Basics.
Level is measured at the position of the interface between phases, where the phases are liquid/gas, solid/gas, or immiscible liquid/liquid. Level is simply a measure of height. It defines the position of the interface, that is, the surface where the two phases meet with respect to a reference point. This measurement is often converted to a volumetric or gravimetric quantity.
· Direct Level Measurement
Direct methods employ physical properties such as fluid motion and buoyancy, as well as optical, thermal, and electrical properties. Direct level measurement does not require compensation for changes in level caused by changes in temperature. Direct level measurements show the actual level of the interface.
· Indirect Level Measurement
Indirect level measurement involves converting measurements of some other quantity, such as pressure to level by determining how much pressure is exerted over a given area at a specific measuring point, the height of the substance above that measuring point can also be determined. For example, the formula used to determine the height of water in an open tank is:
h = P / .433 psi
where:
h = height,
p = pressure indicated on a gage,
.433 psi = pressure exerted by one square inch of water, one foot high.
For substances other than water, the liquid's specific gravity (the ratio of the liquid's density to water's density) must be factored into the level calculation:
h = P / .433 psi (G)
where:
G = specific gravity
Temperature can also affect the accuracy of indirect level measurement. Substances have a tendency to expand when heated and contract when cooled. Gases are greatly affected by changes in temperature, while solids are affected very little. Because indirect level measurement is sensitive to specific gravity and the effects of temperature, it is necessary to compensate for these factors to ensure accurate measurement.
· Continuous Level Measurement
In many processes, continuous level measurement is required because it is necessary to know at all times the exact position of the interface in relation to one or more specific reference points. A gage or sight glass, can be used to continuously observe the position of the interface.
· Point-to-Point Level Measurement
Certain processes require only that the level of a substance be maintained between two points. Frequently these two points are a high level and a low level. When this is required, a point-to-point level measurement system is used. Such a system activates control devices only when predetermined levels are reached.
· Selecting Measurement Devices
Some level measurement methods and devices are better suited to point measurement. When selecting a measuring device, it is important to consider the operating parameters and the physical and chemical properties of the process materials.
Visual sensors.
· Dip Sticks and Lead Lines
A dip stick is essentially a stick or rod that is calibrated to indicate level. The dip stick is lowered vertically into a tank or vessel until it reaches a reference point. Usually the bottom of the tank is used to ensure that the dip stick is inserted to the correct depth. The dip stick is then withdrawn and the level is ready by determining where the interface last made contact with the dip stick. Reading the scale on the dip stick indicates the level measurement. A lead line acts in the same way as a dip stick. A steel measuring tape with a weight attached, the lead line can be used in most places that the dip stick can. Since the lead line can be rolled up into a smaller, compact unit, it is often easier to handle than a dip stick.
· Sight Glasses and Gage Glasses
The sight glass is an important method for visually determining level. The sight glass is a transparent tube of glass or plastic mounted outside the vessel and connected to the vessel with pipes. The liquid level in the sight glass matches the level of liquid in the process tank.
In process systems that contain a liquid under high pressure a reflex sight glass is used. This device is armored, to permit it to tolerate higher temperatures and higher pressures. Gage glasses are typically glass covered ports in a vessel that make it possible to observe the level of the substance in the vessel. Many gage glasses will have a scale mounted on the tank that allows the level to be read.
· Float Devices
These devices operate by float movement with a change in level. This movement is then used to convey a level measurement. An object of lower density than the process liquid is placed in the vessel, causing it to float on the surface. The float rises and falls with the level, and its position is sensed outside the vessel to indicate level measurement.
· Magnetic-Type Float Devices
Floats can also be used with magnets to detect and indicate level. This type of measurement system uses the attraction between two magnets to follow the level of a process liquid.
Variable displacement sensors.
When a body is immersed or partly immersed in a liquid, it loses weight equal to the liquid weight displaced. Variable displacement level devices utilize this principle by measuring the weight of the immersed displacer.
· Archimedes' Principle
Archimedes' Principle states that a body immersed in a liquid will be buoyed up by a force equal to the weight of the liquid it displaces. This upward pressure acting on the area of the displacer creates the force called buoyancy.
· Principles of Variable Displacement
The float displaces its own weight in the liquid in which it floats. It will sink into the liquid until a volume of liquid is displaced that is equal in weight to that of the float. When the specific gravity of the liquid and the cross-sectional area of the float remain constant, the float rises and falls with the level. So, the float will assume a constant relative position with the level and its position is a direct indication of level. The amount of liquid displaced by variable displacers depends on how deeply the device is submerged in the liquid. With variable displacement devices, the amount of displacement varies with the level of the liquid.
