Designers use liquid level sensors—also called liquid level switches or float sensors—in critical roles for safe and efficient system operation across a growing range of industrial applications from heating ventilation and air conditioning (HVAC), water and wastewater treatment, and chemical and petrochemical processing systems, to food and beverage production. While it’s possible to design liquid level sensors from scratch to meet increasingly demanding application requirements in terms of accuracy, energy efficiency, and ruggedness, doing so can quickly become a complex, time-consuming, and ultimately costly process as designers sort through the sensing technology options, packaging, interfaces, and regulatory requirements.
Instead, designers can use pre-packaged solutions with reed switch-based sensing that come complete and usable out-of-the-box, with regulatory approval for UL and IP65 already acquired. Also, because they use reed switch technology, they are often rated for over 10 million cycles, can handle high-power loads with low contact resistance for increased efficiency, and have zero power consumption.
This article reviews key design considerations when selecting float liquid level switches. It then discusses the benefits of using reed switch technology before introducing several liquid level/float sensor solutions from TE Connectivity (TE) and how they’re applied.
Selecting a float liquid level sensor
Float liquid level switches are used for a variety of purposes, such as providing alarms if liquid levels rise or fall to potentially dangerous levels, helping protect equipment from overheating, maintaining proper ratios of materials being mixed, and reducing the risk of fires. Selection of a float liquid level switch for a specific application requires a clear understanding of the design conditions and requirements in terms of:
- What is the liquid and what is its temperature and pressure?
- Does the application require a switch that is normally open (N.O.) or normally closed (N.C.)?
- Is single-pole single-throw (SPST) or single-pole double-throw (SPDT) switching needed?
- What switch orientation is needed: horizontal from the side, mounted to the top, or mounted to the bottom?
- Is a single level condition indicator such as “full,” “partially full,” or “empty” sufficient, or do multiple levels of the liquid need to be monitored?
The liquid and its condition are important considerations; different switch body materials are suited to different application needs. Demanding applications such as high-temperature water, fuels, and oils may need a glass-filled polyphenylene sulfide housing rated up to +130 degrees Celsius (°C) and 4.7 bar pressures.
Vertical liquid level switches can have bodies up to one meter long and use rigid materials such as various plastics, brass, or stainless steel. Less demanding applications such as water tanks can use relatively inexpensive acetal and foamed polypropylene bodies that are rated up to +60°C and 0.34 bar. In addition to employing the appropriate switch body material, the selection of the switching technology is an important consideration when specifying liquid level sensors.
Benefits of reed switch technology
Reed switches are a mature and reliable technology. Float liquid level switches using reed switch technology are passive devices and need no external power source to function. The switching action in these sensors is initiated by the interaction of a permanent magnet in the float with the stationary reed switch.
These sensors utilize a movable float with an embedded magnet to activate one or more reed switches in the sensor body (Figure 1). The magnet moves (floats) from the bottom to the top of the switch as the liquid level rises and falls when the liquid level drops. As the magnet moves toward or away from the reed switch, it turns ON or OFF, depending on the configuration.
Figure 1: The float travels up and down the stem (left) as the liquid level rises and falls. The stem contains the stationary reed switch (center), and as the magnet in the float (right) comes close, the switch opens or closes depending on its design. (Image source: TE Connectivity)
The reliability of these sensors results from several factors: they have a single moving part, they are built with liquid-compatible body materials, and they have a hermetically sealed reed switch with Ruthenium contact points rated for over 10 million switching operations.
Reed switches can operate reliably in high-temperature environments. They have a high insulation rating when the switch is OFF, with a current draw that is lower than solid-state switch technologies. Meeting electromagnetic compatibility (EMC) requirements with reed switches is simplified and only minimal testing is required.
Reed switch based liquid level sensors
Designers can turn to liquid level sensors from TE to realize the benefits of reed switch technology. TE enables designers to choose from nearly 20 vertical and horizontal liquid level switches that are available in six families, and are made using a wide range of materials with various switching, fitting, and cable options (Figure 2).
