Adjusting Vacuum Reading for Gauge Position

• There is no practical reason to go below 10mm of water gauge precision.
• A pressure gauge is only accurate to a few percent.

How do these concepts affect the sensitivity of the calculations above? Do they remove the need for the calculations?

Thank you for your comment on precision.

As a general rule, I round off the numbers to the required precision after the calculations are done because I have found that if I round off the numbers first, before doing a calculation, then the result can, on occasion, be different than if I had used the actual number in the calculation and is only the same if I reduce the precision, sometimes to less precision than required. Well, OK, but then what is the required precision?

The pump manufacturer supplied performance specifications with a precision of +/- 0.05 meters H2O; i.e. the TDH at the duty point is 21.4mH2O so I interpret this to mean observed performance should be between 21.35 and 21.45mH2O

This would be about 0.05/21.4*100 = 1/4%

Yes, then, accuracy of the reading from a general use gauge, with a precision of say, 2%, will be less than this specified precision.

With regard to the effect of the gauge not being at the center line of the pump; however, the physical distance can be measured fairly precisely. In this case, the 8 inches would be 8" +/- 0.25 inches so the pressure due to 8"H2O would be 0.2032 meters +/- 0.0064 meters. This precision is more precise than the 0.05 meter manufacturer's specification precision.

[I just noted that in the original question I mistakenly said "So far I got -8"Hg = -0.2032 meters...." What I was referring to was than the gauge that read in "Hg was 8" above the centerline so I thought I would need to subtract 8" of water pressure, i.e. add a negative number -8"H2O = -0.2032mH2O pressure to the gauge reading.]

So to review.

The effect of having the inlet gauge 8" higher than the center line of the pump will change the reading of the inlet (suction) gauge by 8"H2O or 0.2mH2O [ I still need to learn if this will increase or decrease the measured value by 0.2mH2O ]

The measurement that I got from the inlet gauge was -6"Hg = -2.072mH2O

Hmm. It looks like the position of the gauge has no significant impact on the Inlet Head measurement. i.e. If I measure the inlet (suction) pressure to be -2.072 m H2O and I round this to -2.1m then adding or subtracting .0064 meters from this, would not change or adjust the reading of -2.1m

Since Total Head is what I am trying to measure, then the vertical distance between the inlet and discharge gauges is perhaps more important than the distance from pump center line to a particular gauge. Here 14" - 8" = 6" or 0.1524 mH2O

Again, perhaps I can conclude that if the inlet and discharge gauges are less than 8" or 0.2m vertical distance apart then no adjustment is needed because the adjustment would not change the calculated Total Head. If gauges are say 60" or 1.5m vertical distance apart then an adjustment for gauge position would be warrented.

Why would you expect pressure on the suction side of a pump?

Elevation = head, it's that simple.

As PWS says, I think you are after too much precision. I also wonder how much confusion you will introduce by changing the units everyone is used to working with now, to something new.

If the gauge is above the pump you should subtract the water column (pressure) in the sensing lines from the absolute pressure indication. If the suction gauge had air in it when installed the air will expand as pressure decreases, thus displacing water in the sensing line and decreasing the head effect to the indication. Do not mistakenly presume that the gauge sensing lines are full of water.

Pls try to Install a pressure gauge, say 10 mtrs above the existing Pressure gauge in discharge.. As per your THEORY, the upper gauge should show a higher pressure by about 14.7 psi.....(1atm= 14.7 psi = 10 mtrs of Water Head= 30 in of Hg)

I think this will not happen..Water is not static, its moving. Pls inform the feed back.

The most effective way to "correctly" measure pump pressures (or any other multiple-source system) is to install ONE very high quality gauge on a header with ALL of the measuring points connected to the header with ball or plug valve isolation. Mount the header and gauge anywhere above ALL of the measurement points and be sure that all lines are "flooded".

Just open one valve at a time and record the reading on a simple chart. Include appropriate "blank" spaces on the chart to note various differences that will be used to compute performance.

We all have missed the point and the question it seems..

Inlet = -6 inches of Hg = -81.6 inches of Water

Outlet = 35.5 psi = 983.35 inches of water

Diff in Elevation of Suction and Discharge = 14 - 6 = 8 inches

Head developed by Pump = 983.35 - (-81.6)+8 = 1072.95 INCHES OF WATER = 38.6 PSI = 27.25 Meters of Water.

Hope it satisfies yr querries.

Pls verify the conversion factors before you conclude.

Sweet!