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What types of float switches are commonly used, and which is best for my application?
Float switches come in a few designs; each float switch type is suited to different situations:
a) Tethered Float Switches: This is the most common type of float switch application. In a tethered application a float switch is suspended by its own cable and rotates up and down with the fluid level. In a “tethered” application the float switch needs a larger basin for its movement. The benefit of tethered float switches is that the wide moving range allows the pump to run longer each time it turns on. The downside is that they require space to move and therefore may not be suitable where space is tight. The “tether” term refers to where the part of the float switches electrical cord between the float and where it is anchored (mentioned above). The moving part of the cord is called the “tether”. If the tether can get caught on anything in the area in which it is moving, it can cause the system not to work as it should. This is common with sump pumps – they often have tethered floats, and there can be up to two pumps, and other various sensors in a sump as small as a 12” diameter (30” is the normal sump pump diameter). Tethered floats are common in sumps and lift stations with enough space, and the floats are sometimes called “wide angle” meaning that they turn on and off generally at 45⁰ angle. You may remember the comment from some of our other blog posts that the name “float switch” can be misleading because internally these work more like a tilt switch. The when the fluid raises an anchored switch, the float switch “tilts” up, when the fluid lowers the float switch tilts down. How does it tilt? This is due to the cable being anchored at some point. The float is actually rotating around its anchor point. But note the tether distance has limits. If the tether is too small, the pump will turn on and off very frequently. If the tether is too great, the weight of the cable can affect the operation. Generally, the most common float switches can have a maximum tether of about 12-14 inches. Because the SludgeBoss® is internally counterweighted, it does not need a tether. Conversely the WaterBoss™ Float Switch is
b) Vertical Float Switches: A vertical float switch generally moves up and down on a vertical rod – the floats movement being limited to up/down, with no opportunity for side-to-side. It works in tight spaces. Vertical floats are often used when the basin is too small for a tethered float to swing freely. Vertical style float switches are often built right into the pump (such as the Zoeller M53) . So why would we not just use a vertical float switch for everything? While possible it is often not practical because six or seven feet of light weight rod (generally fiberglass) between the “float” and the “switch” may bend over long distances or become brittle over time. Companies like Allen-Bradley offer the head unit for these types of products. If we change the material to metal, the float at the end of the rod must get bigger (i.e. more buoyant) to lift the increased weight of the rod. For this reason, the vast majority of vertical float switches offered are for sump pumps. That said, we have a 12” diameter sewer pipe that is 15 feet deep in our inground liner pool. High level is detected with a Sump Alarm Wi-Fi unit with conductivity probes. I was telling another engineer how the probes work beautifully in the chlorinated water. He mentioned that he just has a “piece of foam and a stick”. When his water level gets too high, it knocks the cover off his pool sump pump, and he calls that his “high level alarm”. Good luck getting an alarm if you’re not home, but he did save some money!
c) Diaphragm (Pressure) Float Switches: You don’t see a lot of these, but Little Giant perfected the art of the diagram float switch. Diaphragm switches are good applications for small spaces and shallow sumps. In this case, when the water rises it goes into a chamber that pushes on (generally a rubber or plastic) diagram. When the diaphragm pushes in, it triggers a microswitch that activates the pump to run. These are fantastic for super small spaces. Little Giant made this with a few versions ending “- LL” that could go down to having about ¼” of water left in the bottom of the tank. Some newer versions (like the ION float switch) have a diaphragm that pushes on a load cell, which makes them adjustable for turn off and turn on times.
There are downsides to diaphragm float switches. First, diaphragm float switches are generally expensive. We covered two types – the load cell and the microswitch. Why are they expensive?
Diaphragm - Tube Type: The picture below is the Little Giant Microswitch type. In part because there is a small air tube built into the cable power cable. This is necessary for the diaphragm float to work, because for the water to displace the diaphragm and trigger the switch, the air must leave from a chamber and be replaced when the water level drops (i.e. the purpose of the tube). When I examined the float, I wondered if the air tube would clog over time and perhaps inhibit the operation after a few years. That never happened. However, the diaphragm itself (a circular rubber disk) eventually broke. They sell replacement kits, and it was not a difficult project (or it would not have been if I had not overtightened them and stripped the plastic housing – eventually leading to the switches demise).
Diaphragm – Load Cell: The load cell type of Diaphragm float switch has a diaphragm that pushes on an electronic load cell. The more water that is above the float, the more pressure the water creates – giving the Load Cell more information about how much water is in the sump. It’s a fantastic technology – and requires a printed circuit board internal to the float switch, and next to the diaphragm. ION makes a great load cell float switch.
Of course, the Load Cell is more expensive than the Tube Type but can do more. The microswitch based models don’t give the same pump run time, but of course if the application is in a small, shallow space, that’s not really an option anyway. In my experience all diaphragm float switches have a moving diaphragm that comes int contact with the fluid. In my application, the diaphragm may have failed due to chlorine exposure – hard to say, but I did replace the diaphragm several times over the life of the float switch (ended by the screw incident above).
d) Mercury Float Switches: were mentioned earlier. Mercury float switches a tethered float switch that has mercury in it rather than a ball and switch. They have become less popular over the years as mercury is considered a toxic chemical that municipalities want to keep out of water systems. Maine, California, and Minnesota banned them at the time of this writing. Mercury float switches are popular because the moving part is mercury - rather than a ball. The mercury itself keeps the electrical contacts clean of debris that builds up slowly with each electrical connection. There is no switch. There are no failure parts. Mercury switches are known for their long life and reliability. Because of the nature of having mercury internal to the float switch, most constructions have a glass capsule internal to the float in which the mercury moves, which does mean they need to be handled with care. Mercury float switches can be more finicky in triggering for orientation – and the key component is a mercury tilt switch an.