Industrial Liquid Level Measurement & Control Application Notes

If you would like would like one of our technical specialists to evaluate your application and provide a recommended equipment quotation then please contact us with a completed Applications Data Sheet. AMI routinely provides dimensioned sensor drawings for customer approval to ensure there are no surprises or interference issues.

The following brief applications information is provided. This is only a partial list of applications for which AMI can provide measurement solutions.

Aerospace Run Tank Management Cooling Towers Diesel Fuel Storage Electrostatic Desalters Gasoline/Water Interface High Pressure Applications Hydraulic Oil Hydrocarbon/Water Separation Liquid Propane and LNG Liquid Oxygen Media Detection Sewage Treatment Vessels Sump Level Control Washdown Area Applications

Applications Considerations

The most effective application of capacitance to measure levels are when the dielectric value of the media remains constant over the operating range of interest. If the transmitter is calibrated to this condition and it remains constant then the level reading will remain accurate. Several factors have the ability to introduce an error into the level reading by shifting the environment away from the calibrated condition.

Composition Changes - Most materials have differing dielectric values. If the composition of a mixture changes significantly over time then this may cause an inaccurate level measurement due to the shift in dielectric value. Low dielectric value materials (non-conductive) are most susceptible to this affect. Mixtures with high dielectric values (conductive materials) are much less affected by this change. Impurities to an otherwise constant composition material will have varying degrees of affect depending on their relative dielectric value difference from the bulk material.

Density Changes - In most plant operations the dielectric value of a material is not a monitored or considered an important parameter. A simple way to decide if changing dielectric values may be a problem is to determine if the density of the material is changing. There is typically a direct correlation between the density of a material and the dielectric value. One obvious parameter that can cause this change is variations in temperature another is composition. If your applications density in shifting by some % then you can expect a similar error in the level reading as referenced to the calibrated condition. If the density (inferred dielectric) is higher than the calibrated condition by 10% then your level reading will be 10% higher than the actual level. The opposite is true in reverse.

Moisture - Water is very conductive and has a high dielectric value. If the bulk material being measured is normally non-conductive but can contain some moisture then any change in the moisture content will affect the level reading. In a constant level condition, as the moisture increases the level reading will read false on the high side. This is a common problem if a hydrocarbon tank collects enough water in the bottom to eventually touch the bottom of the capacitance probe - at this point it will likely jump to a 100% reading. Other hygroscopic materials (i.e. those that absorb moisture from the air) like powders or granular solids (such as polymer pellets) can also be susceptible to this effect.

The other common moisture related issue occurs when water enters the probe cover or transmitter housing through the installed plant conduit. This will cause the circuit to read a false high level and remain at 100%. The culprit is normally found to be a poorly sealed junction point or cover in the conduit. One very quick and simple solution to this problem is to backfill the nipple section directly on top of the probe with a common silicon RTV caulk. This area must of course be dried thoroughly before doing so. This will effectively create a barrier and prevent water from shorting out the sensors' signal to the transmitter. This problem can be avoided in many cases if the conduit routing is such that the transmitter housing is not at a low point in the wiring system.

Coating or Material Build Up - In some processes material adheres to the capacitance probe and/or tank wall forming a coating over time. Normally this reaches some equilibrium state where the coating layer thickness remains basically constant. In these cases it is best to delay calibration of the transmitter until the level probe is fully coated. If this is not done then a false high reading will be seen periodically as the coating builds up and recalibration will be required each time to bring the instrument back into operation. Plant personnel and I&E technicians should also be aware that the level reading will conversely read low after a coating process is cleaned and put back into operation. If possible it is best not to recalibrate and simply wait for the coating to re-develop.

The adverse affects of coating are much more pronounced and difficult to deal with when the material is conductive or water based. An example of a compounded problem would be where a thick, moist conductive coating formed on the probe, a new calibration was done and the coating on the exposed probe dried out (became much less conductive). In this case the probe would give a false low level signal. Such cases are difficult to address and capacitance may not be the optimum technique to use. Systems where a periodic cleaning cycle occurs can often keep buildup within acceptable levels. Operators will often recognize the gradual creep of the signal due to coating.

