How to select the correct type of float switch – 7 steps

 

Why is this important?

When one considers that float switches are usually used to protect against damage when high or low liquid situations occur, it is important to use the right type of float switch. The single biggest mistake that is made in float switch selection is when a control signal needs to be provided that uses low voltage and or low currents. Why? Read on.

 

There are many different types of float switches.

In general they are either:

    Tethered (attached to the end of a wire or cable), or

    Non-tethered (fixed to the side, bottom or top of a tank)

These can be broken down into two other classes:

    Switches used to directly switch pumps and other electrical devices.

    Switches used to provide control signals to control panels, relays. PLC's ect.

 

Step 1 - What voltage, what current

The most important one. So take note!

 

Low voltage, low current (Pilot duty switches)

Contrary to popular belief, you cannot just use any type of float switch to switch low voltages with low currents. Using the wrong type of switch here will result in a failure at some stage. The reason is that some float switch contacts are designed to switch arching loads.

 

An arching load is any voltage or current that will cause a small spark to be generated when the switch contact closes or opens. This arc keeps the mating surfaces of the contact clean. When you switch low voltages (less than 12V and/or low currents of less than 500mA), arching does not take place and over time a high resistance deposit accumulates on the contact. In time this deposit will result in the contacts not being able to conduct reliably. So, for low voltage and current applications, choose a float switch that has been specifically designed for the application.

 

Good quality control switches usually use gold plated contact. These types of float switches are called float control switches or pilot duty switches.

 

High voltage, high current

Ensure that you have selected a float switch that is capable of handling the current and voltage requirements. When selecting a float switch to control a pump ensure that the float switch is capable of handling the initial startup current. For instance, our SJE PumpMaster float switches will handle a running current of 13amps, however is capable of handling a startup current of 85amps. All pumps have a higher current when starting. This current falls back to a running current once the motor is up to speed.

 

Step2 - Choose Pump Up (normally closed) or Pump Down (normally open)

You will now need to decide how your float switch is going to be used. To explain the Pump Up and Pump Down concept we are going to use the situation where there is a tank with a pump connected to it.

 

If you want the pump to start when that tank is full and to stop when the tank is empty, then you will require a float switch that is Pump Down (you want to stop the pump at some stage when the level is going down).

 

If the tank is low and you want the pump to start and then to stop when the tank is full, you will require a Pump Up switch (stop the pump at some stage as the level goes up).

 

Float switches that are pump up or pump down have 2 wires attached to them. There is another type of float switch that is called a SPDT float switch. This float switch uses 3 wires and can be used to pump up or down depending on which 2 wires are used. One wire is common to both contacts. SPDT stands for Single Pole, Double Throw.

 

S tep 3 - Select the pumping range or switching range

F loat switches are designed to work over a range. For tethered float switches, the length of the tether dictates the level range. A longer tether results in a larger range between the float switch switching on or off. There is always a minimum and a maximum range that a float switch can handle.

 

The minimum is range is limited by the stiffness of the float switch cable. If the tether is too short, the float switch cannot bend up or down.

 

The maximum range is generally dictated by the weight of the cable. If the tether length is too long, the float cannot support the weight of the cable.

 

All good makes of float switches will have an installation document that will show the pumping range for different tether lengths.

 

Step 4 - Narrow angle or wide angle switch

Tethered switches can be either narrow angle of wide angle. Narrow angle means that the switch will switch over a very short range. These switches are usually used to maintain an accurate level in a tank.

 

Never use a narrow angle switch to control a pump. The pump will turn on and off with the slightest wave action (called short cycling) resulting in pumps and/or switch damage. Use a wide-angle float switch for pump control.

 

Step 5 - Turbulent or calm conditions

Some float switches are only suitable for use in calm conditions. Using float switches that are not designed for use in turbulent conditions, can result in damage to the equipment attached. The reason for this is that the contacts in the float switch will toggle on and off as the float switch approaches the level at which it will switch.

 

Float switches suitable for turbulent conditions are usually mechanical switches and the manufacturer’s specification will state that the switch is suitable for turbulent conditions.

 

Step 6 - Mercury or mechanical switches

The internal switching mechanism in float switches is usually mercury or some form of mechanical activation using a ball. Mercury switches are falling out of favor due to potential toxicity concerns and ROHS compliance issues in Europe. Mercury switches are usually cheaper than mechanically activated float switches. Mercury switches are generally not suitable for turbulent environments and high current applications.

 

Step 7 - Pole mounted or weighted

Tethered float switches can be mounted in one of two ways:

 

Pole mounted: here the floats switch cable is attached to an inlet/outlet pipe located inside the tank. The cable is usually attached to the pipe by using a specially designed stainless steel clamp.

 

Weighted: if the tethered float switch can be suspended from the top of the tank, then you can use a cable weight to hold the float switch at the correct level. Cable weight is usually available as an optional extra when ordering float switches. These weights are clipped onto the cable and are easy to adjust. You need to apply some caution in using suspended floats switches in tanks where there is a strong swirling action.

 

Be careful when installing float switches near submerged object such as pumps and pipes. Ensure that the float cannot wedge itself under, on-top or against any object.