How to Choose the Best Liquid Level Switch

A level switch is an electromechanical device used to monitor the level of liquid in a tank or reservoir. They can be used to maintain the liquid level between a low and high point, or they can also be used to trigger an alarm when a low level or high level is reached. Level switches are used everywhere and are very safe and reliable. They are probably in your coffee maker, automobile, RV, basement sump, toilets and water storage tanks. They usually either have an electrical switch, which is either a microswitch or a very tiny reed switch. There is usually a floating mechanism that triggers the switch as it moves up or down with the liquid level.

You should consider the electrical requirements, wire type, switch orientation, type of liquid, wetted materials and mounting method when choosing a level switch. The correct good quality switch and proper installation provide years of uninterrupted service.

Common uses of liquid Level switches
  • Maintain liquid levels between a low and high level. These are called “Pump Up” or “Pump Down” situations. In a “Pump Up” application a water tank may be maintained fully by the well pump turning on when water in the tank reaches the low-level switch and then turns the pump off when the water level reaches the high-level switch. A ‘Pump Down” empties the tank when it gets full, like a sump pump or condensate pump. A controller (latching relay) is usually required to coordinate the function of the two-level switches such as the Harwil LC-1.
  • Trigger a warning when a liquid level reaches a high limit or low limit. This could be to keep a reservoir from overflowing or running dry. Often a tank will have 4 level switches. Two to maintain the water level between the low and high-level switches and then high limit and low limit level switches. If the level control is using a different technology, high and low-level alarms are even more important acting as a backup if the other technology fails.
  • Wide Differential Level Switches can be used where the distance between high and low is not too great. If the distance between the on and off level is narrow enough, a single switch can be used. The Harwil L-21 can have a 1, 2, 3 or 5-inch differential and special models can be up to 24 Inches. Normally the distance between on and off is very narrow, about ¼ of an inch, which would cause a pump or valve to cycle too rapidly.
  • Level switches can be used to indicate that liquid is present. For example, a level switch can be installed in a spill containment reservoir to warn if there has been a leak. Liquid filling the reservoir would cause an alarm. A level switch is often used to trigger a signal or event, such as “refill the tank”. The level switch could be simply connected to a light that turns on and notifies the operator “go fill the tank” or “turn off the pump”.
  • Specific Gravity Level Switches can indicate the presence of water in oil or the presence of 2 liquids which have different specific gravities. Harwil manufactures switches that float in the barrier between oil and water. For example, these can be used to warn that the diesel fuel holding tank is contaminated with water. L-8 and L-40
  • Types of Level Switches and Installation methods

    Level Switches are either mounted horizontally or vertically. They can be mounted through the tank wall at the top or bottom of the tank. When mounted horizontally through the tank wall, a bulkhead connector is used. You need to make sure the float mechanism can move freely once mounted. Most of the Harwil level switches can be submerged in the liquid so that they can be attached to the end of a PCV pipe placed in the liquid from the top or bottom of the tank. Most can be installed either vertically or horizontally. Some vertical switches use a floating donut, that is more likely to stick or catch on debris in the liquid. Another switch has a floating ball on a wire, that is not as reliable and until recently these used a mercury switch inside the floating ball.

    Material selection

    Level Switches are made from a wide variety of materials. The material must be compatible with the liquid and the environment where it will be installed. Harwil’s switches are made of brass, 316 stainless steel, Noryl or Fortron plastics. The wetted materials can also be brass, stainless steel, Hastelloy C, Noryl and Fortron. Because these materials are compatible with most liquids, the Harwil switch can be used in just about any liquid. There is a Compatibility Chart in the Harwil Catalog and on the Harwil Website.

    Electrical Requirements

    The level switch may be controlling a large water pump or a tiny relay on a circuit board. The switch you choose and the electrical connection must be capable of handling the power requirements. The switch and wiring may be exposed to the liquid or outdoors exposed to the weather. Each of these must be considered. You can always call Harwil to get your questions answered so that you choose the proper switch.


    Harwil is over 60 years old and Harwil Switches are used by just about every pool and spa equipment manufacturer. Harwil switches are used by the Navy, dozens of municipal water companies, and hundreds of water treatment service companies. If you have a pool or spa, you probably already own a Harwil switch. Some of our switches commonly last over 25 years, often longer than the system where they are installed.

    You can rely on Harwil to help you choose the correct switch for your application. Harwil specializes in designing and building custom switches for original equipment manufacturers.