that the head will be 25% of the full speed head.
 Reply

AJB  Posted: 5/8/2018 

Wouldn't the speed you were running your pump at effect the #s? Should I run my pump at full power when I am taking these readings?
 Reply

Inyopools  Posted: 3/26/2018 

Hello JD - As long as the filter is clean, the filter gauge should give you a good enough reading to get you in the ballpark.
 Reply

JD  Posted: 3/26/2018 

The pump I wish to test is a Astral Maxim 5 hp it only has a drain plug on the suction side. in this case can I use the pressure reading from the sand filter?
 Reply

Cosme  Posted: 7/8/2017 

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

InyoPools Product Specialist Dennis R.  Posted: 12/2/2016 

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

InyoPools Product Specialist Dennis R.  Posted: 12/2/2016 

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

Todd  Posted: 11/29/2016 

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

Anonymous  Posted: 11/29/2016 

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!
 Reply

InyoPools Product Specialist Dennis R.  Posted: 7/5/2016 

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.
 Reply

Anonymous  Posted: 7/5/2016 

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.
 Reply

InyoPools Product Specialist Dennis R.  Posted: 4/29/2016 

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

eb1  Posted: 4/28/2016 

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
 Reply

InyoPools Product Specialist Dennis R.  Posted: 3/7/2014 

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.
 Reply

dell  Posted: 3/1/2014 

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?
 Reply