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【Introduction】Reed switch-based liquid level sensors offer designers a reliable and long-life option for harsh and demanding applications. These switches come in a variety of configurations and can be used to measure whether the tank is full or empty, or when the liquid is at a predetermined level in between, some switches can measure multiple liquid levels. In addition, they do not require an external power supply and can easily meet EMC requirements.
Liquid level sensors (also known as liquid level switches or float sensors) play a key role in the safe and efficient operation of systems and are increasingly used by designers in a variety of industrial applications, including heating, ventilation and air conditioning (HVAC) ), water and wastewater treatment, chemical and petrochemical treatment systems, and food and beverage production. While you may choose to design a level sensor from the ground up to meet increasingly demanding application requirements for accuracy, energy efficiency, and ruggedness, doing so can quickly drag you into a complex, time-consuming and ultimately costly process as the design The reader should be familiar with sensing technology selection, packaging, interface and regulatory requirements.
Instead, designers can use prepackaged solutions based on reed switch sensing technology that are ready out of the box and have UL and IP65 regulatory approval. Additionally, because they use reed switch technology, which is typically rated for more than 10 million cycles, they can handle high power loads, and have the benefits of low contact resistance for energy efficiency and zero power consumption.
This article reviews key design considerations when selecting a float level switch. It then discusses the benefits of using reed switch technology, and concludes with several TE Connectivity (TE) level/float sensor solutions and how they can be used.
Choosing a Float Level Sensor
Float level switches are used for a variety of purposes, such as sounding an alarm when liquid levels rise or fall to potentially dangerous levels, helping to protect equipment from overheating, maintaining proper proportions of materials being mixed, and reducing the risk of fire. When selecting a float level switch for a specific application, a clear understanding of design conditions and requirements is required, namely:
· What is the liquid and what is its temperature and pressure?
· Does the application require a normally open (NO) or normally closed (NC) switch?
· Do you need a single pole single throw (SPST) or single pole double throw (SPDT) switch?
· What orientation of the switch is required: horizontal when viewed from the side, top mounted, or bottom mounted?
Is a single level condition indicator such as “full”, “partially full” or “empty” sufficient, or do you need to monitor multiple levels?
The liquid and its condition are important considerations; different switch body materials suit different application needs. Demanding applications such as high temperature water, fuels and oils may require a glass filled polyphenylene sulfide housing rated up to +130 degrees Celsius (°C) and 4.7 bar pressure.
The bodies of vertical level switches can be 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 expanded polypropylene bodies, which are rated up to +60°C and 0.34 bar. In addition to the proper switch body material, the choice of switch technology is an important consideration when specifying level sensors.
Benefits of Reed Switch Technology
Reed switches are a proven and reliable technology. Floating level switches using reed switch technology are passive devices that do not require an external power source to function. The switching action of these sensors is activated by the interaction of a permanent magnet in the float with a fixed 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). When the liquid level rises, the magnet moves (floats) from the bottom to the top of the switch, and when the liquid level falls, the magnet goes down. When the magnet moves toward or away from the reed switch, it turns on or off, depending on the configuration.
Figure 1: The float moves up and down the valve stem (left) as the liquid level rises and falls. The valve stem contains a fixed reed switch (middle) that opens or closes when a magnet in the float (right) approaches, depending on its design. (Image source: TE Connectivity)
The reliability of these sensors comes from several factors: they have only one moving part, they are fabricated from liquid-compatible body materials, and they have a hermetically sealed reed switch with ruthenium contacts rated for over 10 million switching operations.
Reed switches work reliably in high temperature environments. When the switch is closed, they have a higher insulation rating and lower current consumption than solid state switching technology. Using a reed switch to meet electromagnetic compatibility (EMC) requirements is simple and requires only minimal testing.
Reed switch based liquid level sensor
Designers can use TE’s liquid level sensors to realize the benefits of reed switch technology. TE enables designers to choose from nearly 20 vertical and horizontal level switches in six series, constructed in a variety of materials, and with a variety of switch, accessory and cable options (Figure 2).
Figure 2: TE Connectivity’s float level sensors are available in a variety of configurations, including vertical and side mounts, up to a 90° angle, and universal mounting accessories. (Image source: TE Connectivity)
TE 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 for Standardization (ISO) certified for industrial applications. Some models are UL listed and the UK Water Regulation Advisory Scheme (WRAS) certified. It is rated for over 10 million switching cycles and is designed to facilitate installation and field service. If one of these switches is damaged, it can be easily replaced.
TE offers level sensors that can measure when a tank is full, empty or partially filled, and some models can measure multiple levels. Vertical level sensors like the VS801-51, with glass-filled polypropylene bodies designed for use in water, are available with NO or NC switches, and in SPST or SPDT configurations. Manufactured from glass-filled nylon 6.6, the VCS-06 is available as a NO or NC switch in SPST configuration (Figure 3). Both sensors can be used in applications involving boiling water and fuel, and some models are rated for operating temperatures up to +130°C and pressures up to 4 bar.
Figure 3: The VCS-06 series liquid level sensor can be installed on the top or bottom of the liquid storage tank, and can achieve NO or NC contact depending on the orientation of the float. (Image source: TE Connectivity)
For applications that need to measure a single level between full and empty, designers can use a horizontal level switch. Examples of UL certification include:
· LS309-32, made of glass-filled nylon 6.6 for use in oils, fuels and non-ionic liquids. It is available in an SPST configuration with a standard activation swing of 40.4 millimeters (mm) and is rated at 200 VDC or 250 VAC with loads up to 70 watts (W) (Figure 4).
· LCS-03, the shell is acetal/polypropylene and the float is foamed polypropylene. This is designed for use in water and wastewater applications where space is limited. It has a compact horizontal short pendulum (35.5 mm) and comes with a cable or integral connector. When the float is level, it is NC, rated at 48 volts DC, and has a maximum power of 40 watts.
· LDS309-11N, housing and float in glass-filled nylon 6.6 for oil, fuel and non-ionic liquids at pressures up to 4.7 bar. It is able to sense small changes in liquid level with a narrow 8.65 mm travel difference between actuation and release. The device uses SPST switches and is rated for loads up to 70 W at 200 VDC or 250 VAC.
Figure 4: The LCS309-32 level switch has a standard activation swing of 40.4 mm and is designed for use with oils, fuels and non-ionic liquids. (Image source: TE Connectivity)
When a single level measurement is not enough, designers can use extended switches up to one meter in length. For example, the EVS312-51N uses two switches to provide three different level status indications (Figure 5). The high switch is an NC device and the low switch is a NO device.
Figure 5: Extended switches like the EVS312-51N use two switches to provide three different level status indications. (Image source: TE Connectivity)
When the float is at the upper limit, both switches are turned on; when the float is at the lower limit, both switches are turned off; when the float is between the upper and lower limits, the high switch is turned off and the low switch is turned on (Table 1).
Table 1: The high and low switches in the EVS312-51N can be used to indicate three different liquid level states. (Image source: TE Connectivity)
The body of the EVS312-51N is nylon 6.6, the float is glass-filled nylon 6.6, rated at 175 VDC or 125 VAC, with loads up to 5W, and available in internal and external mounting configurations.
Reed switch-based liquid level sensors offer designers a reliable and long-life option for harsh and demanding applications. These switches come in a variety of configurations and can be used to measure whether the tank is full or empty, or when the liquid is at a predetermined level in between, some switches can measure multiple liquid levels. In addition, they do not require an external power supply and can easily meet EMC requirements.