The span of the displacer is the distance that the displacer will respond to the forces of buoyancy. Buoyant force depends on the amount of liquid displaced and the density of the liquid. It is important to note the relationship of specific gravity to the change in weight of the displacer as the level changes. Displacers used in liquids with lower specific gravity will not change weight as dramatically as those used in liquids with higher specific gravity. This is why displacer level measuring systems are not used in applications where they could be immersed in liquids of varying specific gravities.
· Liquid-Liquid Interface Measurement
An advantage of variable displacers is that they are capable of detecting liquid-liquid interfaces as well as liquid-gas interfaces. When a displacer is used to determine the level of an interface between two liquids, it is always completely submerged.
· Variable Displacement Level Measuring Devices
A displacer must be connected to a measuring mechanism which, when sensing the changes in buoyant force, converts this force into an indication of level. A displacer body can be suspended directly in a tank, or installed in a float chamber on the outside of the vessel. Torque tube displacer level instrument is suspended from an arm that is attached to a torque tube or torque rod. A knife-edge bearing supports the movable end of the torque tube. This type of bearing provides an almost frictionless pivot point. The torque tube must be sufficient strength to support the full weight of the displacer in the absence of buoyancy, or when the level is at minimum. It is a solid or hollow tube that transfers displacer motion to an electronic instrument or a pneumatic instrument that will produce a signal proportional to the changes in the weight of the displacer. Spring balance displacers are devices similar to torque tube displacers. In these devices, the torsional spring of the torque tube is replaced by a conventional range spring. The motion of the displacer is transferred to the indicator by means of magnetic coupling.
· Applications
Variable displacement level devices are most often used for local level indication or control. Because displacers are immersed in process fluids, their material of construction must be compatible with the process. Displacers are also extremely sensitive to changes in the density of process liquids. Provisions must be made to measure and compensate for such changes in density when variable displacers are used.
Pressure sensors.
Since level can be determined by pressure, or head, many pressure measuring devices are used for indicating level.
· Hydrostatic Pressure
A liquid at rest in a vessel exerts a pressure on the walls of the vessel. At any given point the pressure on the wall of the vessel is proportional to the vertical distance between that point and the surface of the liquid, and varies with the height of the liquid. The relationship between the weight produced by the vertical height of a column of water and the pressure exerted on the supporting surfaces of the vessel can be used to determine level. The relationship between pressure and level makes it possible to convert hydrostatic measurements directly to level in feet or inches. In the following equations, "WC" stands for water column and is usually omitted from equations as understood in discussions of hydrostatic pressure.
1 lb./in.2 = 2.31 feet water
= 27.7 inches water (WC)
1 psi = 2.31 feet
= 27.7 inches
· Open-Tank Head Level Measurement
If level is to be determined and indicated by measuring pressure, the specific gravity of the liquid must be known. The specific gravity of water is 1.00. If the liquid has a lower specific gravity, the pressure exerted by the column of liquid will be less than that exerted by a column of water of the same height. For liquids with a specific gravity greater than 1.00, the pressure exerted by the column of liquid will be greater. To compensate for the difference in specific gravity, the following equation is used:
h = (p (2.31 ft.)) / G
where:
h = height in feet
p = pressure
G = specific gravity
· Diaphragm Box
The diaphragm box is submerged in the process liquid and connected to a pressure gage by a gage line. The hydrostatic head produced by the level of the liquid in the tank exerts pressure on the bottom of the diaphragm causing it to flex upward. This action compresses the gas in the box and the gage line. The pressure is applied to a gage or other pressure element that is part of an indicator assembly calibrated to indicate liquid level units.
· Air-Trap Sensors
As the liquid level rises, the hydrostatic head forces liquid up into an air trap sensor, or inverted bell. As the level of the liquid rises, it compresses the air trapped in the bell and the gage line until an equilibrium between the air pressure and the pressure exerted by the hydrostatic head is reached.
· Air Bubble or Surge Tube
Known by various names, including an air bubble, a surge tube, an air purge and a dip tube, this type of system uses a continuous air supply that is connected to a tube that extends into the tank to a point that represents the minimum level line. An air regulator controls the air flow. It increases air flow to the tube until all liquid is forced from the tube. At this pressure and flow rate, the air begins to bubble out of the bottom of the tube. This indicates that the air pressure forcing the liquid out of the tube is equal to the hydrostatic head produced by the height of the process liquid being forced into the tube. The air pressure acting against the hydrostatic head provides the pressure indication to the gage.
