What is meant by switching differential in level switches?  

Switching differential also termed as hysteresis, is a characteristic of a switch. It is applicable to all types of mechanical and electronic switches used to detect various parameters such as level, pressure, flow or temperature.

In view of level switches, it is a distance or difference between make and break of switching contacts. Switching differential/ hysteresis is important and useful function, mostly required in every process industry as it prevents chattering of switching contacts or instability around a particular switching set point.

Will see the demonstration of practical example with float pivoted switch.

Below shows a float pivoted level switch consists of magnetic systems, one carried by float stem and other by switch carriage inside the enclosure. A micro switch lever operates due to force created by magnetic linkages (repulsion due to like poles of magnet).

Here the float switch is used to detect liquid level at high level (L1) to switch on the pump.

During rising of liquid level at point ‘L1’, microswitch actuates and pump starts, it remains actuated on further rise of liquid level.

In case, there is turbulence in liquid and if the switch does not have differential, it will chatter (switched on and in small succession) around point ‘L1’, which may lead to pump failure in long run. But here switch remains actuated for a distance between pt. L1 & L2 irrespective of turbulence in liquid. Distance between ‘L1 & L2’ (15 ± 5 mm) is differential of the switch. Switching differential depend on magnets and microswitch characteristics.

During falling of liquid level, at point ‘L2’, switch de - actuates and pump stops.

In mechanical switches, differential is decided during its design and is generally specified by the manufacturer. However, in electronic switches it is generally configurable over the entire measuring range through software. Set and reset point of electronic switches are programmable using keypad & display.

The switching differential is widely used in most of the industrial applications. However, in some cases, or applications large differential may be disadvantage, as it limits ability to detect small change in level.