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Staging method refers to the arrangement of regenerators and pulse tube(s) in a multi stage pulse tube cooler(PTC). This paper describes the study of two staging methods for a 25 K two stage pulse tube cooler. Staging method 1, a thermally coupled, inline staging design, was studied using SAGE and by developing an experimental PTC. A no load temperature of 40.4 K was achieved by the experimental PTC. The effect of regenerator configuration was studied by varying the regenerator length. Experimental results show a detrimental interplay of pressure drop and regenerator effectiveness resulting in higher no load temperatures. Staging method 2, consisting of thermally linked single stage 80 K PTC and two stage 25 K PTC, was studied using a SAGE model. The mass flow from the pressure wave generator is bifurcated to the PTCs; thus reducing pressure drop and regenerator ineffectiveness losses by 79% and 42% respectively. With staging method 2, a design cooling power of 1.98 W at 25 K is obtained.

The paper is based on a study to develop carbon-glass epoxy hybrid composites with desirable thermal properties for applications at cryogenic temperatures. It analyzes the coefficient of thermal expansion of carbon-epoxy and glass-epoxy composite materials and compares it with the properties of carbon-glass epoxy hybrid composites in the temperature range 300 K to 125K. Urethane modified epoxy matrix system is used to make the composite specimens suitable for use even for temperatures as low as 20K. It is noted that the lay-up with 80% of carbon fibers in the total volume fraction of fibers oriented at 30 degrees and 20% of glass fibers oriented at 0 degrees yields near to zero coefficient of thermal expansion as the temperature is lowered from ambient to 125 K. (c) 2010 Elsevier Ltd. All rights reserved.

A three-dimensional numerical model of Ranque–Hilsch vortex tube has been developed using the commercial CFD code (Star-CD) to analyze the flow parameters and energy separation mechanism inside the tube. Investigations have been done on the variation of fluid properties and flow parameters as the fluid particles progress in the flow field by tracking different particles exiting through the hot and cold end. Fluid properties like stagnation temperature, static temperature, static pressure and total pressure and flow parameters like axial, radial and swirl velocities are obtained along the axial and radial directions to understand the flow behaviour inside the tube. The presence of free vortex zone inside the tube also has been investigated. Possible energy transfer mechanisms are discussed and an estimate has been made on the magnitude of energy transfer from the cold end exit flow to hot end exit flow. Effects of secondary circulation and length of the tube on energy separation also have been evaluated.

Sustained efforts have been made in our laboratory to improve the performance of an indigenously developed pressure wave generator by reducing the mechanical losses and the required input power. An acoustically matching pulse tube cooler, with a design target of 0.5 W at 80 K, was designed using Sage and experience gained from previous studies. The pulse tube cooler was fabricated and tested. The effect of regenerator stacking pattern on the cooler performance was studied by filling the regenerator with mesh of the same size # 400 and with multi meshes # 250, 325, 400. In present experiments, regenerator with # 400 mesh at 30 bar filling pressure performed better with more energy efficiency. A no load temperature of 74 K was achieved with input power of 59 W corresponding to a cooling power of 0.22 W at 80 K. Parasitic heat load to the cooler was measured be 0.68 W. This heat load is primarily by heat conduction through the regenerator and pulse tube wall. By reducing the wall thickness from 0.30 mm to 0.15 mm, the parasitic loads can be reduced by 50%. (C) 2015 The Authors. Published by Elsevier B.V.

An experimental study on pulse tube cryocooler is presented with a twin pulse tube configuration. The study is conducted with a dual piston head linear compressor design which is developed indigenously. The two identical pulse tube cryocoolers are operated by a single linear motor which generates 180 degrees out of phase dual pressure waves. The advantages of the configuration being the reduction in fabrication cost and the increased cooling power. The compressor is driven at a frequency of 48 Hz using indigenously developed PWM based power supply. The CFD study of pulse tube cryocooler is discussed along with the experimental cool down results. A detailed experimental and FEM based studies on the fabrication procedure of heat exchangers is conducted to ensure better heat transfer in the same.

