The 40H-3 level switch is designed for internal mounting through the top of the process vessel. The main advantage of these level switches is lower cost. Mounting connections are offered in as per table on page no. 3 and in a variety of flange sizes and pressure ratings.

a) For Single Switch Mechanism (Series 311) (Refer Fig. 1)

The operating principle provides for magnetic switch action resulting from a change in liquid level, which moves a magnetic attraction Sleeve into the field of an externally located magnet. The illustrations given indicate the operating principle using a Float or Displacer to provide the operating motion. A falling level causes a downward movement of a magnetic Attraction Sleeve moving it below the magnetic field generated by the externally mounted alnico magnet. The bias spring then causes the magnet to pull away from Enclosing tube in turn actuating the Switch. The reverse function takes place on rising level with the Attraction Sleeve being moved into the field causing the magnet to pull in towards the Enclosing tube in turn actuating the Switch.

b) For Double Switch Mechanism (Series 321) (Refer Fig.2)

The Stainless-Steel Magnetic Attraction Sleeve (3) actuated by the float/Displacer, is outside the field (2) of the Alnico permanent magnet (1) the switch is in the Released position & an electrical circuit is open for (L-C) connection of Micro Switch (4). The switch is held in the “Released" position by gravity and the tension of spring which provides the snap action when the spring operates. Refer figure 2A

When the Float/Displacer causes the Attraction Sleeve to enter the field of permanent magnet associated with a given switch, the magnet swings into contact with the non-magnetic Enclosing Tube (5) & the switch is then “Actuated" (Refer figure 2B). In the actuated position of the switch there is a closed electrical circuit, between L-C of Micro Switch.

Switch mechanism is interchangeable and may be fitted with any Level Switch provided the Switching gap does not exceed the maximum of 115 mm.

Each switch unit has a nominal differential of maximum 30mm. If a differential greater than 30mm is required for the pump control, two switch unit must be used with a holding circuit arranged for the starter control circuit. With the switches set at the minimum operation the combined differential will be between 28mm to 35mm. Wider differential may be obtained by increasing the separation between the switches Contact L-C is normally open contact and H-C is normally closed contact and these are marked on micro switch. The L-C circuit closes on a rise of water level, the HC circuit closes on a fall of water level. Each switch unit has a normal differential of maximum 30mm of liquid at SG 1.00

The basic operating principle provides for magnetic switch action resulting form a change in liquid level, which moves a magnetic attraction sleeve in to the field of an externally located magnet. This principle eliminates problems associated with flexing diaphragm seals, bellows seals as well as ‘coating’ problems encountered by prove type devices. Side mounting units employ permanent magnetic force as the only link between the float and the switching element. As the pivoted float follows liquid level change, it moves attraction sleeve into or out of the field of a switch actuating magnet causing switch operation. An enclosing (non-magnetic) tube effectively isolates the switch form the controlled liquid.

The 40H-5 level switches are designed for mounting externally on the process vessel and when used with isolation valves allows the switch to be isolated from the process. This feature is useful during start-up, or when maintenance is required. The chambers are offered with NPT, Socket weld and flanged connections as standard. The External cage can be welded type which makes is less prone to leakage, lighter in weight and consequently less expensive. Optionally Bolted type External cage is available which is comparatively heavier, but has an edge over former for ease in cleaning, inspection and has replacement feature of float Assemblies.