Cryogenic Liquid Level Measurement & Control Application Notes

• Radiation Detector Cooling - Continuous reading capacitance sensors offer a much better solution for liquid nitrogen level control in radiation detector dewars compared to traditional point sensing based systems. Capacitance sensors have no moving parts and are fabricated with a bend such that they can be molded into the rubbery dewar collars. This allows the sensor to exit in the radial direction avoiding interference with the detector device. Existing point sensor based units can easily and affordably be converted to continuous reading level control. 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. This type of solution is applicable over the full range of dewar sizes commonly in use from 5 liters to 50 liters. It is important to communicate the exact detector make, model and dewar type you are using so that AMI can provide the correct sensor, collar or other necessary items. Installations using a bank of several detectors commonly require a liquid nitrogen header system which is also provided by AMI. Solenoid fill valves are connected to this manifold and therefore ach controller allows each dewar to call for a fill cycle independently of the other dewars. Autofill solutions for radiation detector dewars requiring all attitude range of motion are also available.

• All-Attitude Autofill Systems - AMI has developed a dewar and level control system that will fill liquid nitrogen dewars in any attitude. The system continuously senses and reports the level in all attitudes and then initiates a fill cycle based on user input set points. This unique design is a combination of dewar, sensor and autofill technology all being brought together to solve a very demanding application. This solution is obviously necessary when the dewar is being manipulated over a range of orientations but this same solution can also be useful in fixed orientations when the fill port entry is below the horizontal plane. Flexible transfer hoses, proper strain relief and range of motion freedom are all issues to be addressed.

Traditional Vertical Filling	Horizontal Filling	Inverted Filling - Amazing !

• Aerospace Run Tanks - 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.

• Signal Update Rates - When monitoring rapid changes in level height it is important to update your signal very quickly. AMI instruments update at rates ranging from 20 mS to 300 mS depending on the model selected. Often times the analog output signal is updated more rapidly than the display. Because of this AMI recommends obtaining an analog output option on any instrument for such applications.

• Outdoor or Washdown Area Applications - With the introduction of an Industrial line of liquid level transmitters AMI can provide both continuous and point sensing of cryogenic or non-cryogenic liquids in a fully weather-proof, corrosion resistant NEMA 4X enclosure. See Industrial Capacitance Probes and Transmitters for details. 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. Sensors with standard conduit adapters are also available to protect this coax connection from moisture. When the active portion of the cryogenic probe itself is exposed to moisture the probe can either be dried (natural evaporation or forced dry gas purging) 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.

• Auto-Changeover Systems - AMI offers an auto-changeover system by using this mode of the Model 286 Multi-Channel Liquid Level Controller. To accomplish this the user should own their liquid supply canisters and replace the existing visual float indicator with a continuous reading electronic capacitance probe. The replacement procedure is as simple as venting the tank, unscrewing the float and screwing in the capacitance sensor. Capacitance sensors are available with matching threads in correct lengths for most makes and sizes of cryogenic supply canisters.

        Visual Float Device                                                 Precision Capacitance Sensor Replacement

After capacitance sensors are installed in two supply tanks, a vacuum jacketed manifold with integral solenoid valves is installed between the two tanks. The Model 286 controller supplies the target dewar from one tank until it senses that a supply tank is near empty based on the user low level setpoint or based on a fill cycle time exceeded setting. Once an empty dewar is sensed the Model 286 will automatically switch to filling from the second supply canister and alert the user that one of the supply canisters is now empty. After the empty canister is replaced the system is reset by simply cycling the power on the Model 286. Any example of an installed system is shown below which monitors a critical device on a beam line. The target dewar in this installation is underground.

Auto-Changeover System in Use

• Biological Specimen Storage - The long term care and management of highly valuable and/or irreplaceable biological specimen samples is a big responsibility. This is common in the case of fertility clinics. Many customers have experienced poor results with cheap single point low level alarm monitors which are notoriously unreliable. AMI does not provide such devices. Instead AMI offers a more reliable, safer and more cost effective solution by equipping such Cryo-Bio dewars with autofilling capability. More reliable because of the advanced and proven features of our instruments such as digital readouts, Hi/Low alarms, remote computer communications, etc. Safer because lab personnel do not have to manually fill multiple dewars and risk burns from liquid nitrogen or back injury from transporting heavy LN2 containers. More cost effective because you eliminate the mundane task of routine manual filling multiple dewars every day. Most importantly, you insure that your valuable samples are always covered in LN2. We can apply our vast knowledge of continuous reading level controls to each installation. Individual portable dewars can be equipped with an autofill system by providing a modified dewar cap. A level sensor, vent and fill port are provided by AMI as part of each new cap assembly. If routine sample access is a requirement, a ruggedized sensor is recommended to avoid accidental bending of the sensor element during handling. A flexible fill line or transfer hose makes removal and replacement of the cap assembly very easy. It is important to note that the lab technician should power OFF the level control instrument before removing the dewar cap. Failure to do so will initiate a fill sequence and begin venting gas which may scare the operator and cause possible injury. An example of a modified wide mouth Barnstead Cryo-Bio sample dewar is shown here for use at a fertility clinic.

• Small Dewar Autofilling - 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. In such cases a fill tube, vent and standard transfer line train complete the autofill system. It is important on small dewars to stain relief the transfer line to prevent unwanted forces on the dewar or fill tube.

A second approach, the Flow-Thru sensor, is used for the majority of applications and will allow existing systems to be retrofitted in the field. The unique Flow-Thru sensor design 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. 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.

 

0.25 Liter IR Dewar Autofill Video

In small dewar applications were the active sensor length is very short and the volume is small, an unexpected rise in the level reading can sometimes be observed. This increase can occur for different reasons depending on whether the dewar has just finished filling or has run dry. If the dewar had just finished filling and the increase occurred quickly, this is likely a result of residual liquid downstream of the fill valve flowing into the dewar after the fill valve has closed. If the dewar had just finished filling and the increase occurred slowly, this is likely a result of the external components of the flow through sensor reaching thermal equilibrium after the fill. A slight offset in capacitance occurs during the fill when the external parts are cooled. As the parts warm up, the offset returns to normal and reading is actually becoming more accurate. If the increase occurred after the dewar had run dry, this is likely caused by an “Air Offset”. Learn more about the Air Offset phenomenon which is common in small dewar autofill applications.

Note that filling a dewar with liquid nitrogen is not necessarily a smooth process. As the nitrogen flows into the dewar, it is a combination of liquid and vapor. This two-phase fluid causes significant bubbling and splashing in the dewar that can manifest itself as fluctuations in the level reading until the upper setpoint is achieved.

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