Tank Agitation - In tanks with agitation it is obviously important to locate the probe away from moving blades to avoid damage to the probe or internal tank components. Agitation may produce a vortex which can raise the liquid level on the probe. This will be seen as an increase in the level reading. Liquid splashing may also occur and been seen as increased and/or erratic level readings. The use of a stilling well probe or electronic damping can minimize these effects.

Specific Applications

• Diesel Fuel Storage
  The AMI Model 175 Industrial Level Transmitter is an ideal way to provide reliable level measurement of Diesel Fuel in a storage tank, day tank or on transport vehicles. In storage tanks the transmitter is commonly calibrated with the tank full of water at 100% (20 mA) and with the tank full of diesel for 0% (4 mA). The transmitter then reads out the level of water in the tank bottom. A separate measurement device using a different technique is used to measure the actual total level of the combined liquid in the tank. The transmitter provides a 4-20mA continuous level signal. The onboard display and keypad provide a simple interface for calibration. There is also a version that is FM Approved as Explosion-proof and Intrinsically Safe. This is a complete liquid level measurement system.

On vehicle systems the transmitter and a dash mounted level readout are both powered from the vehicles 12VDC circuit. The rugged NEMA enclosure is well suited for direct mounting on the trailer tank either on top or remote mounting on the side for easier access.

• Sewage Treatment Vessels
  The AMI Model 175 Industrial Level Transmitter is able to monitor conditions in sedimentation basins and sludge tanks. This can insure throughput and prevent over-spill conditions. It is also well suited for lift station or sump level control + multi-pump operation (up to 4ea.). Capacitance is well suited because of it's relative immunity to errors caused by foam accumulation. Digester level sensing is also common. The transmitter can be used for level monitoring via the onboard display or level control with the 4-20mA signal.

• Sump Level Control
  The AMI Model 175 Industrial Level Transmitter can be used to control the level in a sump containing almost any liquid ranging from Hydrocarbons to water. A continuous 4-20mA signal, proportional to the level in the sump, is the standard output. Setpoints are achieved with the use of "current trips". These setpoints can be placed anywhere along the length of the probe. Pump control function is also available.

• Hydrocarbon/Water (Oil/Water) Separation
  The AMI Model 175 Industrial Level Transmitter can easily detect the interface between any hydrocarbon and water. It allows you to draw off the water from the bottom of vessel, without draining hydrocarbons into the line. This liquid level measurement system is inexpensive protection against unwanted excursions and EPA regulation violations in Oil/Water Separators (OWS). The Model 175 is ideal for use in the last stage of a multi-stage separator.

• Media Detection
  The AMI Model 175 Industrial Level Transmitter can be used for more than just liquid level measurement. The RF Capacitance technology can also be used to detect a specific media in a vessel or pipeline. For example, placing a Teflon-sheathed probe into a pipeline, the level transmitter can detect when the pipe is empty; full of hydrocarbon; or full of water. The 4-20mA output would indicate 4mA for empty, 20mA for water, and in between (6-15mA) for hydrocarbon (depending upon the type of fluid). The Model 175 provides this function at a significant savings over a nuclear densitometer. Monitoring metals content in gearbox oil can detect when critical oil changes are needed prior to costly engine component wear or failure.

• Hydraulic Oil
  The AMI Model 175 Industrial Level Transmitter is able to measure the level of hydraulic oil in pressure vessels. The most common use is in the monitoring of "pressure pots" used in electric turbine power stations. An RF Capacitance transmitter is not affected by the pressure changes that occur in these vessels. Feed oil chambers for turbine bearings is an example. This Liquid Level Measurement system provides an accurate, reliable way of controlling the level in these vessels.

