To qualify as a universal level technology, there are a number of important criteria which must be met. The first test is the ability to measure both point level (on/off) and continuous level (analog, typically 4-20 mA).
measurements, as the name suggests, are those which indicate whether the material level has risen or fallen to some predetermined point in the vessel. Some typical examples of these measurements are "over-fill" protection, pump cavitation protection, pump start and stop control, and "plugged chute" detection. Historically, the output mode of a point level control has been one or more pairs of DPDT dry relay contacts. However, the trend is continually moving in the direction of a switched change between high and low current in the 4-20 mA range. This current change is then transmitted to a receiver in the control room which may be a process controller, PLC, or other similar device. The receiver then converts the current signal to an alarm or control action. A universal point level device should be able to provide either output mode.
measurements, on the other hand, provide a continuous reading of the actual material level in a vessel. The output mode of a continuous level measurement is typically a 4-20 mA signal that is proportional to the level. Because users frequently want the information in their units of choice, the continuous level signal should indicate how many feet of level or gallons (volume) or pounds (weight) are present. Obviously, a successful level technology will be able to provide the answers in the most useful form. While a 4-20 mA signal is still the most common format for this class of instrumentation, other forms such as the digital formats of RS-232 and 422 and the not yet finalized Fieldbus, are gaining acceptance as we move toward increased automation.
The second test of universality is the ability of the technology to measure all four types of materials found in industry today. The four types of materials are: liquids, granulars, slurries, and interface.
The third test of universality is the ability to measure level under the wide range of process conditions commonly found in industry. The conditions of most concern are: temperature, pressure, agitation, corrosion (chemical compatibility), foaming, explosion hazards, and potentially lethal materials.
Physical and Chemical Properties
The fourth test of universality is making a measurement independent of changes in material density, chemical composition, build-up, viscosity, or electrical properties. Certain technologies depend on some of these specific properties to make their measurements and are, therefore, sensitive to changes in them.
What is RF Admittance? Why is it a Universal Technology?
is a universal technology because when measuring level it is measuring something common to everything in the universe. All things, even a vacuum, have an electrical property called dielectric constant
(k). The other electrical property is conductivity
(g) which is the nemesis of capacitance
Radio frequency admittance
is a technology that takes both properties into account when computing the level. The measurement is made at a predetermined radio frequency rate. That frequency is usually between 15 Khz and 400 Khz. Hence the name radio frequency admittance. For information on how more materials affect the measurement, see the section on "Properties of Materials" in this article.
2.1 Two-Terminal Capacitance Point Level Controls
A discussion of the operating theory of an on/off level control should begin with the two-terminal capacitance-type system. Please refer to Fig. 2-1A which shows a block diagram of a typical twoterminal on/off capacitance level control and the connecting cable and sensing element that make it a level control system.
The level control is typically powered by a 120-volt 60 Hertz power source, although 50-cycle and 230- volt are available as options. Internally, there is a power supply that converts ac to 24 Vdc in order to run various parts of the circuit. The oscillator circuit generates a sinusoidal radio frequency wave form of about 100 khz, and is connected to a bridge circuit.
The bridge circuit contains the tuning adjustment which balances out the capacitance of the probe shown on Cprobe. The bridge circuit is connected directly to the sensing element (probe) via a connecting coaxial cable. The shield of the coaxial cable is connected to ground which is one side of the measurement. The sensing element end of the cable has its shield connected to the condulet, protecting the terminations of the sensing element. Because the condulet and probe body are threaded into the vessel, the vessel is also grounded through this connection.
During the calibration procedure, the capacitance of the probe in the vessel is balanced out by the tuning capacitor and bridge balance is obtained. When the coaxial cables are more than a few feet long, it is also necessary to add a capacitance equal to the effect of the capacitance of the coaxial cable itself. This capacitance is shown on the drawing as Cpad.
In the tuning process, the capacitance of the cable and the probe are balanced out, and out put to the demodulator is essentially O volts. When the level in the vessel rises, a greater capacitance occurs. This causes a change in the signal being sent to the demodulator, where it is converted to a de voltage proportional to the unbalance.
This change in signal, caused by the increase in level, is amplified and used to energize the relay shown in the diagram. The relay contacts are then used to sound an alarm or activate the appropriate control.
The simple capacitance level control system has several drawbacks and has been almost completely phased out in favor of the threeterminal or Cote-Shield type level control systems. The main drawback of simple capacitance is illustrated in Fig. 2-1 B.
After the level has once covered the probe and then fallen away, there may be a coating left on the probe that can cause the instrument to react as though the level was still on the probe. A problem usually occurs when the material leaves a conductive coating that goes all the way to the mounting threads. In general, coating problems are less severe with insulating materials. If there is a problem, it can sometimes be avoided by mounting the probe vertically, so that the coating never reaches the mounting threads. In this case, even if the material leaves a conductive coating, the operate point will be at the tip of the probe.
The second drawback to the two-terminal capacitance system is that the connecting cable acts as a large capacitor which is not stable with temperature. Changes in cable capacitance will appear to be the same as changes in vessel level. When this happens, there can be considerable errors in applications involving low dielectric materials. The errors and shortcomings found in two-terminal capacitance systems have been overcome by the use of three-terminal or Cote-Shield systems.
