What type of float switch should I choose?

There seems to be a vast choice of pump float switches available in the marketplace, with various facts, figures and pricing available, but are they all essentially the same?

Typically, a float switch is a device used in a liquid to control the operation of something, be it a pump, valve, alarm or other devices.  The main elements being housing, switching device and fixing method or material.

Firstly, let's look at the construction

Most sump pump float switches available are constructed using either polypropylene (pp float switch) or plastic housing which offers a good all-round combination of strength and resistivity to most liquids.  More uncommon materials are stainless steel and rubber, more suited to specific applications and therefore higher in price.  

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 FLO 101 Float Switches (ATEX Rated) installed and ready for action.

What liquid will it be installed in?

The next thing to consider is what is the medium the level control is to be installed in.  If it’s purely water, consider if this is Potable (drinking water) or purely waste/greywater.  Potable water floats will need to be constructed from WRAS approved materials, essential if the float is to be used in a clean drinking water supply application.

Next, taking into account the specific gravity, viscosity and density of the liquid to float is to be installed in. 

The selected float must be buoyant enough to operate and switch and be constructed to withstand immersion pressure at the required depth, along with any impact from turbulent installations.

More specific liquids such as fuels, and heavy sludge liquors, will again require specific attributes from the potential float switch used. We have a range of oil float switches specifically designed for use in diesel oil.

Cables

Cable choice plays a bit part in the selection process when reviewing suitability for use.

Rubber cables such as HO7RN8-F and HO5RN-F, are particularly suited for submergence in water and damp atmospheres, whereas some PVC compounds are not suited to applications where hydrocarbons are present, as the cable becomes stiff and therefore hinders the operation of the float switch. 

Applications such as diesel and fuel again require a more specialist PVC cable resistant to oils which causes rubber cables to swell.

FLO 102 Float Switch

Operation

The switching mechanism internal to the float is crucial to the life expectancy of the product.  Most submersible float switches on the market now use a chrome-plated or stainless-steel ball secured in a rolling chamber to then, in turn, operate an electrical microswitch. 

Older products still in existence typically used mercury to make and break an electric circuit, however since the Restriction of Hazardous Substances Directive 2002/95/EC came into effect 13 August 2004, this is now no longer used, and therefore any new product offered as a replacement must be RoHS compliant.

Consideration must be given to the number of likely operations required by the float switch, which then has some bearing on the manufacturer's chosen micro switch in terms of longevity of use.   The internal construction of the ‘rolling’ chamber in which the ball operates should limit the Nm (Newton metre) of force exerted on the microswitch when contact is made.

Electrical

Lastly the electrical element needs to be understood, from operating via a safe extra low voltage source, to any protective devices such as an RCD (residual current device) to protect against the risk of electrical shock if safe extra-low voltage is not employed.

If the float switch is to be installed in a hazardous area environment (or zoned application) where explosive gasses could be present, such as methane in a sewage well, then an ATEX rated float switch is required and should be installed with a matching galvanic isolation barrier, or relay.