A high frequency two stage pulse tube cooler (PTC) has been designed using Sage software for a JT-PTC hybrid helium recondensation system. The cold end of the first stage regenerator is anchored at 80 K using a liquid nitrogen supply. Such a thermally coupled design simplifies the design without the need for considering flow distribution between the stages and ensures that entire flow is available to produce cooling power at 25 K. The pulse tube cooler is designed for maximal utilization of the 900 W PV power provided by the Pressure Wave Generator (PWG). With the first prototype, a no load temperature of 40.4 K was achieved at a filling pressure of 24.1 bar. The effect of filling pressure on the acoustic matching of the PTC and PWG was investigated. It is observed that filling pressure has a significant effect on the PWG piston stroke amplitude. Using phasor analysis, it is shown that the phase relationship at different sections of the two stage PTC is detrimentally affected by the pulse tube volume. A scheme for achieving advantageous phasing by reducing the pulse tube volume is proposed. This involves maintaining the hot heat exchanger, inertance tube and buffer volume at 80 K. With the modification it is shown that a beneficial phase reversal across second stage Regenerator is achieved. The method is currently under experimental investigation.

For space applications, the weight of the liquid level sensors are of major concern as they affect the payload fraction and hence the cost. An attempt is made to design and test a light weight High Temperature Superconductor (HTS) wire based liquid level sensor for Liquid Oxygen (LOX) tank used in the cryostage of the spacecraft. The total resistance value measured of the HTS wire is inversely proportional to the liquid level. A HTS wire (SF12100) of 12mm width and 2.76m length without copper stabilizer has been used in the level sensor. The developed HTS wire based LOX level sensor is calibrated against a discrete diode array type level sensor. Liquid Nitrogen (LN2) and LOX has been used as cryogenic fluid for the calibration purpose. The automatic data logging for the system has been done using LabVIEW11. The net weight of the developed sensor is less than 1 kg.

Earlier version of an indigenously developed Pressure Wave Generator (PWG) could not develop the necessary pressure ratio to satisfactorily operate a pulse tube cooler, largely due to high blow by losses in the piston cylinder seal gap and due to a few design deficiencies. Effect of different parameters like seal gap, piston diameter, piston stroke, moving mass and the piston back volume on the performance is studied analytically. Modifications were done to the PWG based on analysis and the performance is experimentally measured. A significant improvement in PWG performance is seen as a result of the modifications. The improved PWG is tested with the same pulse tube cooler but with different inertance tube configurations. A no load temperature of 130 K is achieved with an inertance tube configuration designed using Sage software. The delivered PV power is estimated to be 28.4 W which can produce a refrigeration of about 1 W at 80 K.

This paper describes the design, fabrication and testing of a moving magnet type linear motor of dual piston configuration for a pulse tube cryocooler for ground applications. Eight radially magnetized segmented magnets were used to form one set of a magnet ring. Four magnet rings of such type were constructed, in which one pair of rings has north-pole on its outer diameter and south-pole on inner diameter, while the other pair is it's complementary. The magnets were mounted with opposite poles together on the magnet holder with an axial moving shaft having a piston mounted on both ends of the shaft. The shaft movement was restricted to the axial direction by using C-clamp type flexures, mounted on both sides of the shaft. The force requirement for driving the compressor was calculated based on which the electrical circuit of motor is designed by proper selection of wire gauge and Ampere-turns. The flexure spring force estimation was done through simulation using ANSYS 11.0 and was verified experimentally; while the magnet spring force was determined experimentally. The motor with mounted piston was tested using a variable voltage and variable frequency power supply capable of driving 140 watts of load.

A moving magnet linear motor compressor or pressure wave generator (PWG) of 2 cc swept volume with dual opposed piston configuration has been developed to operate miniature pulse tube coolers. Prelimnary experiments yielded only a no-load cold end temperature of 180 K. Auxiliary tests and the interpretation of detailed modeling of a PWG suggest that much of the PV power has been lost in the form of blow-by at piston seals due to large and non-optimum clearance seal gap between piston and cylinder. The results of experimental parameters simulated using Sage provide the optimum seal gap value for maximizing the delivered PV power.

