Standard AMI Liquid Level Sensors Capacitance-based liquid level sensors, used in conjunction with Models 175, 185, 186, and 286 cryogen monitors, are typically manufactured of 3/8 inch OD stainless steel tubing and will operate with virtually all common laboratory cryogenic liquids including nitrogen, liquid neon, and liquid xenon. Sensors for common Industrial Use Cryogenics are also available for materials such as liquid oxygen (LOx), liquid hydrogen LH2), liquid carbon dioxide (liquid CO2), liquified or liquefied natural gas (LNG), liquid argon, liquid propane, and liquid butane. Upon request, special assembly techniques can be applied for sensors required for liquid oxygen or hydrogen measurement. Sensors can be supplied in single-section overall lengths of up to 50 feet. Alternatively segmented sensors are available that can be bolted together during installation. Three standard sensor mounting configurations are available. The typical configuration includes a hermetically sealed BNC connector with an adjustable 3/8 NPT nylon feed-through. For higher pressure or vacuum applications, a welded stainless steel 3/8 NPT BNC connector fitting or conflat flange BNC connector fitting is available. Twelve feet of connecting coaxial cable and in-line oscillator/transmitter are required to interface the sensor with a non-industrial instrument. See more about specific cryogenic level Applications or Industrial Level Applications Contact AMI with your specifications for a quotation.

Custom Liquid Level Sensors

Custom sensors are available from AMI to meet your individual application requirements. Below is a partial list of the type custom capacitance sensors that AMI provides. As an additional service AMI can provide materials certifications, vacuum leak rate certifications or pressure test certifications.

• Bent Sensors - Useful in systems with molded dewar collars or requirements for radial exiting connections. These continuous reading level sensors offer much improved performance over previously available point sensor based units. Such requirements are often very useful in germanium gamma ray detector (HPGe) liquid nitrogen autofill systems and Silicon (Li) x-ray detector liquid nitrogen autofill systems. See photos to right. AMI can provide such systems as part of a new installation or, in many cases, retrofit existing equipment with a brand new continuous reading autofill system.

• LOX Sensors - 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, no special cleaning required for liquid oxygen service is provided. Certified LOx cleaning is the responsibility of the customer.

• Point Sensors - 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 or in a smaller package as gas/liquid indication in closed pipe systems such as heat exchangers or phase separators.

• Rugged Service Sensors - AMI can provide a number of specialty sensors depending upon the application requirements. In industrial environments AMI recommends the use of our 1/2" O.D. Rugged Service sensors. These sensors are built to a more robust standard and are built to survive rough handling. These sensors are also recommended in applications were a metal compression fitting is used to prevent crimping of the metal outer tube.

• Semi-Flexible Sensors - Continuous reading sensors can be made using special materials that allow the probe to be bent in almost any configuration and hold its shape. This allows for precise continuous level measurement in areas that were previously impossible to monitor. Such sensors can be inserted and extracted through a slightly larger smooth walled guide tube. These sensors are ideal for horizontal bore magnet systems with a curved nitrogen annulus. Sizes provided are either 1/4" or 3/8" O.D.

High Pressure Sensors - 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.

• Industrial Use and Conductive Liquid Sensors - With the introduction of an Industrial line of liquid level transmitters AMI has also introduced over 10 styles of probes specifically designed to meet the level measurement needs of the Process Industry. These probe types can be used with any AMI capacitance based instrument but are specifically designed for use with our industrial capacitance transmitters. See Industrial Capacitance Probes and Transmitters for details. Flanges and fittings of almost any type or size can be provided. A sensor with welded VCR fitting is shown here on right.

