Choosing a float switch

Minature Vertical Floats
Our float switches

 

OVERVIEW

Float switches work by switching reed switch located inside the stem part of the float switch. When a magnet in the floating cylinder comes into the right position, the magnetic field causes the reed switch to change state. Typically the contact surfaces are sealed in a glass ampoule. Because the switch contact surfaces are in a vacuum, oxidation is eliminated and they are expected to last for millions of cycles.

Float switch circuit

CHOOSING A FLOAT SWITCH

To choose a float switch, the material must of course be compatible with the chemistry.  (chemical compatibility chart here)  After this, the secondary considerations are the size, cost, durability, and current capacity.  

SMALL OR LARGE FORMAT?  Large vertical float switches have the advantage that there is more physical space and more buoyancy to allow heavier-duty reed switches in order to handle higher current carrying capacity.  This is needed if you are running current through the switch to directly control a device. However this not a good idea to run them this way as it shortens the life of the reed switch, which will lead to switch failure. This benefit completely goes away in an electronic circuit passing just a few micro-amps.  If used this way, the lifetime of the switches is measured in millions of cycles.

Most of the vertical float switches we offer are miniature, or subminiature float switches. Although there is no official definition of what constitutes a “miniature” float switch, it makes sense that “miniature” should be reserved for the smallest possible version of product available.  By this definition, a polypropylene float switch with a diameter of less than 18 mm would reasonably be called a miniature or subminiature float switch.  For stainless steel, that size would be less than 23 mm diameter.

Vertical polypropylene float switch
Polypropylene or ‘F’

(Its amazing that many manufacturers call a 1.5 inch diameter plastic float “miniature”!)  Miniature float switches have the advantage that they can fit in small openings like a 3/4″ drum bung. Also, they can be conveniently surrounded by a float guard to protect them from physical damage. If the switches are protected from chemical attack, electrical failure, and physical damage, what’s to stop them for lasting almost forever? That’s why we like miniature switches!

AQUEOUS LIQUIDS

In general, polypropylene floats are great for aqueous chemicals, including strong acids and caustic chemicals. The smallest float switch made (of any material) measures just 16 mm diameter. It has a very light specific gravity of just 0.5.  This float switch is made from foamed polypropylene and we call our ‘P’ float.

Vertical polypropylene float switch
Kynar or ‘K’ type

The disadvantage of this ‘P’ float switch is that it is not reversible, and it is a foamed construction which is does not repel some chemicals as well as molded polypropylene.  
So for these applications we have another polypropylene float called our ‘F’ style. It is slightly larger diameter of 17.5 mm. In addition, it is a hollow molded part and therefore has a quite slippery surface with rounded edges, which is better at repelling liquids.   It also has the advantage that it is reversible. The S.G. is 0.85 which suitable for aqueous chemicals, but not for most hydrocarbons. Both of these polypropylene floats are low cost and are the first choice if they are compatible with the chemistry. Link to float switch specifications here.

In addition there is a PVDF float switch (Kynar® by Arkema) which we call our ‘K’ float.  This is suitable for very aggressive chemicals. At 19 mm diameter, It is also small enough to fit in small drum bungs and resistant to the most aggressive chemicals.

HYDROCARBONS

316 Stainless Float Switch
Stainless or ‘S’ type

Choosing the a float switch for hydrocarbon chemicals usually comes down to stainless steel or BUNA, although our light-weight polypropylene ‘P’ style has been used successfully if the chemistry is right.

BUNA vertical float switches go by many names such as Nitrile rubber, NBR rubber, BUNA-N, acrylonitrile-butadiene and others.

The floating BUNA cylinder is actually a hard crystalline material that doesn’t seem at all like “rubber”, but is named so because it is derived from the same chemical reaction as rubbers. It is compatible with most aliphatic hydrocarbons, and many other chemicals. However it is difficult to classify broad categories of compatible chemicals so the chemical compatibility must be checked
Stainless steel vertical float switches consist of a hollow cylinder.  It can be made from different grades of stainless but we use only 316 stainless for maximum chemical resistance.  We prefer the narrow vertical float switches which measure 23 mm diameter because they can fit inside of the ¾ inch bung in drums, and protecting it with a float guard is easy.  The S.G. or this float is 0.73, which is suitable for most hydrocarbons. 

Light Weight Stainless Float Switch
‘U’ ultra light float switch

For very light density hydrocarbons we have a larger diameter 316 stainless steel float switch with a density of just 0.5.  However it is necessarily larger in order to achieve lighter weight.  The diameter of this float (which we call ‘U’ for ultralight) is 38 mm.
Specifications on the above float switches can be found on this page