sonic ranging sensor

Image Credit: ABB Measurement Products | Honeywell | Campbell Scientific

 

Level sensors are used to detect liquid or powder levels, or interfaces between liquids. They are an important part of a fluid system because the position and displacement of an object or media is critical to know in many applications. The substance to be measured could be inside a container or in its natural form such as a lake or river. A sensor is able to detect a change and communicate that change with a user. Level sensors are generally designed for a specific application, rather than general application.

 

How Level Sensors Work

There are two basic level-measurement types, point level and continuous level sensors. Below is a video which explains each measurement.

 

Video Credit: unclecudesac

  • Point level sensors or multi-point sensors mark a specific level and communicate to the user if the media is above or below that level. They are used generally as a high alarm or switch. These level sensors can also be integrated in a single device to add a low alarm, or to serve as a stepped version of a continuous level.

Example of point level measurement.

Video Credit: madisonsensors

  • Continuous level sensors measure liquid or dry material levels within a specified range and provide outputs which continuously indicate the level. They are more sophisticated devices than point level sensors because they measure over a range of levels instead of at one point to let the user know the exact amount of a substance. This produces an analog output that directly correlates to the fluid level in the container.

Example of continuous level measurement.

Video Credit: madisonsensors

 

Level Sensor Devices

There are three major device classifications for level sensors:

  • Sensor-only devices provide outputs for processors, controllers, or data acquisition systems; however, they do not have a display or user interface. Typically, sensor-only devices are used in applications that require a switch.
  • Level sensor systems are simple instruments or gauges. These level sensors provide more capabilities than sensor-only (transducer) devices, and may include a display and/or user interface. Generally, level sensor systems are designed for applications that require a simple gauge, a locally-programmable (stand-alone) unit, or a controller/totalizer.
  • Sensor transmitters are designed to transmit data from the sensor via a two-wire current loop. The loop has an external power supply, and the transmitter acts as a variable resistor with respect to its input signal.

Product Selection

There are several important specifications to consider when selecting a level sensor. These include: the media you will be measuring, contact or non-contact measurement, type of level measurement technology, temperature and pressure ranges of the system, and output needs.

 

Contact or Non-contact

Level sensors measure using contact or non-contact methods. Contact methods involve physical contact between the device and the media, while non-contact methods measure without touching the media. Non-contact methods are the best choice for servicing corrosive media. 

 

Media

Media is the material that the sensor needs to measure. Level sensors can be responsible for measuring a wide variety of media in fluid level systems in many industries. The two general media types include liquid and dry materials.

  • Liquid media can include
    • Water (hot or cold, clean or dirty, fresh or salt)
    • Gasoline (diesel fuel)
    • Hydraulic fluid
    • Highly viscous or gummy fluids
  • Dry materials can include
    • Bulk solids
    • Powders 

Type of Measurement Technology

Level sensors are available with many different meter technologies. Choices include

 

Technology

Description

Application

Air bubbler

air bubbler

Image Credit : Chipkin Automation Systems                                  

They have a source of compressed air, an air flow restrictor, sensing tube, and pressure transmitter. The device works with a small amount of air metered into a dip tube in the tank. A pressure transducer monitors the air pressure in the tube to make sure the pressure in the tube equals the pressure at the bottom of the tank. The liquid's level is determined by dividing the pressure inside the tube by the liquid's density. The system is able to measure the liquid's level as directly proportional to the pressure as long as the material's density remains constant.

Advantages: only the dip tube touches the media and needs to be compatible, point of contact does not have electrical parts; controls are located outside the tank making the system safer.   

An air bubbler system does not include any moving parts making them ideal for measuring the level of liquids, suspended solids, and sludge. Harsh environments, such as cooling towers, reservoirs, vented fuel tanks, drain sumps or air washers.

Capacitive or radio frequency (RF) sensors

 

Image Credit: Sensors.mag

The sensor is composed of a coaxial capacitor. All surfaces of the conductor are coated with a thin isolating layer to prevent an electric short circuit when it is immersed in the water tank. As the level of the tank increases water fills more and more space between the sensor’s coaxial conductors. This changes the average dielectric constant between the conductors and therefore changes the sensor’s capacitance. There is a level-capacitance dependence which provides a linear relationship. The slope depends on the liquid. Capacitance depends on temperature so a temperature sensor should always be present. 

Capacitive sensors are used in many industries including (but not limited to) chemical, food, water treatment, power, and breweries.

