Level Measurement

Level sensors are the devices which are used to detect the level of flowing substances such as liquids, slurries, granular materials, and powders etc. Due to gravity, all these substances flow to form a basic level in their physical boundaries. These flowing substances can have their boundary either in the form of a container (vessel) or natural form like a river or a lake.

Level Measurement Methods

The two major methods of level measurement are:

  1. Continuous level method: In this method, continuous level sensors are employed which determine liquid level within a given range instead of at a single point. These sensors usually indicate the correct amount of substance in the specified area. These sensors are extra sophisticated and capable enough to perform level monitoring for the complete system. Via this method an analog output is generated which directly correlates to the level in the container.
  2. Point level method: In this method, point level sensors are used which can only detect whether the substance level is above or below the sensing point. This method is typically used to deal with fluid levels which are exceptionally high or low.

Selection of Level Sensors

While selecting an optimum type of level sensor for a particular application, one must resolve following issues:

  1. Which substance need to be measured, for example, liquid, slurry, solid, interface, granular, or powder.
  2. Specify whether the level sensor can be inserted into the tank or kept outside.
  3. Decide upon the suitability of continuous level or point sensor for the given application.
  4. Specify whether the substance being measured is electrically conductive or not.
  5. State the type of output required i.e. analog, relay or digital display.
  6. Indicate the required temperature and pressure ranges depending upon the application.
  7. Specify the required level measurement range.
  8. Identify whether turbulence, foam or vapor arises at the surface of the liquid or not.
  9. Make out whether material under consideration coats or builds up on surfaces.

In short, various physical and application variables which may affect the selection criteria of liquid level measurement sensors consist of:

Physical Constraints:

  • Phase of the substance under measurement i.e. liquid, solid or slurry
  • Temperature
  • Pressure or Vacuum
  • Chemistry
  • Dielectric constant of medium
  • Density or Specific gravity of medium
  • Agitation
  • Noise either acoustical or electrical
  • Vibration
  • Mechanical shock
  • Tank or bin size and shape

Application Constraints:

  • Cost
  • Accuracy
  • Appearance
  • Response rate
  • Ease of calibration or programming
  • Physical size and installation of the instrument
  • Monitoring or control of continuous or discrete i.e. point levels

Types of Level Sensors

Level measurement is not merely the determination of the presence or absence of a liquid at a particular level. It is a much more complex process than this. Nowadays many advance level-measuring devices are available which comprise of diagnostics as well as configuration. They are competent enough to transfer process data over a network for remote monitoring and control instrumentation. Some of the level measurement devices include PID control as well. Most widely used liquid level sensors are given below:

Float Switches

These are point level sensors which include a magnetic float moving with the liquid surface. It activates a hermetically sealed “reed switch” in the stem. This is a simple type of flow sensor which offers many advantages like

  • Less maintenance
  • Easy installation
  • Minimized shock, vibration, and pressure

A reed switch is mainly of two types i.e. single pole, single throw (SPST) or single pole, double throw (SPDT).

Non-Contact Ultrasonic Sensors

These types of sensors consist of following elements:

  • Analog signal processor
  • Microprocessor
  • Binary coded decimal i.e. BCD range switches
  • Output driver circuit

The microprocessor generates the transmit pulses and a gate signal which passes through the analog signal processor to the sensor, which then transmits an ultrasonic beam to the fluid surface. An echo is returned from the surface which is detected by the sensor and transferred back to the microprocessor. This is done to digitally represent the distance between the sensor and the surface level. The microprocessor keeps a constant record of received signals and hence calculates averaged values for measurement of liquid level.

Contact Ultrasonic Sensors

These types of ultrasonic sensors consist of no moving parts and hence need no calibration. These sensors normally find their use in vessels or pipes to automatically run pumps and activate solenoid valves and high/low alarms. These sensors are well-suited for almost all liquids and are not affected by coatings, clinging droplets, foam, and vapor. However, these sensors pose problem when they are employed with extremely aerated liquids and liquids viscous enough to clog the sensor gap.

Capacitance Level Sensors

Just like ultrasonic sensors, these types of sensors can also offer both point and continuous level measurement. These sensors basically consist of a probe to examine liquid level changes in the tank. Electronically outputs are conditioned to capacitive and resistive values, which are then transformed into analog signals. The probe employed for measurement purpose can be either rigid or flexible. In general, rigid probes are applied in turbulent situations, since they tend to offer higher stability in such cases. However, flexible probes come into use when there is not adequate clearance available for a rigid probe or in applications where very long lengths are required.