Level instruments are a class of instruments designed to display and/or transmit a signal that corresponds to the level of the material that is being measured within a containing vessel.
Integral to process control in many industries, there are three primary applications for level instruments: transmitting, switching, and indicating/controlling.
Types of Level Instruments:
Level Transmitters provide continuous level measurements over the range of the system rather than at a single point and produce an output signal that directly correlates to the
level in the vessel. The output signal generated can be used to display the depth or to actuate control functions.
Level Switches are point level measurement instruments used to mark a single, preset level condition such as high or low level condition that would trigger an alarm or switch.
Level Indicators / Controllers : Level indicators utilize a mechanical device, such as a float, or a sensor with output signal to indicate the level within a tank. Level controllers
are, essentially, indictors with integrated control output which allows for functions such as on/off, alarms, etc.
Level Instruments Technology
Float
Float level instruments are mechanical devices that move in conjunction with the level of the liquid medium. The movement of the float indicates the level and can actuate a switch or create an
output signal. Float level instruments are an economical and practical solution.
Depth / Pressure
Pressure based level instruments use pressure sensors at the bottom of a tank to measure the force exerted per unit area. For level applications, that force is the weight of the medium to be
measured plus any pressure acting on the surface. By knowing the density of the medium and compensating for the effect of ambient pressure, the resulting value can be converted to a level
measurement. The output signal generated can be used to display the depth or to actuate control functions.
Radar
Radar level instruments measure the time required for a microwave pulse and its reflected echo to make a complete round trip between the transducer and the sensed material level. That value is
then converted into a level measurement.
Radar level instruments utilize non-contact measurement unaffected by changes in temperature, pressure or the presence of vapor. Accuracy of radar level instruments depend upon bandwidth and
frequency but is unaffected by density, conductivity or dielectric constant of the medium.
Ultrasonic
Ultrasonic level instruments use sound waves to determine the level of liquids, solids, and slurries. Consisting of two elements, a high efficiency transducer and an electronic transceiver,
these non-contact systems measure the time for an ultrasonic pulse and reflected echo make a complete round trip between the transducer and the sensed material level. Ultrasonic systems can also
measure open channel flow, volumetric output, and differential level.
While highly accurate, ultrasonic level instruments must compensate for temperature. Applications involving heavy dust, foam or surface turbulence are not advised as the ultrasonic pulse can be
absorbed or otherwise distorted affecting accuracy.
Capacitive
Capacitance level instruments operate on the electrical characteristics of a capacitor. A capacitor is made of two conductive plates isolated from one another by a dielectric. Capable of storing
an electrical charge, that charge varies depending upon the conductivity of the dielectric.
In level applications, the medium to be measured serves the dielectric while the tank wall and sensor are the conductive plates. As the tank fills the sensor probe is exposed to a more conductive
material than air and the capacitance value of the probe increases. This value is converted to an output signal that can be used to display the level or activate a switch or control.
Capacitance level instruments do not work well with low capacitance materials like plastic, sand, etc. or with materials having a large particle size (and thus a lot of air pockets). Material
build-up along the probe can also make readings inaccurate.
Conductive
Conductivity level sensors use a low-voltage, current-limited power source applied across separate electrodes to detect the medium's resistance when their electrodes are covered by the process
material. Normally used with water, wastewater, water-based liquids; conductive level probes are simple to install and use. They have the added benefit of being solid state.
Conductivity level sensors provide point level detection and are adept at switching functions for minimum, maximum or differential applications. If buildup insulates the probe from the medium,
it will stop working properly.
Vibrating Fork
Vibrating fork level instruments are designed to vibrate at its frequency by a pair of piezoceramic discs. When the forks come in contact with the medium the frequency and amplitude changes.
That change is detected and converted into a switch signal. Tuning forks are particularly good for high viscosity materials. Models with high excitation frequency ensure interference free
applications in turbulent surfaces.
Optical
Optical level instruments contains an infrared LED and a light receiver. Light from the LED is directed into a prism which forms the tip of the sensor. With no liquid present, light from the
LED is reflected within the prism to the receiver. When rising liquid immerses the prism, the light is refracted out into the liquid, leaving little or no light to reach the receiver. Sensing
this change, the receiver actuates electronic switching within the unit to operate an external alarm or control circuit.
Points to consider when selecting level instruments:
- What is the material to be measured?
- Is the sensor to be placed inside or outside of the chamber?
- Do you need contact or non-contact measurement?
- What degree of accuracy is required?
- What are the pressure and temperature ranges needed?
- Is point monitoring or continuous measurement needed?
- What measurement range is needed?
- What type of output is needed?
- Is there any build-up of dust or foam to consider?
- What is the material of the tank?
If you have any questions regarding level instruments please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