Figure 2: Float liquid level sensors from TE Connectivity are available in a variety of configurations including vertical mount and side mount, up to 90° angle, and universal mounting fittings. (Image source: TE Connectivity)
TE liquid level switches are rated up to 250 volts alternating current (AC) or 200 volts direct current (DC). They are ISO/TS 16949 certified for automotive products and International Organization of Standardization (ISO) certified for industrial applications. Some models have UL approval and UK Water Regulations Advisory Scheme (WRAS) approval. They are rated for over 10 million switching cycles and are designed for easy installation and field service. If one of these switches gets damaged, it can be easily replaced.
Liquid level sensors are offered by TE that measure when a tank is full, empty, or partially filled, and some models can measure multiple liquid levels. Devices such as the VS801-51 vertical level sensor with a glass-filled polypropylene body are designed for use in water, are available with N.O. or N.C. switches, and SPST or SPDT configurations. The VCS-06, which is made using glass-filled nylon 6.6, is available with N.O. or N.C. switches in a SPST configuration (Figure 3). Both sensors are available for use in applications involving boiling water and fuels, with some models rated for operation up to +130°C and 4 bar pressure.
Figure 3: The VCS-06 series level sensor can mount at either the top or bottom of a liquid storage tank and the float can be oriented to provide an N.O. or N.C. contact. (Image source: TE Connectivity)
For applications that need to measure single liquid levels in between full and empty, designers can use horizontal liquid level switches. UL approved examples include the:
- LS309-32, made with glass-filled nylon 6.6 and intended for use in oil, fuel, and non-ionic liquids. It has an SPST configuration, a standard activation swing of 40.4 millimeters (mm), and is rated for 200 volts DC or 250 volts AC, with loads up to 70 watts (W) (Figure 4).
- LCS-03, with a housing of acetal/polypropylene and a float of foamed polypropylene. This is designed for use in space-constrained water and wastewater applications. It comes with a compact horizontal short-swing (35.5 mm), and with a cable or an integral connector. It is N.C. when the float is horizontal and is rated for 48 volts DC up to 40 W.
- LDS309-11N, with a housing and float of glass-filled nylon 6.6 and intended for use in oil, fuel, and non-ionic liquids with pressures up to 4.7 bar. It is capable of sensing small changes in the liquid level with a close differential travel of 8.65 mm between operation and release. The device uses SPST switching and is rated for loads up to 70 W and voltages to 200 volts DC or 250 volts AC.
Figure 4: The LCS309-32 horizontal liquid level switch has a standard activation swing of 40.4 mm and is designed for use in oil, fuel, and non-ionic liquids. (Image source: TE Connectivity)
When a single level measurement is insufficient, designers can turn to extended-range switches that can be up to one meter long. For example, the EVS312-51N uses two switches to provide an indication of three different level conditions (Figure 5). The High Switch is an N.C. device, and the Low Switch is an N.O. device.
Figure 5: Extended-range switches such as the EVS312-51N use two switches to provide an indication of three different level conditions. (Image source: TE Connectivity)
When the float is at the top limit, both switches are open; when it is at the bottom limit, both switches are closed; and when it is in between the top and bottom limits, the High Switch is closed and the Low Switch is open (Table 1).
Table 1: The High Switch and Low Switch in the EVS312-51N can be used to indicate three different level conditions. (Image source: TE Connectivity)
The EVS312-51N has a body of nylon 6.6 and a float of glass-filled nylon 6.6 and is rated for 175 volts DC or 125 volts AC with a load up to 5 W. It is offered in internal and external mount configurations.
Liquid level sensors based on reed switches give designers a reliable and long-life choice for harsh and demanding applications. The switches come in a variety of configurations for measuring when a liquid holding tank is full or empty, or when the liquid is at a predetermined level in between, and some can measure multiple liquid levels. Additionally, they need no external power source and can easily meet EMC requirements.
- Material Selection for Liquid-Level Sensors