How to measure water level in wells and boreholes?

When we have a well or borehole with a submerged pump, whether electric, shaft or simply direct suction, one issue that always worries us is the water level in the well and if for some reason it drops.

A well or boreholeand therefore the pump that is usually mounted on it, is a very important point in the chain. It is essential for any activity that requires it, be it industrial, agricultural (irrigation or farm supply), municipal (water supply), recreational, etc... to be carried out. If the level of the well falls too low, the activity will have to cease or look for much more expensive alternatives (carrying water in cisterns, new pipelines, etc...).

It is therefore of vital importance for the owner of the well and the borehole to know the water level in the well.

Static level and dynamic level in wells

In this post we are going to look at different options for measuring water in a well or borehole, but first we are going to define what is known as the dynamic level and static level of a well.

Static Levelwater level in the well with the pump switched off and coming from rest (at least 1 hour without the pump running). It indicates the level of the water table (water level) at rest. On the one hand, it indicates the submergence level of the pump and on the other hand, by difference with the dynamic level, it will give us a clue as to the operation of the well.

Dynamic Level: is the water level in the well with the pump running (i.e. pumping a certain flow rate). It can be variable since, depending on the flow rate of the pump, it may drop continuously until the level of the pump is reached (technically, a dry well).

Focusing on the systems for measuring the level of the well, we can make a first differentiation into two distinct groups:

Point measurement or continuous measurement.

The one-off measure is one that is carried out on an ad hoc, independent, non-continuous basis.

Depending on the type of well it can be:

• De visu: in shallow wells of large diameter can be measured directly with a tape measure or even have a "metric scale" in the well and just by looking we can see the level. This is a very rare case and only occurs in traditional shallow wells (up to 10m maximum) and large diameter wells (at least 100cm).
• With level probesThe test reel is usually a reel with a stainless steel electrode at the tip and a ground which is connected to the well column. When the electrical circuit is closed by the water in the well, it emits a sound or gives a reading on a "multimeter" type indicator. It is usually powered by batteries or a small battery.
• By means of indirect media with an intermediate fluid: normally compressed air, the system works as follows in a tube or piezometer placed up to the height of the pump we apply compressed air until it is completely empty, once the tube is empty we close a tap and stop applying air to the piezometer, at that moment the water will begin to enter the piezometer until it stabilises, if at the top of the piezometer we have a manometer the pressure in bars that marks x 10 will be approximately the metres of water that there is above the pump. It is necessary to open and close the tap every time we want to check the level, so the measurement is punctual.

The measure continues On the other hand, as its name suggests, it is a measurement that is taken continuously and can certainly be exported to a controller, PLC or inverter for use in the management of the sounding. There are two main ways of taking it

• By means of directly measuring submersible pressure probesThe pressure transducer is usually fitted with a special cable with a small "hollow" piezometer which indicates the atmospheric pressure value to the probe and thus by means of the differential pressure the value is usually given in the form of 4-20ma.
  It is a very effective and simple system but has two main problems One is the price of the cable with piezometer tube, which can be prohibitively expensive at great depths, and the other is the interference that can be caused by modern pumping equipment control systems such as variable speed drives, which can introduce "electrical noise" into the system and cause the reflected values to fluctuate and become unusable once the system is running.

By means of indirect measurement controllers: The SONDELEVEL system is based on the indirect measurement system using compressed air described above and, although it requires a piezometer tube and a compressed air compressor, it is a more economical system and much more resistant (it is not affected) to "electrical noise" than the previous system. It also shows the depth value on the display and by means of two simple potentiometers it allows us to control a relay and its hysteresis to control the pumping equipment. It also includes an analogue output with the depth of the well, which can be exported (for example) to an inverter to avoid overexploitation of a borehole (as an example).

In conclusion, we can say that indirect measurement systems, whether point or continuous, based on compressed air, are not only simple to implement and operate, but also very resistant to the effects of "electrical noise", which can render expensive differential pressure transducer installations unusable (as is often the case).