2.2 Three-Terminal Cote-Shield Point Level Controls
In comparing Fig. 2-2 with Fig. 2-1A, the major difference between the two circuits is the presence of a Cote-Shield amplifier in the three-terminal system. This amplifier is designed to have a gain of exactly "1 ", with its output being of voltage and phase identical to the input, but at a very low impedance, "Z". The output is connected to the shield of the coaxial cable and then to a shield ring on the probe that is called the Cote-Shield element. Ground is carried along the same cable, as a separate wire, and connects the chassis ground of the electronics to the vessel. This causes the outside of the vessel to be ground.
Observing the waveform diagrams in this figure, both the center wire and the shield wire of the coaxial cable are at exactly the same voltage and phase at all times. Since they are always at the same potential, there can be no current flow through the insulation capacitance of the cable. Thus there is no change in calibration due to the temperature effects of the cable. This makes it possible to lengthen or shorten the cable connecting the probe to the electronics without changing the original calibration. This is not true with two-terminal capacitance systems.
Fig. 2-3A shows an exaggerated view of how a coating may look in a level control system. Fig. 2-38 shows an electrical equivalent circuit of this coating left on the probe.
The center wire of the coax is connected to the center rod of the probe, and the shield of the coax is connected to the middle element of the probe, called the Cote-Shield element. The ground wire of the cable is connected to the condulet and, therefore, to the vessel body. As in the case of the coax, since there is no difference in voltage between the probe element and the CoteShield element, there can be no current flow through any resistance due to the coating that may exist on the insulator.
The electronic instrument measures only the current that travels from the probe or sensing element center wire to ground, and this can only occur through the actual material being measured. There will be a current flowing from the Cote-Shield element to the vessel wall because of the potential difference that exists there.
However, this current is not measured, and will not cause the instrument to indicate high level. When the level in the vessel does rise and touch the center rod of the sensing element, it causes a current to flow that is sensed by the demodulator and causes the relay to change states.
2.3 Three-Terminal, Two-Wire Cote-Shield Point Level Controls (LCT Level Control Transmitter)
The next logical development in on/off level controls is the three-terminal control which transmits data by means of the classic two wire loop. Theory of operation is very similar to the threeterminal control described earlier, except the power supply and output portions of the circuit are remotely mounted in the control room where they are normally used. This is particularly true when the LCT is used as an alarm system for overfill/ high level protection. See Fig. 2-4.
Although the field-mounted unit functions much like the line-powered unit described earlier, there are some unique differences. The two wires carrying the 24-volt power to the electronic unit in the field also indicate whether the level is above or below the sensing element set point. The receiver system measures the current drawn by the transmitter. For instance, when the current is 15-20 mA, the transmitter is in the "normal" condition. When the "alarm" condition occurs at the sensing element, the current drops to 4-10 mA and the receiver system signals "alarm".
If the current being drawn by the transmitter is other than the normal or alarm range, there is a loop malfunction. There are two basic types of receivers. One is the simple line fail-safe type. It provides continuous self-checking diagnostics with a common output for both alarm and loop malfunction conditions. The other type, known as "line-monitoring", has an additional status output which differentiates a fault condition (instrument failure or signal loop malfunction) from an alarm condition.
There are a number of important benefits to this three-terminal two-wire approach.
2.3.1 Intrinsic Safety
The transmitter and signal wires are intrinsically safe with or without separate barriers, and need not be in rigid conduit.
2.3.2 Remotely Mounted Relay Contact Outputs
Usually the alarm outputs are needed in the control room to actuate alarms and annunciators. With this system, the outputs are already in the control room.
2.3.3 Greater Personnel Safety
There is greater personnel safety because only 24 volts de is present in the field mounted transmitter. It can be checked safely when powered ("hot").
2.3.4 Self Checking/Diagnostics
The LCT two-wire system has self checking diagnostic capabilities and will set off an alarm if there is a loop or instrument malfunction.
2.3.5 Push-button Calibration and Level Verification
Some models of the LCT two-wire point level control are available with digital, remote calibration and level verification for maximum calibration security and operator convenience. See Section 5.5.
2.3.6 Verify System Function Validation Circuit
In theory, a high level alarm system is installed in a vessel to indicate when material has reached a given point The intent is for the system to prevent dangerous overfill conditions.
In practice, however, a vessel may go for months or even years without ever experiencing a high level condition. It is imperative that the high level alarm actually function when called upon to do so.
The Verify switch
offered by Drexelbrook Engineering simulates the effects of a high material level by imposing a calibrated amount of capacitance to the level sensor. By actuating the switch, the entire control loop is checked and verified. If any part of the system is non-functional, the Verify circuit will no indicate an alarm condition.
When comparing system check features offered by different manufacturers, it is important to understand exactly what is being offered. Many companies offer "self-check" or "system function validation" options which only check the function of the transmitter. This type of test does nothing to insure the integrity of the sensing element, wire connections, or calibration. The Verify circuit
offered by Drexelbrook Engineering meets or exceeds the following regulations:
• NFPA 30, section 10.2
• NJ uniform fire code, NJAC 5:18-1 et seq
• EPA oil pollution prevention act of 1990
• EPA storm water run off requirement
• EPA spill prevention control & countermeasure (SPCC) plan
• Superfund amendment & reauthorization act (SARA) section 313
• OSHA process safety management of highly hazardous chemicals
All of the above advantages lead to the single great advantage of lower installed cost. In the not-too-distant future, most point level controls will probably be of the three-terminal, two-wire type.
2.4 Three Terminal Cote-Shield Multipoint Level Controls
The Multipoint Series electronic unit
is designed for use with a two-terminal sensing element and three-terminal coaxial cable.