Level Switch

Hi Vicky,

Whatever application I have seen, two sensing points are available. i.e. 2 NO, 2 NC.

The vendor amy be contacted and asked to supply more.

But if done also problem will be in cable glanding and a same input used at different purpose will decrease the reliabilty.

Hope this may help you.

vikky_rathi, when you take an analog sensor you can have (theoretically) unlimited number of sensing points according to your requirement - its only depending on sensors quality (resolution determined by noise and other influences). Ultrasonic is sometimes a good solution because it is contactless sensing with resolutions in the sub-mm-range.

Selection of sensors depends on your goods - is it a fluid ?
Regards Uwe

you can multiply the binary signal i.e. open or close contact for using in other applications.regarding sensing points in one level sensor i cann't understand can u explain in detail what u want exactly.

Well, it might be more convenient to use a level transmitter to give an analog signal, and determine the trip points individually elsewhere. One might use a number of trip-amplifiers within the control panel for example, depending on loop resistance and voltage, or one might program the trip points within a PLC based upon the engineering units from the scaled input signal.

The P&I diagram for this plant will, if it is correctly drawn, determine the exact number of trip points needed. Look for the abbreviations LSH/LAH, LSHH/LAHH, LSL/LAL and LSLL/LALL in the instrument bubbles as these, together with a well-written Control Philosophy document, will determine the correct approach needed from the Controls & Instrumentation Designer.

Your question: What is the maximum number of switching points on a level switch.

If I am right to assume, you want to build a level alarm or control system. You can build a simple system with SCR (or Traics) and DPDT relays. Wire up the SCR to work as a Flip-Flop, and trigger the flop action by the contact of one electrode in the tank, and the flip action by another electrode in the tank.

You can cut two stainless steel rods of lengths suitable for your application. The longer electrode is for the low level trigger, and the shorter electrode for the high level.

I build this simple circuit to work a pump for the sump on-off control with a dead band. The sump to be pumped out must be conductive, mainly water and the electrodes insulated and shield to water-tight and weather proof.

Alternatively, you can use a simple float switch with a rugged cable. Attach the cable to a pole dipped into the tank. Clamp the cable with hose clip, some distance away from the float switch to allow free play for the float to move up and down a distance. This distance is your cut-on and cut-off points for the pump control. If your pump rating is small, then you can connect the switch contact directly to operate the pump without another relay. These float switches are sold off the shelf and available in the catalog for level control.

TIJIT^{digital is our name, portable is our game}

wow you made me think about how many level switches a SEGWAY uses. It is after all constantly trying to level it's self.

In boiler applications, in addition to the analog level transmitter, you're required to have level switches to protect the boiler from too high or too low feedwater levels. I believe your question involves this or something similar.

Technically speaking there's no limit to how many level sensors you can install. There are practical limits, however, and these depend on your application. Let's start with simple and move on from there:

• A high and low level switch (LSH, LSL) to start or start a pump.
• A low-low level switch (LSLL) to back up the low-level switch in case it fails.
• A high-high level switch (LSHH) to back up the high-level switch in case it fails.
• A high level switch to send an alarm (LAH) that the tank is close to overflowing. This could start a backup pump to help in bringing down the level.
• A low level switch to send an alarm (LAL) that the tank is close to emptying which may cause the discharge pump to run dry and damage it. This can stop the pump in the same way as the LSL. It's a backup and sends an alarm at the same time.
• A high-high level sensor that sends an alarm (LAHH), these will usually cause the whole system to shutdown and even cause the operator to hit the emergency stop switch.
• A low-low level sensor that sends an alarm (LALL) that causes the whole system to shutdown and even cause the operator to hit the emergency stop switch.

Note that not all of these are required in a level switching or alarm application. In some cases, a level switch can be used for both alarm and switching functions. How many you use depends on your application. If this is this a safety requirement then redundancy and separate function may be mandatory.