Using air line to test wells

Using air line to test wells

Be sure all joints and connections are airtight, then pump air into the line until the maximum possible pressure is reached.

Air forced into the line creates pressure which forces the water out the lower end, leaving the line full of air.

Readings should not be taken while the air is being pumped in.

The head of water above the end of the line maintains the pressure and the gauge registers the actual pressure or head above the end of the line.

A gauge graduated in feet shows directly the amount of submergence of the end of the line.

Subtract this figure from the length of the line and you have the water level.

Just as drawdown is measured, so is recovery measured after pumping has stopped.

In fact, recovery can be observed directly from the action of the needle as it moves to the right due to the column of water in the well rising to the static level when pumping is stopped.

For active wells in good, open aquifers this rise is very rapid, occurring in a few minutes.

In poorer formations the recovery of static level may be a matter of hours.

In any case, no well ever recovers the last few inches until a period of from 24 to 72 hours has elapsed.

Practical example

Let us say that the gauge is an altitude gauge reading in feet and it takes 46 feet of head or pressure equivalent to this amount of head to force all of the water out of the air line.

We have created a condition whereby the air pressure inside the air line is just sufficient to balance the pressure of the water on the outside of the line.

In this case it is 46 feet by direct reading on the altitude gauge.

If the distance from the center of the gauge to the open end of the air line is 95 feet, then the distance from the gauge to the static level is the difference between 95 feet and 46 feet, or 49 feet.

If the gauge is above or below the ground surface, this must be considered if measurements are being referred to the surface.

During pumping test

Suppose now we start our test pump.

It will be noticed that the pressure on the gauge drops as the water is lowered in the well.

This is as it should be since the water in the well is lowering and there is not as much water pressure above the open end of the air pipe as there was before pumping started.

In most gauges the needle actually moves to the left, or counter clockwise, to indicate a drop in pressure.

Most altitude gauges have two needles, one red and the other black.

It is customary to set one needle, usually the red one, at the first reading - the reading taken before starting to pump the well.

Then by reading the movable, or black, needle, the difference between the two needles can be read directly and the drawdown determined instantly.

Example calculation

Let us say our movable needle reads 22 feet after pumping has started.

The difference between 46, our first reading, and 22 is 24 feet, or the drawdown at the quantity being pumped.

A few trials will be sufficient for anyone to become familiar with the use of an altitude gauge and air line.

Pressure gauge alternative

If a pressure gauge is used, it operates exactly the same way except that the readings are in pounds per square inch and must be converted into feet of head for practical use.

To do this, multiply the readings in pounds by 2.31 to obtain feet in head.

For example, if the gauge reads 40 psi, then the head in feet would be 40 X 2.31 or 92.4 feet.