The Basics of Float Switches | Guide


What is a float switch?

A float switch is a type of contact liquid level sensor that uses a float to operate a switch. Float switches are commonly used to control other devices such as alarms and pumps when a liquid level rises or falls to a specific point. While there are some mechanical float switches (similar to the float in a toilet tank that turns off the incoming water when the tank is “full”), this article will focus on electrical float switches, floats that are used to open and close (i.e., turn off and on) electrical circuits.

Float switches are also often sought after because they are cost-effective, reliable, and can be used with a wide variety of liquids. In this article, we will dive into common uses of float switches, both normally open float switches and normally closed float switches.

Types of Float Switches

There are two types of float switches: stem-mounted float switches and cable-suspended float switches.

APG's FS-410 Miniature Stainless Steel Float SwitchAPG's FLE Multi-Point Stem-Mounted Float Switch

Stem-mounted Float Switches

Stem-mounted float switches constrain the movement of the float to up-and-down along a stem, and operate on a one-float-one-level basis. Stem-mounted float switches can be as simple as a single-point switch with a single float on one stem or they can be complex multi-point switches with as many as seven floats on a single stem. Single-point float switches can be found in both vertical and horizontal orientations.

APG's KARI Cable-Suspended Float Switch

Cable-suspended Float Switches

Cable-suspended float switches, on the other hand, are as free as the cables they are attached to. If you don’t tether a cable-suspended float switch, it will go with the flow, as far as the cable allows! While cable-suspended float switches are all mechanically pretty equivalent (float and cable), they can be much more electrically diverse than stem-mounted float switches. Cable float switches can be simple, with a single point that controls a switch, or more complex, with up to four switch points and built-in hysteresis.

How do float switches work?

Almost all electrical float switches work by using a magnet to open and close a reed switch.

How Stem-mounted Float Switches Work

stem-mounted float switch diagramStem-mounted float switches use a magnet in a float that passes over the reed switch with the rising or falling action of the liquid, either opening or closing the switch. On horizontal stem-mounted float switches, the float rotates away from the reed switch on a hinged arm, rather than free-floating on a central stem.

How Cable-suspended Float Switches Work

Cable-suspended float switches have both the magnet and the reed switch located in the float. As the float rises and falls with the liquid level, the magnet moves closer to or further away from the reed switch, causing it to open or close. In both cases, the changing liquid level is translated to an electrical on-off signal by the movement of the magnet.

Normally Open Float Switches vs. Normally Closed Float Switches

normally open vs normally closedThe trickiest part of float switches is figuring out whether a situation calls for a Normally Open or Normally Closed switch. “Normally” refers to the lowest position the float can be in where the float is not floating. “Open” means an open circuit, which is off. “Closed” switch completes a circuit, turning it on. For any given application, you need to think about whether you want rising or falling liquid levels to trigger an action, and whether that action should be turning a circuit (think a pump, or an alarm) on or off. A normally open switch does not allow any electricity through it when the float is freely hanging. For example, this means that as the water flows into a tank and raises the float, its contacts will close to turn on an emptying pump.. Normally open float switches are used for dewatering, sump and sewage points.

Normally open switches turn on with rising liquid levels and off with falling levels. This means that they are great for High Level alarms and controlling emptying (or pump down) pumps, such as sump pumps and sewage lift station pumps.

Conversely, a normally closed switch will turn off with rising liquid levels and on with falling levels. They are often used for Low Level alarms and filling, or pump up, pumps. As an example, if you want your float switch to trigger a low-level alarm, you would use a normally closed float switch. However, if you need to turn off an emptying pump when the level in a tank or reservoir gets too low, then you would use a normally open switch.

It is important to know whether you need a normally open or normally closed float switch because most float switches come with the normally open or normally closed operation selected at the time of purchase. There are some single-point stem-mounted float switches that are user reversible.

Float Switch Setup and Installation Best Practices

Some float switches are "plug and play", and most are fairly easy to install, but it's worth taking a moment to go over some best practices for setting up and installing float switches.

