Level Measuring Instrumentation
Local level-measuring instruments can include diagnostics as well as configuration and process data that can be communicated over a network to remote monitoring and control instrumentation. Some of the most commonly used liquid level measurement methods are:
- RF capacitance
- Conductance (conductivity)
- Hydrostatic head/tank gauging
Radio Frequency(RF) technology uses the electrical characteristics of a capacitor, in several varies configurations for level measurement. Often stated as RFcapacitance or simply RF. The method is suited for detecting the level of liquids, slurries, granulars, or interfaces contained in a vessel. Designs are available for measuring process level at a specific point, at multiple points, or continuously over the entire vessel height. Radio frequencies for all types range from 30 kHz to 1 MHz.
With the tank or vessel is empty, the insulating medium between the two conductors is air. With the tank full, the insulating material is the process liquid or solid. As the level rises in the tank to start covering the probe, some of the insulating effect from air changes into that from the process material, producing a change in capacitance between the sensing probe and ground. This capacitance is measured to provide a direct, linear measurement of tank level. When the process material is conductive, the sensing probe is covered with an insulating sheath such as Teflon or Kynar. The insulated probe acts as one plate of the capacitor, and the conductive process material acts as the other. The latter, being conductive, connects electrically to the grounded metallic tank. The insulating medium or dielectric for this application is the probe´s sheath. As the level of conductive process material changes, a proportional change in capacitance occurs. Note that this measurement is unaffected by changes in the temperature or exact composition of the process material.
The conductance method of liquid level measurement is based on the electrical conductance of the measured material, which is usually a liquid that can conduct a current with a low-voltage source (normally <20 V). Hence the method is also referred to as a conductivity system. Conductance is a relatively low-cost, simple method to detect and control level in a vessel.
A usual way to set up an electrical circuit is to use a dual-tip probe that eliminates the need for grounding a metal tank. Such probes are generally used for point level detection, and the detected point can be the interface between a conductive and nonconductive liquid.
Typically two dual-tip probes that detect maximum and minimum levels are inserted. When the level reaches the upper probe, a switch closes to start the discharge pump; when the level reaches the lower probe, the switch opens to stop the pump.
One of the most common methods of measuring liquid level is to measure the pressure exerted by a column (or head) of liquid in the vessel. The density of a liquid varies with temperature. For the highest precision in level measurement, the density must therefore be compensated for or expressed with relation to the actual temperature of the measured liquid. This is the case with hydrostatic tank gauging (HTG) described below.
DP-type instruments were used to measure liquid level. Orifice meters, originally designed to measure differential pressure across an orifice in a pipeline, readily adapted to level measurement. Today´s smart DP transmitters adapt equally well to level measurements and use the same basic principles as their precursors. With open vessels (those not under pressure or a vacuum), a pipe at or near the bottom of the vessel connects only to the high-pressure side of the meter body and the low-pressure side is open to the atmosphere. If the vessel is pressurized or under vacuum, the low side of the meter has a pipe connection near the top of the vessel, so that the instrument responds only to changes in the head of liquid.
Hydrostatic Tank Gauging
A specialized application for systems that involve hydrostatic measurements is hydrostatic tank gauging (HTG). It is a standard way to accurately gauge liquid inventory and to monitor transfers in tank farms and similar multiple-tank storage facilities. HTG systems can provide accurate information on tank level, mass, density, and volume of the contents in every tank. These values can also be networked digitally for multiple remote access by computer from a safe area.
Mass (weight) of the tank´s contents can be calculated from the hydrostatic head (measured by PT) multiplied by the tank area (obtained from a lookup table). The liquid´s temperature-density relationship can be used to calculate the volume and level, provided the tank is not under pressure. Data fed into a computer system make it possible for all calculations to be automatic, with results continuously available for monitoring and accounting purposes.
The level transmitter, with its probe installed at an angle into the bottom portion of the tank, is an innovative way to detect accumulation of water, separated from oil, and to control withdrawal of product only. Moreover, by measuring the water-oil interface level, the LT provides a means of correcting precisely for the water level, which would incorrectly be measured as product.
Though the DP transmitter used to measure hydrostatic pressure for level measurement other methods are available. For example, a pressure transmitter in the form of a stainless steel probe that looks much like a thermometer bulb. The probe is simply lowered into the tank toward the bottom, supported by plastic tubing or cable that carries wiring to a meter mounted externally on or near the tank. The meter displays the level data and can transmit the information to another receiver for remote monitoring, recording, and control.