• Aerospace Run Tank Management - Run tank installations are typically very custom and demanding with requirements for high pressures, fast drain rates, long sensor lengths, high vibration and often multi-liquid compatible probes. AMI offers engineered solutions to each of these issues. High pressures are dealt with by using appropriate flanges with high pressure feedthrus compatible with cryogenic temperatures. When necessary AMI utilizes finite element analysis to model the stresses and verify that the design is within acceptable limits of the materials of construction. Hydrostatic pressure testing and special LOx cleaning services are available. Customers should specify the required cleaning specification number in each case. Hydraulic dynamics play in important role when fast drain rates are involved and AMI addresses these issues with a combination of special sensor construction and fast electronics sampling.

  • LOX Compatibility - Made using virgin Teflon and stainless steels, these sensors contain no epoxies or other materials that could act as a fuel source in liquid oxygen systems. Special care is taken to minimize residual oils during the construction process, however, special cleaning required for liquid oxygen service is normally provided by the customer although AMI can provide this as a third party service via qualified subcontractors.

  • Redundant Point Sensing - Point sensors can be provided using the same proven capacitance technology used in our continuous sensors. Uses for such sensors can be as a redundant backup reading to a continuous sensor (pictured below) in highly critical applications. Alternatively, cryogenic range platinum RTD temperature sensors can be mounted in specified locations to provide a level of redundancy when required. In either case these point sensors are brought out to a separate monitor from the continuous sensor reading.

• High Vibration, Long Lengths, Turbulence - AMI recommends the use of our Rugged Service sensors in aerospace applications. These sensors are built to a more robust standard and will survive demanding applications or rough handling. To withstand high vibration construction techniques are used to ensure no parts can fall off the sensor and become a contaminant inside the tank. Lock wires and tack welded fasteners are used for this purpose. Long sensors of over 50 feet have been fabricated that are rigid under their own weight, easy to install and shippable. These sensors also be provided in sections and are either bolted together with a miniature flange or screwed together. Sensor sections are factory assembled before shipment and re-assembly to ensure proper fit. Each sensor section is match marked for proper identification and assembly instructions are normally provided. Turbulence in the run tank can be caused by formation of a large vortex under high flow conditions. Lateral restraint of long sensors is recommended at several points along the length of the sensor. This can be accomplished by clamping or welding the sensor mast to an existing internal ladder or by installing radial spokes inside the tank. The sensor can also be inserted inside a heavy walled pipe that is secured at the top and bottom of the tank if provisions are made for proper venting to insure the sensor sees the true liquid inside the tank. In less turbulent installations a simple pipe stub welded to the bottom of the tank can serve to capture and restrain the tip of the level probe preventing lateral movement.

• High Flow Rates - Certain level applications require a very fast electronic signal update rate combined with specially designed sensors to provide a suitable level signal under extremely rapid level change conditions. Many of AMI's level instruments update the analog output reading every 50 mS and are therefore suitable for such fast fill or drain rates. By engineering the hydraulics of these high flow sensors a minimum lag between the sensor liquid annulus and bulk tank level (level reading Lag) is kept below 1 mm. 

• High Pressure - Level measurement systems for cryogenic applications up to 10,000 psig have been provided by AMI. By using commercial or custom designed flanges, high pressure feedthru connectors and custom sensor technology, almost no application is impossible. As needed the pressure boundaries are modeled and analyzed for stress using state of the art finite element programs. Hydrostatic pressure tests and certificates are readily available. When required, ASME certified pressure vessel welders and specialized welding techniques are employed.

• Washdown Area Applications - In food service or pharmaceutical environments requiring routine wash down cleaning, equipment ability to withstand moisture is especially important. With the introduction of an Industrial line of liquid level transmitters AMI can provides a fully weather-proof, corrosion resistant NEMA 4X enclosure. Additionally our internal circuit boards are conformal coated and sealed within a separate internal canister or "puck". Sensors with standard conduit adapters are also available to protect this coax connection from moisture. Capacitance sensor signals can be effectively "shorted" by the presence of electrically conductive water getting in the connection between the capacitance plates. This is most commonly a problem in the coaxial connection at the upper end of the rigid sensor. AMI offers several solutions to handle system moisture and fully understand the requirements to alleviate such problems.