  • First, and this is true for all float switches, make sure the float switch you purchased is the float switch you received, and that your float switch is compatible with the liquid you will be putting it in. If it's not the one you ordered, or it's not compatible, then it won't work, no matter how well you follow the rest of these steps.
  • Second, check that your float switch will physically install correctly. Almost all stem-mounted float switches penetrate the vessel containing the liquid they will be in, since most stems are not made to be submersed. So, whether mounted from the outside or the inside of the vessel, make sure the float switch mounts correctly and that the mount is liquid tight. A leak around the mount of the float switch will tend to adversely affect the performance of the float switch.If your float switch is a cable-suspended float switch, verifying your physical setup might be a bit more challenging, but it is no less necessary. You'll need to check several things:
    1. Is the junction box your cable-suspended float switch will connect to above the high-water (or whatever liquid you are monitoring) mark?
    2. When connected to that junction box, will your cable-suspended float switch reach the desired lowest switch point?
    3. Does your cable-suspended float switch need to be tethered to keep it from floating out of position or into the path of inlet/outlet pipes, agitators, etc.?
    4. Is the weight of your cable-suspended float switch adequately supported, so that it does not pull out of the terminals in the junction box?
  • Third, check that your float switch will integrate correctly with your control system. If you only have a single switch, you only need to hook up two wires. Each additional switch or switch point should only add one wire. Do you have the appropriate number of terminals for the number of wires coming from your switch(es)? Are your switches connecting to your control system or control circuitry, or are they directly in-line with the controlled load? If your switch(es) is(are) directly in-line, do you have adequate protection for your switch(es) from current/voltage spikes?

Once you can answer all of these questions affirmatively, you are ready to install your float switch!

What applications are Float Switches used for?

Float switches are used for point-level detection; that is, float switches provide indication that a fluid level is at-or-above (normally open), or at-or-below (normally closed) a specific level. This is in contrast to continuous level measurement sensors, which give a continuous level reading. Float switches are a great solution if you need to trigger an alarm, or turn a pump on or off, based on a specific liquid level. Need to know when your home heating oil tank is down to 15%? Float switch! Want to automatically turn on a basement sump pump when the water level gets to a certain point? Float switch! Need to control alternating emptying pumps, with low and low-low level alarms? Float switches!

Because of the variety of materials, arrangements, and capabilities of float switches, a float switch can be found for almost every situation. Small spaces call for small, stem-mounted switches, while cable-suspended float switches are ideal for spaces with wider hysteresis bands. Aggressive chemicals require float switches made of stronger, more resistive materials (like plastics, instead of stainless steel). Explosive atmospheres or high-pressure, high-temperature processes need switches with appropriate certifications.

Float switches are also helpful for reducing complicated automation. For some process systems, a central PLC that monitors and runs everything is necessary for smooth, cost-effective operation. In these situations, a float switch is merely one more input, one more data point in a necessarily complex scheme. But for smaller applications—say a pump or two on each of a couple of tanks—float switches can be the key to on-going success. Hooking float switches up to the relays that control the pumps, with a backup alarm or two, might be all the automation necessary: empty (or fill) the tanks when required, provide “Hey, the pumps didn’t turn off!” or “Hey, the pumps didn’t turn on!” lights or horns, and you could be set. If you don’t need a constant, continuous level reading, if you just need to know when specific high or low levels are reached or exceeded, then a float switch is the answer you are looking for.

In the end, the type, material, and operation of the float switch you choose comes down to what best matches the application. Whether complicated or straight forward, small vessels or large reservoirs, common liquids or really aggressive chemicals, there is a float switch that can fit the job.

Are you curious about what kind of float switch will work best for your situation? Are you ready to find out how to integrate float switches into your control system? Drop our Measurement Experts an email, or give them a call right now! Their expert knowledge will help match you up with a float switch that will make liquid level measurement and control one less thing you have to worry about.

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