The effect of natural convection on the oscillatory flow in an open-ended pipe driven by a timewise sinusoidally varying pressure at one end and subjected to an ambient-to-cryogenic temperature difference across the ends, is numerically studied. Conjugate effects arising out of the interaction of oscillatory flow with heat conduction in the pipe wall are taken into account by considering a finite thickness wall with an insulated exterior surface. Two cases, namely, one with natural convection acting downwards and the other, with natural convection acting upwards, are considered. The full set of compressible flow equations with axissymmetry are solved using a pressure correction algorithm. Parametric studies are conducted with frequencies in the range 5-15 Hz for an end-to-end temperature difference of 200 and 50 K. Results are obtained for the variation of velocity, temperature. Nusselt number and the phase relationship between mass flow rate and temperature. It is found that the Rayleigh number has a minimal effect on the time averaged Nusselt number and phase angle. However, it does influence the local variation of velocity and Nusselt number over one cycle. The natural convection and pressure amplitude have influence on the energy flow through the gas and solid. (C) 2011 Elsevier Ltd. All rights reserved.

The oscillating flow and temperature field in an open tube subjected to cryogenic temperature at the cold end and ambient temperature at the hot end is studied numerically. The flow is driven by a time-wise sinusoidally varying pressure at the cold end. The conjugate problem takes into account the interaction of oscillatory flow with the heat conduction in the tube wall. The full set of compressible flow equations with axisymmetry assumption are solved with a pressure correction algorithm. Parametric studies are conducted with frequencies of 5-15 Hz, with one end maintained at 100 K and other end at 300 K. The flow and temperature distributions and the cooldown characteristics are obtained. The frequency and pressure amplitude have negligible effect on the time averaged Nusselt number. Pressure amplitude is an important factor determining the enthalpy flow through the solid wall. The frequency of operation has considerable effect on penetration of temperature into the tube. The density variation has strong influence on property profiles during cooldown. The present study is expected to be of interest in applications such as pulse tube refrigerators and other cryocoolers, where oscillatory flows occur in open tubes. (C) 2011 Elsevier Ltd. All rights reserved.

In space application the precision level measurement of cryogenic liquids in the storage tanks is done using triple redundant capacitance level sensor, for control and safety point of view. The linearity of each sensor element depends upon the cylindricity and concentricity of the internal and external electrodes. The complexity of calibrating all sensors together has been addressed by two step calibration methodology which has been developed and used for the calibration of six capacitance sensors. All calibrations are done using Liquid Nitrogen (LN2) as a cryogenic fluid. In the first step of calibration, one of the elements of Liquid Hydrogen (LH2) level sensor is calibrated using 700mm eleven point discrete diode array. Four wire method has been used for the diode array. Thus a linearity curve for a single element of LH2 is obtained. In second step of calibration, using the equation thus obtained for the above sensor, it is considered as a reference for calibrating remaining elements of the same LH2 sensor and other level sensor (either Liquid Oxygen (LOX) or LH2). The elimination of stray capacitance for the capacitance level probes has been attempted. The automatic data logging of capacitance values through GPIB is done using LabVIEW 8.5.

The integral diaphragm pressure transducers machined out of precipitation hardened martensite stainless steel (APX4) are widely used for propellant pressure measurements in space applications. These transducers are expected to exhibit dimensional stability and linearity for their entire useful life. These vital factors are very critical for the reliable performance and dependability of the pressure transducers. However, these transducers invariably develop internal stresses during various stages of machining. These stresses have an adverse effect on the performance of the transducers causing deviation from linearity. In order to eliminate these possibilities, it was planned to cryotreat the machined transducers to improve both the long-term linearity and dimensional stability. To study these effects, an experimental cryotreatment unit was designed and developed based on the concept of indirect cooling using the concept of cold nitrogen gas forced closed loop convection currents. The system has the capability of cryotreating large number of samples for varied rates of cooling, soaking and warm-up. After obtaining the initial levels of residual stress and retained austenite using X-ray diffraction techniques, the pressure transducers were cryotreated at 98 K for 36 h. Immediately after cryotreatment, the transducers were tempered at 510 degrees C for 3 h in vacuum furnace. Results after cryo treatment clearly indicated significant reduction in residual stress levels and conversion of retained austenite to martensite. These changes have brought in improvements in long term zero drift and dimensional stability. The cryotreated pressure transducers have been incorporated for actual space applications. (c) 2010 Published by Elsevier Ltd.