Differential pressure

Differential pressure sensor

Image Credit: Sensors.mag

A differential pressure (DP) level sensor measures the difference in pressure between the top and bottom of a tank of fluid. The higher pressure, caused by the fluid, is compared to a lower reference pressure (generally atmospheric). Since the tank is open to the atmosphere, only the high pressure connection needs to be connected to the transmitter. The low pressure is released to the atmosphere so the pressure differential is the hydrostatic head (weight) of the liquid in the tank. The maximum height of liquid about the transmitter determines the maximum level measured. If the tank is not open to the atmosphere or is filled with a fluid other than water a different configuration is required.

DP sensors are the most common sensor.

Electrical conductivity

or resistance

Conductivity sensorImage Credit: Gems 

Conductive sensors are used for point-level sensing conductive liquids such as water and highly corrosive liquids. They use low-voltage, current-limited power source applied across separate electrodes. Higher voltage sensors are designed to operate in less conductive (higher-resistance) media. These sensors are very safe due to the use of low voltages and currents. They are also simple to use and install but they must be checked for medium buildup on the probe so it continues to function properly.

 

Mechanical or magnetic float

magnetic level float

Mechanical, magnetic or other float sensors involve the opening or closing of a mechanical switch. This occurs through direct contact with the switch or magnetic operation of a reed. Magnetic floats use a permanent magnet sealed inside a float rises or falls to the actuation level. Mechanical floats use a miniature switch that the float moves against. Float level sensors cannot be used with highly vicious materials. Advantages: Low cost, simple, dependable. Image Credit: Babbitt Inc   

Float level sensors can be used to determine the interface level in oil-water separation systems, in multi-parameter sensor systems

Optical sensor

optical level sensor

Image Credit: Gems 

Optical sensors use visible, infrared, or laser light to detect fluid level. They rely upon the light transmitting, reflecting, or refracting abilities of the material. Optical sensors can be used in contact and non-contact sensing. In non-contacting systems, the light aimed down on the surface of the fluid and the reflected light is detected by a photocell. Different fluids can be detected at different levels if multiple photocells are used. These sensors have a simple, straightforward design and are reliable sensors which do not be to be recalibrated between batches. They can be integrated in systems where they must be compatible with a variety of process materials and process conditions. Their response time is virtually immediate and highly accurate.

Optical sensors can be used to detect high foam levels or specific materials. They can also be used to determine if a material has reached a specific viscosity, density, opacity, or thermal conductivity condition. 

Pressure membrane

pressure membrane

Pressure membrane meters have a pressure-sensitive switch. They transmit pressure to an internal sensor via an organic or thin-metal membrane. Pressure membranes measure level based on the principle that pressure is proportional to the level of liquid multiplied by the specific gravity. In these devices the level equals the hydrostatic head (pressure) measurement divided by the density of the liquid. Image Credit: National Instruments

 

Radar or microwave

Image Credit: Babbitt Inc.

Microwave (aka RADAR) sensors use electromagnetic energy instead of air molecules to transmit the energy so they will penetrate temperature and vapors layers and can be used in a vacuum. Electromagnetic energy is reflected by objects with high conductive properties like metal and conductive water. There are two basic processing techniques. Time-Domain Reflectromety (TDR) is a measurement of time of flight divided by the speed of light. FMCW techniques can be found in Doppler systems. Microwave sensors are executed at different frequencies. Generally, the higher the frequency, the more accurate and expensive the sensor. Non-contact technology.

Microwave sensors are used in moist, vaporous, and dusty environments. They are also used in systems in which temperature varies.

Radio frequency

Radio frequency (RF) devices emit a high-frequency/low-power radio signal from a probe. When the material contacts the probe, the bridge becomes unbalanced and a comparing circuit realizes the change. This causes a relay output to change state.

 

Sonic or ultrasonic meters

ultrasonic level sensor

The sensors emit high-frequency acoustic waves that are reflected back to and detected by the emitting transducer. The sensor’s response is affected by turbulence, foam, steam, vapors, and changes in concentration. Proper mounting of the transducer is important to ensure a clear response to reflected sound and the tank should be free of objects that will distort the echo response. They can be used for continuous and point-level sensing as well as communicating with other devices. Ultrasonic level sensors are inexpensive and high functioning. Image Credit: Sensorsmag.com                       

Ultrasonic level sensors are used for non-contact applications with highly viscous and bulk solids. They are used in systems which require remote, wireless monitoring and plant network communications.

