Water Pressure

Brooke Clarke 2012

House Water Pressure Regulator Water Pressure OP6638 Analog Guage
                Oil Pressure Sensor
                  P2A Watter Pressure Gauge

    Resistance Measurements
Rain Bird P2A Water Pressure Guage with 3/4" Male Garden Hose fitting


Every now and then I hear a toilet filling for some number of seconds.  There are a couple of reasons why that might happen.  Maybe there's a slow leak and depending on the amount of hysteriesis between the stop filling level and the start filling level on the float the toilet is topping up.  Maybe the water company has a large variation in the pressure that's feeding my house and even after the pressure regulator (Fig 2) there's still a consideerable amount of variation in the pressure inside the house.

So I'd like to monitor the water pressure.  But there's no off the shelf solution for measuring your water pressure so on this page I'll document what I'm trying.


The first idea is to use the oil pressure sender for a motor vehicle.  It turns out that there are two kinds.  One is a switch that drives the red OIL light and that will not work for this application.  The other is a sender for an oil pressure guage and that's what I got.
The box is marked:
Switch/Commuateur Interruptor
UPC: 7   07390 84219   6

Ant the sensor is marked:
U.S. Pat 4079351

This sensor (and I suspect most of them) has 1/8" NPT threads.  There are a number of places that the sensor might be connected.  The first thought was on an outside hose bib, but that would mean everything would be exposed to rain and large temperature variations.  Some sink faucets have male threads (kitchen) or female threads (bathroom) so that was a possiblity, but I didn't want to give up the kitchen sink and getting the correct male threaded adapter for a bathroom sink may be a problem.  Since there is a spare shower that seemed to be the logical place to install the sensor.  The shower has a male 1/2" NPT fitting so all that's needed is a Reducing Coupling 1/2" x 1/8" NPT (UPC: 0   48314 18031   5).
Some Teflon tape was used on both joints.  See Fig 1.

Fig 1
              Pressure OP6638 Analog Guage Oil Pressure Sensor
Fig 2 Pressure Regulator
House Water
              Pressure Regulator

Resistance Measurements

Using the Fluke model 87 Digital Multimeter in MIN-MAX Resistance mode will allow measuring the pressure variation.  It's starting out at 35.2 Ohms.
At this time I don't know how to convert from Ohms to PSI, but using an analog meter on another location should solve that.
1 June 2012 - Max 89.9, Min 31.4 average 38.3 Ohms.

Bought a new "Watts" model IWTG made in China pressure gauge from Home Depot (UPC: 0  98268 15995  0) that fits a hose bib and the dial is marked 0 to 200 PSI.  It measured 100 PSI even when the pressure regulator screw was all the way out or all the way in.  At first thought the pressure regulator was bad, but then noticed the the water pressure in the house was very high.  The black needle moves from 0 up to 100 and back smoothly so the problem seems to be a design defect rather than a calibration problem.  The red tattle tale high pressure pointer does not have enough friction to hold it's position so is of no use.

Backing off the regulator screw lowered the pressure.  After doing all that the readings are:
2 Jun 2012: Max 89.9 Ohms, MIN: 28.6 Ohms, Avg: 51.0 Ohms

When there's no pressure the resistance is 0 Ohms.  Note with 0 Ohms for no pressure an Oil Warning light can easily be turned on in addition to being used to drive an analog gauge.

So that's one point on the pressure vs. resistance plot.
If the sensor is linear then one other point will set the scale factor.
NOTE: maximum reading is about 90 PSI even if pressure is higher.

With kitchen sink running pressure is about 55 PSI.
House Water
With no water running pressure is about 110 PSI.
House Water

Using a 3/4" x 1/8" Brass Reducer the pressure guage from the broken air compressor regulator was attached to a hose bib using Teflon tape.  With the kitchen faucet running full on the house water pressure was adjusted to 50 PSI.  With the kitchen faucet still running the oil pressure resistance was 50.0 Ohms.

So the scale factor for the OP6638 looks like 1 PSI/Ohm making it direct reading on an Ohm meter.

Maybe I need to drain the house to re-establish the air traps?

Rain Bird P2A Water Pressure Gauge with 3/4" Male Garden Hose fitting

Got this gauge to check the house water system as part of the planning for an external home fire protection sprinkler system.  I feel it is much more reliable than the Made in China gauge.

This unit is composed of a 3/4" female GHT to female 1/4" NPT adapter and a pressure gauge with a 1/4 NPT male fitting.
Rain Bird P2A
            Water Pressure Guage with 3/4" Male Garden Hose


4079351 Pressure responsive sender, General Automotive Speciality Co., Inc., Mar 14, 1978, 338/36; 73/723; 338/42 - they claim much better performance and built-in calibration when compared to other analog pressure sensors (NAPA p/n OP6638).


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