King Flowmeter

Measuring waterflow rates – Joe

SUMMARY: Adding tubing runs and watercooling components can substantially impact waterpump flowrates.


One of the pieces of test gear to measure waterblock performance is a liquid flowmeter – this measures flow through a system, in this case, in gallons per minute (gpm). The flowmeter I bought is made by King Instruments¹ and measures flows from 0 to 3 gpm – just the range I’m interested in.

This is an analog instrument that basically is a tapered tube with a stainless steel float that is pushed up by water pressure – the more pressure, the higher the gpm – simple concept, lots or precision manufacturing to do it accurately. The stainless steel float


looks like a mushroom. Once it stabilizes, you read the gpm off the sight glass. I found it quite easy and use a ruler for reading between the markings on the scale.

To give an example of what I can use this for, I tried a number of waterpumps to familiarize myself with using it. What follows is what I found for an EHEIM 1250 waterpump using combinations of tubing size, radiator, CPU and GPU waterblocks (just measuring flow – not pressure).


Note the GPU waterblock uses a “swirl” design.

The EHEIM 1250 is rated for 317 gph, or 5.28 gpm, with a delivery head of 6’7″. It features a ¾” intake and ½” on the output side. As soon as you hook up hoses etc., flow rates² drop quickly:

Flow Test Measurements – EHEIM 1250



% of Spec

Spec Rating



1/2″ Tubing



3/8″ Tubing



Waterblock Alone



Radiator Alone



GPU Waterblock Alone



WB + Rad



WB + Rad + Gpu



Note: 3/8″ tubing was used with radiator and waterblocks, which feature 3/8″ nipples.

Obviously this is only for this setup and does not address pressure, but I think it’s representative of typical watercooling systems many of our readers are using. Whatever waterpump you have, figure at a minimum actual waterflow through your system is something like 15 – 30% of the pump’s rating with just a radiator and waterblock.

The more that’s included in the cooling loop, including GPU waterblocks and 90 degree bends, the more it reduces waterflow. In addition, the smaller the tubing diameter, the lower the flow. Waterblocks, GPUs and radiators with anything below 3/8″ fittings are not conducive to high flow rates.


Overall, the larger the diameter, the better. However, I feel that anything over 1/2″ tubing is difficult to work and fit, but anything below 3/8″ ID will severely compromise flow rates. Placing a small diameter GPU in a large diameter cooling loop will slow things down appreciably.

¹King Instrument Company – Stainless Steel, Liquid Flowmeter. It has 1″ Female NPT connections, both in and out, and has viewing windows on two sides. The overall length is 19-1/2″ long, with the flowtube box, 14-1/2″ long x 4-1/2″ square. The viewing windows are 2-1/2″ x 11″, and the scale for the flowmeter, is 0-3.0 GPM, marked in 0.05 GPM divisions. The float is # 3/4-GPV-S2 for a liquid with a specific gravity of 1.0 (water).

²I should also note that the instrument itself will impact flow rates (5″ H2O).

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