A further hydrostatic measuring device is a dry-cell transducer that aims to prevent the pressure cell oils from contaminating the process fluid. It incorporates special ceramic and stainless steel diaphragms and is apparently used in much the same way as a DP transmitter.
Radar or Microwave
Radar methods of level measurement are sometimes referred to as microwave types. Both use electromagnetic waves, typically in the microwave X-band (10 GHz) range. This technology is modified and refined for level measurement. Most applications have been designed for continuous level measurement. All types operate on the principle of beaming microwaves downward from a sensor located on top of the vessel. The sensor receives back a portion of the energy that is reflected off the surface of the measured medium. Travel time for the signal (called the time of flight) is used to determine level. For continuous level measurement there are 2 main types of noninvasive systems, as well as one invasive type that uses a cable or rod as a wave guide and extends down into the tank´s contents to near its bottom.
One type of noninvasive system uses a technology called frequency-modulated continuous wave (FMCW). From an electronic module on top of the tank or vessel, a sensor oscillator sends down a linear frequency sweep, at a fixed bandwidth and sweep time. The reflected radar signal is delayed in proportion to the distance to the level surface. Its frequency is different from that of the transmitted signal, and the two signals blend into a new frequency proportional to distance. That new frequency is converted into a very accurate measure of liquid level.
The second noninvasive technology, pulsed radar or pulsed time-of-flight, operates on a principle very similar to that of the ultrasonic pulse method. The radar pulse is aimed at the liquid´s surface and the transit time of the pulse´s re turn is used to calculate level. Because pulse radar is lower power than FMCW, its performance can be affected by obstructions in the tank as well as foam and low-dielectric materials (K < 2).
Guided-wave radar (GWR) is an invasive method that uses a rod or cable to guide the micro wave as it passes down from the sensor into the material being measured and all the way to the bottom of the vessel. The basis for GWR is time-domain reflectometry (TDR) which has been used for years to locate breaks in long lengths of cable that are underground or in building walls. A TDR generator develops more than 200,000 pulses of electromagnetic energy that travel down the waveguide and back. The dielectric of the measured fluid causes a change in impedance that in turn develops a wave reflection. Transit time of pulses down and back is used as a measure of level.
Ultrasonic and Sonic
Ultrasonic and sonic level instruments operate on the basic principle of using sound waves to determine fluid level. The frequency range for ultrasonic methods is ~20200 kHz, and sonic types use a frequency of 10 kHz. A top-of-tank mounted transducer directs waves downward in bursts onto the surface of the material whose level is to be measured. Echoes of these waves return to the transducer, which performs calculations to convert the distance of wave travel into a measure of level in the tank. A piezoelectric crystal inside the transducer converts electrical pulses into sound energy that travels in the form of a wave at the established frequency and at a constant speed in a given medium. The medium is normally air over the material´s surface but it could be a blanket of nitrogen or some other vapor. The sound waves are emitted in bursts and received back at the transducer as echoes. The instrument measures the time for the bursts to travel down to the reflecting surface and return. This time will be proportional to the distance from the transducer to the surface and can be used to determine the level of fluid in the tank. For practical applications of this method, you must consider a number of factors.
A few key points are:
- The speed of sound through the medium (usually air) varies with the medium´s temperature. The transducer may contain a temperature sensor to compensate for changes in operating temperature that would alter the speed of sound and hence the distance calculation that determines an accurate level measurement.
- The presence of heavy foam on the surface of the material can act as a sound absorbent. In some cases, the absorption may be sufficient to preclude use of the ultrasonic technique.
- Extreme turbulence of the liquid can cause fluctuating readings. Use of a damping adjustment in the instrument or a response delay may help overcome this problem.
To enhance performance where foam or other factors affect the wave travel to and from the liquid surface, some models can have a beam guide attached to the transducer.
Although it is a relatively expensive solution, ultrasonic or sonic methods can also be used for point level measurement, An ultrasonic gap technique is an alternative way to measure point level with low-viscosity liquids. A transmit crystal is activated on one side of a "measurement gap" and a receive crystal listens on the opposite side. The signal from the receive crystal is analyzed for the presence or absence of tank contents in the measurement gap.