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Process Automation

Municipal water authorities are required to measure the groundwater level in several locations for a number of reasons. This is to monitor the water table, water in streams and lakes, and potential deterioration of water quality. The locations can range anywhere from wells, streams, dams and aquifers.

In one instance, a municipal water authority had to measure the groundwater from several aquifers. In this case, the sample well consisted of a 4-inch well casing that was approximately 100 feet deep and periodic measurements were taken by a visiting technician monthly.

What are some of the ways you can measure your sample wells?

It depends on how often you are required to measure the groundwater. Some municipal water authorities are required to continuously monitor the sample wells whereas others only need to occasionally measure the sample wells.

If you’re occasionally measuring your wells, you can have a technician drive out to the well site with a gas-powered air compressor, connect the compressor to a flow controller to the dip tube and then run the compressor for a few minutes until he/she is able to get a stable reading. If you choose this method, you will need to have an air supply and install a pressure gauge to get the results.

What types of air supplies are applicable for well sites?

A portable air compressor is one source of air that allows you to run the purge system to make it work, or you can run from a nitrogen bottle and that would act as an alternative source. If you needed a continuous reading, however, a nitrogen bottle would not be a suitable solution.

Occasional water monitoring has its positives too: you only need to buy one set of equipment. You can then transport the equipment from well to well. An alternative would be to outsource the equipment with a third-party provider and pay a fee each time you need to monitor your levels. Groundwater levels don’t change too quickly, so if you do choose to do this yourself, it’s important to note that sediment may block the dip tube. Remember to always clean out the dip tube when you monitor to avoid sedimentary blockage.

If you need to measure your wells continuously, your site may have a small equipment shack with electrical power to run the small air compressor continuously, connect the electronic pressure sensor and get a continuous signal that can be sent back (either wired or wirelessly) to the main location. Like occasional measurement, sediment can block the dip tube. Automatic systems sometimes have a timer to blow high-pressured air into the dip tube to clean it out. The primary benefit of continuous measurement is that you don’t need to commit resources to going out in the field all week to take measurements. Once it’s set up, the central location has all of the information it needs.

In one instance, a site periodically took measurements and needed an alternative solution for a portable, low maintenance system that could operate without electrical power. A bubbler system, which is an air-flow controller that maintains a constant air purge to the dip tube, was selected because it could be operated by a gas-powered air compressor and mounted on a pickup truck. Here, the dip tube was placed in the well to a lower depth than the minimum level measurement.

To take the well’s measurement, we connected the air compressor to the dip tube so that a constant flow of compressed air was delivered into the dip tube. The pressure at the top of the dip tube was measured with a pressure gauge, and the back pressure at the top of the dip tube was proportional to the height of the water in the well.

To minimize errors due to purge flow variations, we needed to control the flow rate. A flow controller consisting of a needle valve and a constant differential relay was used. The constant differential relay maintained a constant pressure drop across the needle valve to keep the constant flow when the back pressure at the dip tube changed or when supply pressure changed. In situations like this, bubblers are often used because they work best in applications containing solid debris or sewage sludge, or where other technologies are difficult to apply.