How to specify a liquid level switch for greater efficiency

Paul Boughton

Ronald A Buchanan looks at the key factors in optimising the performance of float-style liquid level switches and ways to prevent the life of the switch being reduced.

The float-style liquid level switch is a very rugged and reliable design which will perform well for many years if it is designed correctly for the application.

Although you may think this is a simple taskthere are many important considerations which need to be accounted for. Otherwisethe life of the level switch may be greatly reduced. This article will cover each of these factors in depthto help you better specify your liquid level switch application.

Key considerations

The key factors you need to consider are the type of liquidtemperature and pressuremounting methodsize constraintselectrical requirements and applicable agency approvals.

  • The Type of Liquid: The first consideration is the type of liquid in which the switch will be used. The type of liquid will determine the choices of material that can be used for the stem and float. Many liquids are corrosive and can damage some of the materials that could be chosen. The specific gravity of the liquid will also need to be considered when choosing the material and size of the float. To ensure that the float doesin factfloatits specific gravity needs to be lower than that of the liquid.

  • Temperature and Pressure: The next consideration is the temperature and pressures to which the switch will be subjected. You need to consider worst-case conditions for the switchnot just normal operating conditions. This should narrow your choices down. Please refer to the table below for general guidelines on some material choices for various applications.

  • Mounting Considerations: Once that you have narrowed down the materials you can useyou must decide how you are going to mount the level switch. There are two basic choicesvertical or side mountingwith many different variations. It must be noted that carbon steel and other magnetic material vessels can affect the magnetic operation of the reed switch. You mustthereforemount the level switch in a way that will avoid this. Vertical-mount switches can be mounted at the top or the bottom of the tankwhile side-mount switches are meant to be mounted to the side walls of the tank. Both can be mounted utilising either male pipe threadspipe plugsflanges or bulkhead fittings. You must also decide whether the switch will be installed from inside the tank or through the outside. If you are mounting from outside the tankthe size of the mounting hole must be larger than the diameter of the float.

  • Size Constraints: Now that you have decided what materials can be used and how you would hope to mount the level switchyou need to determine what size switch will fit your application. The three most commonly used sizes are: full sizeminiature and subminiature. These sizes refer to the size of the float portion of the switch. The diameter of the float depends on the material used. For examplea stainless steel float has a larger diameter than a polypropyleneBuna-N or Kynar float. The subminiature size is available with a polypropylene or Buna-N float. The operating point (height of the liquid in the tank) of the switch will determine the length of the stem for vertical mount switches. To determine the total length of the stemyou must account for the distance from the switch mounting surface to the operating pointheight of the float and the float retaining method.

  • Electrical Requirements: The length of a switch's life is directly related to the electrical requirements of an application. The level switch uses a magnetically operated reed switch. The magnet is placed into the floatandas the float moves with the liquid levelthe magnet will close or open the reed switchdepending on how the switch is configured. The reed switch can either be a form A (SPST) or form C (SPDT) contact type. You need to determine if you require the switch to be normally open or normally closed. Reed switches are typically rated by their maximum power ratingeither Watts (dc) or VA (ac). These ratings can be very misleading and have caused many a user to choose a switch with the wrong rating. The most important rating of a reed switch is its maximum switching current (which cannot be exceeded). The next factor is the maximum switch voltage. If a switch is rated higher than your requirements for both of thosethen there is just one more rating to check. Multiply your current requirement by the voltage requirement. Their product must be less than the Wattage or VA rating of the switch.

  • Agency Approvals: You will want to know if the level switch has the right agency approval. There are many different approval agencies whose requirements apply within different countries and industries. Such approvals include ULCSANSFCE and use in hazardous environments. Only you can decide what is required for your applicationand you should check local codes or standards to be sure. You should also consider if the approval is needed for the liquid level switch as a component of your system.

Sample applications

Application number one: Water tank fullness. A need to measure when a cooling water tank is full. The switch will be mounted from the inside of the tankat the topin an 1/8-in NPT hole. A 12 VDC50 MA signal will be switched off when the tank reaches full. The maximum temperature the application will see is 85° C (185° F)and maximum pressure is 5 PSIG.

We start first with the liquid to be measured. In this caseit is water. There is a choice of several switch materials for this applicationso then we look to temperature and pressure. The maximum temperature is 85°C (185°F).

Since there should be some margin of safetyI would recommend using a switch rated for at least 100°C (212°F). There are many alternativesexcept for the plastic PBTwhich will not work in water over 65°C (149°F).

The low pressure of this application does not eliminate any choicesso the material choice can now be based on cost.

The lowest-cost material would be Polypropylene for both stem and float.
Nextwe look at the mounting methodwhich is at the top of the tankfrom the insidein a 1/8-in NPT hole. So we would chose a vertical-mount stem with 1/8-in NPT male pipe threads.

Nowwe check the voltage and current requirements: 12VDC and 50MArespectively. The switching current and voltage are lowand the power required would be 0.6Watts. This application does not call for any agency approvals.

Oil temperature

Application number two: Cooking oil level. A need to turn off the heating element in a deep fat fryerwhen the cooking oil drops below a minimum level. The level switch will mount from the outsidethrough the side of the tankin a 1-in NPT hole.

The level switch will control a relay at 120 VAC0.1A. The oil temperature will be a maximum of 177°C (350°F). NSF approval and UL listing are also required here.
Againthe first consideration is the liquid to be measured. This timeit is cooking oil. Because there are several material choices availablewe look to temperature and pressure.

To ensure a margin of safetyI would recommend using a switch rated for at least 200°C (392°F)even though the maximum temperature of the applications is 350°F (177°C). This reduces our material choices down to stainless steel.

Nextwe consider the mounting methodand we see that we need a side-mount style with a 1-in NPT fitting to mount from the outside of the tank. The electrical requirements are 0.1A and 120VACso we calculate the VA ratingwhich is 12VA. There are also NSF and UL requirements.

You will find that standard liquid level switches are available to meet a broad range of application conditions.

Butshould you fail to find one that is exactly right for your needsit is very likely that the manufacturer can modify a standard product to meet your requirements. Sodo not give up without making an inquiry.

Ronald A Buchanan is Vice PresidentMadison CompanyBranfordCTUSA.


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