• Electrostatic Desalters
  The AMI Model 175 Industrial Level Transmitter is an ideal way to provide reliable interface level measurement in electrostatic desalters. The Teflon®-sheathed probes are not affected by the AC/DC current passed through the fluid. A 4-20mA continuous level signal is standard. Onboard diagnostics insure that the Model 175 is always working perfectly.

• Cooling Towers
  The AMI Model 175 Industrial Level Transmitter is easily able to monitor the level of low-temperature water used in cooling towers. This is used to insure that the “hot” coils of the air conditioning unit are always covered. Lime scale buildup will not negatively affect the performance of the Model 175. The transmitter can be used for level monitoring via the onboard display or level control with the 4-20mA signal.

• Liquid Propane and LNG
  The AMI Model 175 Industrial Level Transmitter can be used to monitor the level of Liquid Propane or LNG. Normal variable pressure conditions inside the storage vessel, due to ambient temperature changes, have no measurable affect on the Model 175 reading. A continuous 4-20mA signal, proportional to the level in the vessel, is the standard output. Extensive experience in the measurement of cryogenic fluids has led AMI to develop what may be the best capacitance probes available at any price. At 1 atm. normal saturated conditions our standard probes provide a 2.25 and 2.88 pf/inch capacitance change from liquid to gas in LNG and Propane respectively. Because the Model 175 circuitry is so sensitive these values are easily translated to a stable, accurate level signal. When the absolute best available accuracy is needed AMI can provide semi-custom probes that are at least twice as sensitive.

• Liquid Oxygen
  The AMI Model 175 Industrial Level Transmitter is able to measure the level of Liquid Oxygen. Made using virgin Teflon and stainless steels, these sensors contain no epoxies or other materials that could act as a fuel source in liquid oxygen systems. Special care is taken to minimize residual oils during the construction process, however, special cleaning required for liquid oxygen service is normally provided by the customer although AMI can provide this as a third party service via qualified subcontractors. One interesting application is in the monitoring of LOx vessels mounted on vehicles used in transporting fish from hatcherys to local streams, ponds, and lakes. The rugged design allows the Model 175 to function flawlessly in this application, replacing a malfunctioning float system.

• Gasoline/Water or Oil/Water Interface
  The AMI Model 175 Industrial Level Transmitter can easily detect the interface between gasoline and water. It allows you to draw off the water from the bottom of the vessel, increasing the storage space for the gasoline. Setpoints are achieved with the use of "current trips". These setpoints can be placed anywhere along the length of the probe. Pump control function is also available.

• High Pressure Applications
  Most standard industrial probes are pressure rated to 2000 psig internal pressure. The specific pressure rating of a probe may be lower than this depending on what flange rating or process connection is used. Flexible cable probes are rated to 125 psig. Standard cryogenic probes are rated for 1000 psig and are readily available in 4000 psig ratings with custom Non-FM approved designs. Level measurement systems for cryogenic applications up to 10,000 psig have been provided by AMI. By using commercial or custom designed flanges, high pressure feedthru connectors and custom sensor technology, almost no application is impossible. In custom applications, as needed, the pressure boundaries are modeled and analyzed for stress using state of the art finite element programs. Hydrostatic pressure tests and certificates are readily available. When required, ASME certified pressure vessel welders and specialized welding techniques are also employed when needed.

• Acids
  A variety of Teflon coated probes are available that work well in most acids even in high concentration and elevated temperatures. Suitable acid applications include Hydrofloric acid (HF), Sulfuric acid (H₂SO₄), Acetic acid (CH₃COOH), Hydrocloric (HCl) and more.

• Low Concentration Caustic Solutions
  In concentrations below 10% the Model 175 Industrial Level Transmitter is well suited for level monitoring. This includes Sodium Hydroxide (NaOH) and Potassium Hydroxide (KOH) solutions which are commonly used in washdown mix tanks. In concentrations above 10% these basic solutions will coat the probe surface with crystals. These crystals create a wicking action that pulls a thin film of solution up the probe shaft which is seen by the level transmitter as a false high level signal.

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