• Open LN2 Bath Sensors - Some applications use open baths of liquid nitrogen (food freezing, cryogenic metals treatment, shrink fitting of parts, etc.). Effectively controlling the level in such baths is complicated by the presence of moisture in the surrounding air. This moisture will easily condense on the exposed portion of the level sensor, ultimately forming an ice buildup that can render the level measurement ineffective. AMI has developed a sensor construction which eliminates this problem and will provide reliable readings in open baths for an unlimited time period. In food service applications sensor construction materials are limited to stainless steels and Teflon. Sanitary flanges, polished surfaces and crevice-free fillet welds are all requirements AMI is familiar with. The attached Tech Note, Air Offset and Moisture Issues, helps explain what happens when open bath systems are shut down.

• Frost-Proof and Water-Proof Sensors - 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 this problem and fully understands the requirements to alleviate such problems. Frost-proof sensors keep the electrical connection out of the ice formation. Our Water-Proof kit seals this junction and prevents moisture from reaching the connection. This is especially useful in food service or pharmaceutical environments requiring routine wash down cleaning. When the active portion of the cryogenic probe itself is exposed to moisture the probe can either be dried or given an initial cooling prior to reintroduction of the level measurement into the control loop. The later is an effective method because ice is non-conductive and no longer effects the measurement after the moisture is frozen.

• Segmented Sensors - When tank heights exceed 15 feet it is worthwhile considering a segmented sensor. These sensors come in sections and are either bolted together with a miniature flange or screwed together. Sensor sections are factory assembled and disassembled to ensure proper fit. Each sensor section is match marked for proper identification and assembly instructions are normally provided. Upon request all fasteners can be predrilled for lock wire to ensure no parts come loose even under high vibration conditions.

• High Flow Rate Sensors - 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. 

• Integral Dewar Sensors - In certain cases it is advantageous to incorporate the level sensor inside the dewar. Doing so provides only a single coax connection point on the outside of the dewar for connection back to the level instrument. In dewars with very small volumes or all-attitude requirements it is especially useful. Incorporation of the sensor in this manner provides almost no added heat load to the system and maximizes times between refilling while adding the benefits of continuous level measurement.

0.25 Liter IR Dewar Autofill Video

• Flow-Thru Style Sensors - For small dewar applications AMI utilizes a unique Flow-Thru sensor design that allows liquid to pass through the middle of the sensor while simultaneously and continuously measuring the liquid level. This invention is invaluable when autofilling small dewars where only a single port exists. These can be provided on dewars with a single port of 3/8" or larger. In addition, venting of the internal vessel can also be built into this design and allows for a complete turn-key solution to the problem of filling even the smallest dewar volumes of less than 0.25 liters in size. Photo to right shows flow-thru sensor with integral venting. Learn more about the Air Offset phenomenon which is common in small dewar autofill applications but is easily explainable.

• Mini Dewar Flow-Thru Sensors ( < 1 to 0.1 liter Dewars) - High sensitivity/low noise detectors are often cooled by liquid nitrogen using very compact dewars due to equipment footprint limitations. AMI provides a special flow-thru sensor concept with a built-in phase separation device which is able to fill even the smallest of dewars with only a few inches of liquid height inside the inner vessel. These can be provided on dewars with a single port of 3/8" or larger. Such systems are used in conjunction with the AMI Model 286 Level Controller precool mode which cools the transfer line prior to directing the flow into the small dewar. By doing so the liquid remaining in the dewar at the beginning of a fill cycle experiences almost no loss in level before filling occurs. These systems will autofill and control levels in even the smallest volume dewars accurately and reliably. Typical fill cycles are about 5-10 minutes depending on transfer line lengths used. The Model 286 Level Controller offers a Fill Inhibit feature which allows suppression of a fill cycle start while sensitive data is being collected. Once the relay trigger signal is actuated by the users system logic, the fill cycle and subsequent autofills resume as normal.

   

• Modified Dewar Cap Assemblies - Dewar cap assemblies can be provided with in a variety of configurations which are adapted to allow autofilling for a particular application. These are available with foam neck plugs, stainless flanged versions or small nylon plug assemblies (some examples shown below). Simply contact AMI with the make and model of your dewar and let us handle the rest.

Wessington Cap	Taylor-Wharton Cap	Oxford Cap w/Low Level Point Sensor

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