Level sensors detect the level of liquids and other fluids and fluidized solids, including slurries, granular materials, and powders that exhibit an upper free surface.
The level measurement can be either
Continuous level sensors measure level within a specified range and determine the exact amount of substance in a certain place,
Point-level sensors only indicate whether the substance is above or below the sensing point. Generally detect levels that are excessively high or low.
Selection criteria of the optimal level monitoring method
Phase (liquid, solid or slurry), temperature, pressure or vacuum, chemistry, dielectric constant of medium, density (specific gravity) of medium, agitation (action), acoustical or electrical noise, vibration, mechanical shock, tank or bin size and shape
Also important are the application constraints: price, accuracy, appearance, response rate, ease of calibration or programming, physical size and mounting of the instrument, monitoring or control of continuous or discrete (point) levels
Most used level sensors
1. Vibrating or Tuning Fork
The vibrating sensor technology is perfect for solid and liquid level control, including sticky materials and foam, as well as powders and fine grained solids. However, the types of applications that can use tuning forks is limited to overfill or run dry type applications and they do not provide continuous process measurement. However can be used in conjunction with continuous level detection systems, acting as alarm points for over-filling and leaks.
Pros – Compact, cost effective
Cons – Invasive, number of uses are limited
Applications – level control of liquid, powders and fine grained solids within mining, chemical processing and food and beverage industries.
2. Float Switch
Float switches are one of the most cost effective but also well proven technologies for liquid level sensing. A float switch includes a magnet within a float and a magnetic reed switch contained within a secure housing. The float moves with the change in liquid and will cause the reed switch to either open or close depending on if it’s in air or liquid. Although simple in design, this technology offers long-term reliability at an attractive price point.
Depending on what mounting style the user chooses heavily depends on the design and construction of the tank or container the switch will be situated. Typically, suppliers will offer a range of mounting options, with the most common being horizontal/side mounting and vertical mounting.
Pros : Non-powered, direct indication, relatively inexpensive, various outputs
Cons : Invasive, moving parts, large in size, large amount of liquid has to be present before the float makes contact.
Applications : Tank level applications where water, oil, hydraulic fluids and chemicals are being used.
3. Conductivity or Resistance
Conductive sensors are used for point-level sensing conductive liquids such as water and highly corrosive liquids. Simply put, two metallic probes of different lengths (one long, one short) insert into a tank. The long probe transmits a low voltage, the second shorter probe is cut so the tip is at the switching point. When the probes are in liquid, the current flows across both probes to activate the switch. One of the benefits to these devices is that they are safe due to their low voltages and currents. They are also easy to use and install but regular maintenance checks must be carried out to ensure there is no build up on the probe otherwise it will not perform properly.
Pros – No moving parts, easy to use, low-cost
Cons – Invasive, liquids need to be conductive, probe erosion
Applications – Tank level measurement for boiler water, reagent monitoring, highly corrosive liquids
Ultrasonic sensors measure levels by calculating the duration and strength of high frequency sound waves that are reflected off the surface of the liquid and back to the sensor – the time taken is relative to the distance between the sensor and the liquid. The length of time in which the sensor takes to react is affected by various elements in the atmosphere above the media such as turbulence, foam, temperature etc. Hence why the mounting position is critical in these devices.
Pros – No moving parts, compact, reliable, not affected by media properties
Cons – expensive, invasive, performance can be affected by various elements in the environment
Applications – Non contact applications with highly viscous and bulk solids. Used in systems that require remote monitoring
Capacitance level sensors operate when process fluids have dielectric constants significantly different to air. They measure the change in capacitance between two plates produced by changes in level. Two versions are available, one for fluids with high dielectric constants and one with low dielectric constants.
Capacitance level sensors work with a range of solids, liquids, and mixed materials. They are also available in contact and non-contact configurations meaning some of which can be attached outside the container/tank. capacitance senor is dependent on the type of material or tank. In addition, the sensor needs to be calibrated to the specific material to excuse the varying dielectric constants and differences in the tank design. As this type of technology is contact based, the reliability of these sensors can be heavily influenced by fluids sticking to the probe.
Pros – Solid-state, can be non-invasive, compact, accurate
Cons – May require calibration, can only be used in certain liquids
Applications – Tank level monitoring in chemical, food, water treatment, power and brewery industries.
### 6. Optical Level Switches
Inside the sensor housing is an LED and a phototransistor. When the sensor tip is in air, the infrared light inside the sensor tip is reflected back to the detector. When in liquid, the infrared liquid is refracted out of the sensor tip, causing less energy to reach the detector. Being a solid-state device, these compact switches are ideal for a vast range of point level sensing applications, especially when reliability is essential. Optical liquid level switches are suitable for high, low or intermediate level detection in practically any tank, large or small. They are also suitable for detecting leaks preventing costly damage. Reflected light, such as in a small reflective tank, mirrored tanks, bubbles, milk or coating fluids can often cause issues with delayed readings.
Pros – Compact, no moving parts, high pressure and temperature capability, can detect tiny amounts of liquids
Cons – Invasive as the sensor requires contact with the liquid, requires power, certain thick substances can cause coating on the prism.
Applications – tank level measurement and leak detection applications
In principle radar works in a similar way to ultrasonic, but the pulses travel at the speed of light and again; the reliability and repeatability can be affected – but this time by the dielectric constant of the fluid. However, radar can provide very precise level information and also compensate for fixed structures within the container. The downside can be that the initial cost of the sensor is relatively high, but several manufacturers are making this technology more accessible to the wider market. These sensors are among the handful of technologies that work well in foam and sticky substances
Pros – very accurate, no calibration required, multiple output options
Cons – expensive, can be affected by the environment, limited detection range
Applications – Moist, vaporous and dusty environments. They are also used in systems in which temperatures vary