High frequency, miniature, pulse tube cryocoolers are extensively used in space applications because of their simplicity. Parametric studies of inertance type pulse tube cooler are performed with different length-to-diameter ratios of the pulse tube with the help of the FLUENT (R) package. The local thermal non-equilibrium of the gas and the matrix is taken into account for the modeling of porous zones, in addition to the wall thickness of the components. Dynamic characteristics and the actual mechanism of energy transfer in pulse are examined with the help of the pulse tube wall time constant. The heat interaction between pulse tube wall and the oscillating gas, leading to surface heat pumping, is quantified. The axial heat conduction is found to reduce the performance of the pulse tube refrigerator. The thermal non-equilibrium predicts a higher cold heat exchanger temperature compared to thermal equilibrium. The pressure drop through the porous medium has a strong non-linear effect due to the dominating influence of Forchheimer term over that of the linear Darcy term at high operating frequencies. The phase angle relationships among the pressure, temperature and the mass flow rate in the porous zones are also important in determining the performance of pulse tuberefrigerator.

The thermal performance of multilayer insulation (MLI), consisting of double aluminized Mylar radiation shields and nylon net thermal spacers, was evaluated using a double guarded cylindrical calorimeter and a tank calorimeter over the temperature range 300-77 K. The degradation in the effective thermal conductivity of MLI was evaluated to be 1.68 using the calorimeters. The optimum number of layers for the MLI was 40 to 50 at a layer density of 25 layers $cm ^{-1}$. The temperature profile and heat flux through the MLI were obtained as a function of vacuum level for different numbers of insulation layers. The temperature profile of the MLI indicates the relative predominance of the conduction and radiation components of heat transfer through the insulation. It is observed that for a given number of layers, the temperature of a specific layer between the cold and warm boundaries decreases with an increase in chamber pressure and vice versa.

Simple analytical methods have been employed for heat transfer analysis of experimental data obtained through calorimetric investigations on multilayer insulation (MLI). Sectional heat transfer analysis has shown that the effective thermal conductivity of the MLI varies from section to section of the insulation structure and it has a peak which lies between the middle and warm boundary regions of the MLI. This could be attributed to a peak in residual gas conduction in this region. The theoretical estimation of heat flux through MLI, using a simple analytical model, is also discussed in this paper. This model takes into consideration the non-linear temperature profile of the insulation. The computed heat flux using this model gives a lower (2 to 4 times) value in comparison with the heat flux estimated from calorimetric measurements. A refined model has been suggested which includes the residual gas conduction also in MLI.

The design of a cryogenic loop heat pipe (CLHP) is presented in the paper. As the wick is required only in the evaporator section, very small pore size wicks can be used in applications with high thermal transport requirements and/or where the heat must be transported over a long distance against gravity. A FORTRAN program to solve the mathematical model and to determine the parameters for various boundary conditions has been developed. The CLHP is designed to transfer 5W heat at 70K using nitrogen or oxygen as working fluid. It will be a self priming type device which can operate against gravity with evaporator above the condenser as well as under microgravity condition. A G-M type single stage double inlet pulse tube refrigerator is coupled to the CLHP to test its performance. The mathematical model, design, fabrication integration of the heat pipe with the pulse tube system and testing with stainless steel wick at the evaporator will be described in the paper.