This is most useful for applications such as underground tanks and water wells. However, as with other hydrostatic pressure systems, the major limitation of these systems is that they are generally limited to open-tank applications.
· Closed-Tank Applications
In open tanks, measurements are referenced to atmospheric pressure. At atmospheric pressure, the pressure on the surface of the liquid is equal to the pressure on the reference side of the pressure element in the measuring instrument. When atmospheric pressure changes, the change is equal on both the surface of the liquid and the reference side of the measuring element. To compensate for the effects on level measurement caused by such pressure variations in closed-tank applications, a differential pressure (d/p) cell is often used to measure and indicate level. The d/p cell only responds to differences in pressure applied to two measuring taps. One pressure tap is the measuring point on the tank, which is usually below the minimum level point for the liquid. The other tap is usually located near the top of the tank. The tap in the liquid region of the tank is referred to as the high-side; the other tap, located above the level of the liquid, is referred to as the low-side. System pressure is sensed by both the high and low sides. In addition to system pressure, the high side also senses the pressure exerted by the height of the liquid. Since both sides are exposed to the same system pressure, the effects of system pressure are canceled and the differential pressure cell only indicates liquid level.
An instrument can be calibrated to compensate for the additional static pressure created by the condensed liquid. This compensation or adjustment is called zero elevation. Other means are also available to eliminate inaccuracies due to wet leg problems. For instance, in what is referred to as a wet-leg installation, the low pressure leg is deliberately filled with liquid. Another method involves the use of a device called a pressure repeater or one-to-one relay. The repeater is installed at the top of the tank and linked by pipe to an air relay. The pressure in the tank actuates the air relay, which is connected to an air supply. When the pressure in the tank increases, the relay increases the air pressure on the low-pressure leg. The relay regulates the air pressure so that it is equal to that of the tank pressure. When the pressure in the tank decreases, the relay vents air from the low pressure leg to maintain the equilibrium. Zero suppression, is the correction adjustment required to compensate for error caused by the mounting position of the instrument with respect to the level measurement reference.
Electrical sensors.
· Capacitance
A capacitor consists of two plates separated from each other by an insulating material called a dielectric. In applications involving capacitance measuring devices, one side of the process container acts as one plate and an immersion electrode is used as the other. The dielectric is either air or the material in the vessel. The dielectric varies with the level in the vessel. This variation produces a change in capacitance that is proportional to level. Thus, level values are inferred from the measurement of changes in capacitance, which result from changes in the level.
Capacitance type level measurement devices offer many advantages. Simple in design, they contain no moving parts and require minimal maintenance. The availability of corrosive resistant probes is also an advantage. Measurement is subject to error caused by temperature changes affecting the dielectric constant of the material. If the probes should become coated with a conductive material, errors in measurement may occur.
· Conductivity
A material's ability to conduct electric current can also be used to detect level. This method is typically used for point measurement of liquid interfaces of relatively high conductivity. Conductivity applications are usually limited to alarm devices and on/off control systems. A common arrangement is two electrodes positioned at the top in a tank. One extends to a minimum level and the other is positioned so that its lower edge is at the maximum level. The tank is grounded and functions as the common, or third electrode. Usually, a stilling well is provided to ensure that the interface is not disturbed and to prevent false measurement.
There are limitations to the conductivity method. The first is process substance must be conductive. Second, only point detection measurements can be obtained. The possibility of sparking also makes this method prohibitive for explosive or flammable process substances.
Advantages include low cost and simple design, as well as the fact that there are no moving parts in contact with the process material. These advantages make this type of system an effective method of detecting and indicating level for many water-based materials.
· Resistance
Resistance type level detectors use the electrical relationship between resistance and current flow to accurately measure level. The most common design uses a probe consisting of two conductive strips. One strip has a gold-plated steel base; the other is an elongated wire resistor. The strips are connected at the bottom to form a complete electrical circuit. The upper ends of the strips are connected to a low voltage power supply. The probe is enclosed in a flexible plastic sheath which isolates the strips from the process material. As the level of the process material rises, the hydrostatic pressure forces the resistance strips together up to the interface. This action shorts the circuit below the interface level, and total resistance is reduced proportionately. Resistance sensing devices can be used for liquid-gas interfaces and for slurries or solids. As with the other electrical level sensors discussed, resistance-type level detectors require relatively little maintenance.