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How Full is My Tank?
by Doug Pushard
It is such a simple question, but sometimes not an easy question to answer. Fortunately, today there are many varieties of depth gauge options for buried and above ground tanks – from home made to high-end multi-function electronic devices.
A quick review on some of the alternatives that are available may help you choose which device is best for your needs.
 Above ground tanks have an option of an external measurement bar. The simplest design is a transparent tube mounted on the outside of the tank which shows the actual level of the water in the tank. On this tube or marked nearby on the tank, gallons are indicated. This provides a quick and easy conversion of water level to gallons in the tank.
No electricity, no maintenance – these devices are very simple, yet functional and can be easily fabricated from parts available at most plumbing supply stores.
A variation on this type of device (shown at the right) is not a tube but instead a marker that is attached to a float inside the tank. As the water level changes in the tank, the external marker goes up and down the side of the tank to show the water level in the tank.
 A more sophisticated version of this float-type measuring gauge has no external bar, but a display unit that mounts on top of the tank. These can be used in above or below ground installations. These devices have a small wire chain that attaches to a float that moves up and down with the water level. As the float moves, the display changes to show the depth of the water. This type of device come in both electric and non-electric models.
Moving up in sophistication are gauges that measure depth by pressure or sonar. Both require electricity and wiring from the display gauge to the unit in the cistern. Wiring from the tank to the gauge is usually accomplished with 4 low voltage wires. This provides the option for the gauge to be mounted in a location not near the next (i.e. 100-200’ away). The displays on the unit show either percentage or a colored bar indicator that depict percentage full. Sonar gauges bounce waves off the top of the water to determine the depth, while pressure (i.e. transducers) gauges measure the pressure of the water at the bottom of the tank to determine water depth. Pressure gauges require the transducer to be submerged in the water near the bottom of the tank. Sonar depth gauge mount on the inside or outside of the lid of the tank and bounce waves off the top of the wave. If using a sonar-type depth gauge, it must be mounted parallel to the water to get an accurate reading (i.e. buried cisterns on a hillside with slanted lids may be a challenge but can work if properly installed) and have clear visual unimpeded view of the water (i.e. no dangling wires or inlet/outlet pipes behind the unit and surface of the water).
For both the sonar and pressure type depth gauges there are usually wireless upgrades to transmit the depth data from the tank to the display unit. These wireless units can be extremely distance and obstruction sensitive; so consequently, they should be tested or specified to work in an environment like yours to ensure they will function properly. Some may have external antennas that will increase the range. Batteries will typically last one to two years.
The top-end depth gauges are integrated with other controls. These are electrical units and come in both wired and wireless models. These high-end devices perform additional functions (e.g. turn on or off pumps, open and shut valves, or communicate with other electronic devices. When combined with controllers, depth gauges can assist in automating some of the maintenance of a rainwater system.
The major drawback of almost all the depth gauge available today are they only show depth of water or percentage full of the water in the tank and not the actual gallons of water in the tank. This shortcoming makes depth gauges harder to use for the casual homeowner. Consequently, it is highly recommended a conversion guide is provided nearby so you can quickly do the conversion (e.g. 33% full equals 1,750 gallons).
Depth gauges are a very useful addition to rainwater and greywater systems. Knowing how much water you have in your tank helps you better manage this precious resource.
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