Pressure Canner Gauge Check. Part 1.

It is recommended that pressure canner gauges be checked at least once per year. Normally one would just take the gauges to the local Agricultural Extension Office and have them checked.  In my area, I had been the only one using this service for the past few years and the office decided to discontinue the service and devote their resources to more popular programs.

I was left with sending the gauges to the manufacturer or doing it myself.  This idea I have shown here might be useful to those who, for some reason, are unable to get their pressure canner gauges checked.

These are some notes that I made on a tablet of the “green engineering sketch paper”.  Hopefully they make sense.  This is only an idea that I have.  I’ve not yet tried it in “the real world”….but I think it’s simple enough to work.

A U-tube manometer is considered a “primary standard” for measuring pressure.  In operation a primary standard device depends solely upon the basic units of the measurement system (mass, time, distance, and so on) to make a measurement.  In other words, the instrument is perfect and any errors found in a measured value are ONLY due to not enough care being taken in making the required measurements.

A U-tube manometer depends only upon distance, mass and time (time is needed to measure gravity) to make a pressure measurement.  In reality, even though gravity on earth varies slightly based on where it is measured, the worst case error is less than 0.3% from the assumed value.  Since the measurements need to be within 3% (1/2 PSI at 15 PSI), using the assumed value for gravity is OK.

Here is a picture of a U-tube manometer.  This manometer measures the difference between the left and right legs of the device.  Since the right hand leg is open to normal atmosphere, it, like the pressure canner gauge, measures pressure relative to atmospheric pressure.

Single_manometer_balancedIn this image, the shaded area represents a liquid (distilled water in my case) and the un-shaded areas within the “tube” represent air.  The small black square is a cork or stopper or some other device used to seal that leg of the manometer.  The small L shaped “thing” atop the “stopper” is a small tube going through the “stopper” to which a small hose can be connected.  The other end of this hose gets connected to the pressure canner and an air pump.

This manometer has an equal pressure on both legs and there is zero difference in the levels of the liquid in the two legs.


Single_manometer_unbalanced

In this image, the pressure is higher in the left leg than on the right leg.  The liquid moves by an amount that can be used to find the pressure.  The vertical distance between the two liquid levels, B and A, is what is measured.  From there it is just looking up the density of distilled water in the proper units, and performing a calculation to determine pressure.

The manometer being able to accurately measure pressure does not depend on anything EXCEPT the density of the fluid, gravity and the distance between B and A.  The tube can be round, square or some other shape, large, small, vary in size and shape, flop in the wind (this will make it inconvenient to make measurement though), have square corners, round corners, widely spaced or  closely spaced legs, legs straight up and down or not (being straight up and down makes it easier to measure the distance) or anything else and the measurement accuracy is not changed.  One can even have a bunch of tubing laying on the ground with only some of both ends of the tubing held up in the air and it won’t hurt the measurement accuracy.

The only problem is that to measure 15 PSI, the manometer legs in a water manometer must be 35 or more feet tall.   I do not have a ladder, tree or pole that tall.  I could take everything to the local high school football bleachers, but I might be viewed with suspicion.

To get around this “tallness” problem, multiple manometers can be connected together.  Like this.

four_manometer_balanced

Again, the darkened area represents water and the empty area represents air.   With the pressure on both sides of the manometer being equal, A is level with B.  C is level with D.  E is level with F.  And, G is level with H.  It has no effect on accuracy if one section of the manometer is more full than another. So even though A is level with B and so on, A and B do not need to be level with C and D or any other section of the manometer.   This will likely happen as each section of the manometer is filled independently and it will be difficult to get *exactly* the same amount of fluid in each section.  So, this is one more thing that does not matter when making a measurement.

What happens with this four section manometer is that for a given pressure, A and B move only one fourth the distance the same points would move in a single section manometer.

four_manometer_unbalanced

Conveniently, the vertical distance between A and B is always the same as the vertical distance between C and D, E and F and G and H.  This is the case even if B, D, F and H and A, C, E and G, themselves are at different levels.  So, one only needs to measure the vertical distance between A and B, multiply by four and put the resultant value into the manometer equation.

If the distance between the Us at the top and the Us at the bottom are 10 feet (I have a ladder tall enough for this), a four stage distilled water manometer can measure about 19 PSI.  If 19 PSI, is exceeded, the water at H will spill out onto the ground and relieve the pressure within the canner.  So there is even a built in safety system.

The formula for a U-tube manometer is

P = \rho g h\

P is the pressure that you calculate.

h is easy.  Measure it with a tape measure and convert the value to inches and decimal inches…ie 32-1/4 inches is 32.25 inches., 32-1/2 inches is 32.5 inches and 32-3/4 inches is 32.75 inches.

g is gravity.  Use 32.15 for this number.  You might be able to find a better value for this, but probably not.  It is feet per second per second, not that this matters to the manometer.

\rho \ (it’s the Greek letter rho, not a small P) is the density of distilled water.  Distilled water is the same, regardless of how it’s made or purchased.  However, distilled water’s density varies slightly with temperature.  Unlike gravity, it changes enough that it’s worthwhile to take this variance into account.  Since the distilled water needs to be a liquid, this means the manometer must be used between just above freezing (32F) and just below boiling (212F).  Even though the manometer will work up to 212F degrees, I don’t want to work above 120F degrees, so I won’t bother with density values above 120F degrees.

One doesn’t need to be exacting with the temperature, a reading within the following zones is good enough.

Between 32F and 75F \rho \ is 0.001122
Between 75F and 90F \rho \ is 0.001118
Between 90F and 100F \rho \ is 0.001116
Between 100F and 110F \rho \ is 0.001114
Between 110F and 120F \rho \ is 0.001111

So, measure the temperature, find the correct \rho \ value for water, measure the distance between A and B (in inches) and multiply these two values together.  Then multiply by 4 and then multiply by 32.15.  This will give you the pressure the gauge should be reading.

An example.  The distance between A and B is 34-3/4 inches and the temperature is 80F degrees.

0.001118 \times 34.75 \times 4 \times 32.15  = 5.0 PSI \

For what it’s worth, if you mess up and measure 35 inches instead of 34-3/4 inches, you’ll only be off by a little more than .03 PSI.  This is an indication as to how accurate and precise a manometer can be.

Tomorrow I’m going to be helping a friend fix a car, so I won’t be able to take all this theory and turn it into a workable thing until early next week.

Again, I think this will work, but I will have to wait a few days to see how many “devils are in the details”. 🙂

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