Tuning forks

Image Credit: Dwyer

These sensors use various technologies (typically piezoelectric) to vibrate a probe at about 85Hz and then constantly monitor the changes in the vibrations. Tuning forks detect the dampening that occurs when a vibrating probe is submerged in a process media. The probe is vibrated by one piezoelectric crystal, while another piezoelectric crystal detects the vibration. There are three designs; reed, probe and tuning fork. The designs are distinguished by their configurations and operating frequencies.   They can be used to detect for certain types of liquids. Tuning forks are simple, reliable, and can be cleaned in place. They can tolerate material build-up and can be coated for self-cleaning purposes.

Vibrating sensors are used to detect solid materials (powered, grains)

 

For specification specific information on each measurement technology see the chart in the Liquid Level Sensor Selection Guide.

 

Level Sensor Specifications

Level sensors carry specifications for measuring liquid in a container.

 

Range, Tank Height, Fill Height, and Dead Band Explained

Video Credit: Liquid Level Solutions

  • Range- Maximum measurement distance of the sensor. Most sensors are offered in two or three different ranges.
  • Tank height- The distance from the installed face of the transducer, down to the bottom of the tank.
  • Riser height- The distance from the face of the transducer to the top of the tank
  • Fill height- The distance from the bottom of the tank to the maximum desired liquid height.

  • Dead band- The minimum distance from the face of the transducer from which the sensor can measure.

Process Specifications

  • Allowable sensor over-range- Overrange specification limits the range of matching products to the specified range multiplied by a selected overrange factor. This keeps returned matches close to the range requirement and filters out products with a wider range than needed. "From," "To," and "Overrange" must all be specified to activate this option.

Example: User specifies a sensor with a range from 0-100. Without overrange specification, sensors with ranges of 0-100, 0-1000, and -10,000 to +10,000 are returned as matches. An overrange specification of 50% will filter out any sensors with a full scale range greater than 150.

  • Process operating conditions- There are two process operating conditions to consider:
    • Maximum operating pressure- The maximum operating pressure of the material for which the device is rated.
    • Material temperature range- The maximum material temperature for which the device is rated.

  • Mounting style- Mounting styles are defined as side-mount, top-mount, and bottom-mount. Mounting the sensor depends on pre-exiting system conditions, container characteristics, and if the sensor is for contact or non-contact measurements.

Electrical Specifications

  • Communication interface- There are two basic communication interfaces:
    • Serial- Serial communication means that data bits are sent in a serial (one after another) way over a single line. The advantages of serial communication are that the data can be sent further, and the cable connection is simpler and uses fewer wires. Serial interfaces use a serial communication protocol such as universal serial bus (USB), RS232, or RS485.  
    • Parallel- Parallel communication means that a number of data bits are sent at the same time. More information can be sent quickly and more reliably, but the processing time is longer.  Parallel interfaces use a parallel communication protocol such as general-purpose interface bus (GPIB).
  • Instrument style- In terms of instrument options for the user interface, level sensors may feature  
    • Non-electrical visual or audio outputs which are non-electrical visual indicators such as a simple float, or an audible click. Non-electrical visuals are inexpensive and the simplest to use and install but they are subject to user error in level reading.  
    • Analog meters or indicators which use an analog meter with a needle or an LED indicator to identify the level of fluid in the system.    
    • Digital readouts which feature numerical or application specific readouts. These displays are easy to read and can display significant digits.
    • Video display terminals (VDT) which include cathode ray tubes and flat panel display. VDT is a term used for the computer display consisting of a computer output surface and projecting mechanism that shows data.
  • For both types of level sensors, choices for output options include
    • Analog voltage signal
    • Analog current signal
    • Frequency/ modulated frequency

    • Alarm or visual indicator

Applications

Level monitoring is important in a wide variety of applications. Any industrial process involving the filling or storing a liquid in a tank or vessel would benefit from the use of a level sensor. They are also an important part of process control systems which manages flow rate into and out of a storage vat or reactor.

 

Image Credit: OMEGA

 

Level sensors can be used for high or low level sensing and alarms, leak detection, overfill shutoff and regulating the interface between levels of different media. They are found in a wide variety of industries-- not limited to-- manufacturing, food and beverage, chemical and pharmaceutical, marine, medical, and fuel/energy management.  

 

Resources

Introduction to Level Measurement  

What's the Right Level Technology for Your Application? 

Fraden, Jacob. Handbook of Modern Sensors: Physics, Designs, and Applications. New York: Springer, 2010. Print.

A Level Measurement Orientation

Differential Pressure Type Level Detector - Instrumentation

What is Serial Communication?

Parallel vs. Serial Data Transmission

Read user Insights about Level Sensors

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