Studies of helium II flow through the narrow pore filters of cylindrical shapes, fabricated by compacting under pressure fine powders of aluminium oxide $(0.05 \mu m)$, nickel and chromium $(1 \mu m)$, have been made by measuring the volume flow rates in a two bath arrangement. The "easy" direction mass flow rates are in general agreement with the Gorter Mellink description, but for large temperature gradients where the observed mass flow rates are lower than predicted by the theory and tend to saturation. However if one can treat the cross-sectional area as a fit parameter, our results show that this parameter decreases with increasing chemical potential gradient, very similar to the ease of non-cylindrical geometries. Can then one conclude that irrespective of the geometrical shape of the filter the superfluid flow behaves analogously to the classical flow especially for large pressure differences. The breakthrough studies confirm the occurrence of "choking effect" in the filter made from very fine size aluminium oxide powder. Our studies point to the limitations in using these filters for practical applications such as fountain effect pumps and helium II transfer devices.

Computational fluid dynamics (CFD) and experimental studies are conducted towards the optimization of the Ranque-Hilsch vortex tubes.Different types of nozzle profiles and number of nozzles are evaluated by CFD analysis. The swirl velocity, axial velocity and radial velocity components as well as the flow patterns including secondary circulation flow have been evaluated. The optimum cold end diameter (d(c)) and the length to diameter (L/D) ratios and optimum parameters for obtaining the maximum hot gas temperature and minimum cold gas temperature are obtained through CFD analysis and validated through experiments. The coefficient of performance (COP) of the vortex tube as a heat engineand as a refrigerator has been calculated.

The pressure wave generator is a critical component of a pulse tube refrigerator. Conventionally either a solenoid operated electromagnetic valve or a rotary valve driven by a hermetically sealed motor is used for this. In this article, we present a novel pressure wave generator design based on cam operated valves for use with pulse tube refrigerators. This, when coupled with an inverter drive (variable frequency drive), forms a versatile pressure wave generator in the range between 1 and 10 Hz. In a single stage double inlet pulse tube using this type of pressure wave generator, refrigeration power of about 6 W at 77 K and a no load temperature of about 42 K were obtained. The above results are comparable to those obtained with pulse tube systems based on other two types of pressure wave generators. While the rotary valve based pressure wave generator is prone to developing leaks at the rotating seal, the solenoid valve based system cannot be operated continuously due to coil failure. The cam operated valve based pressure wave generator overcomes the above shortfalls of rotary valve and solenoid valve type pressure generators and is well suited for continuous operation of the refrigerator.

A single stage Pulse Tube Refrigerator has been designed, fabricated and operated with a 3kW GM type helium compressor with an indigenously developed rotary valve in the frequency range from 1.2 to 2.3Hz. The system produces a lowest temperature of 37.5K and a refrigeration power of 7.3W at 77K. The cold end temperature Tc of the Pulse Tube refrigerator depends on its orientation angle $\theta$ with respect to gravity vector and the increase in Tc arises due to the convection in the Pulse Tube. The lowest temperature occurs at $\theta = 0°$. The increase in Tc with change in $\theta$ is small till about 70°. However, beyond this angle, Tc varies strongly and reaches a maximum value at $\theta ~ 120°$ and decreases thereafter upto 180°.For a better understanding of convection phenomenon, detailed experimental studies have been performed on two pulse tubes with the same i.d. 13.3mm, but with lengths 250 and 300mm. The orientation dependence of Tc and the cooling power characteristics have been studied at different frequencies (1.2 Hz to 2.3 Hz) at two different pressures. The results have been analysed to obtain the experimentally observed heat flux due to convection, $Q_{conE}$.The studies show that these values are much larger than those theoretically calculated, $Q_{conT}$ based on natural convection in the pulse tube. Due to the absence of a detailed theory to understand the experimental results, it is assumed that $Q_{conE}$ is a sum of two parts, one arising from the natural convection and the other due to the forced convection. The latter is found to depend on the operating frequency. Empirical analysis has been carried out for the frequency dependent component of convection heat flux.

One of the critical factors in the design of a cryogenic tank is the method of suspending the inner vessel within the outer vessel. A thermally inferior suspension system can nullify the effect of using high performance thermal insulation. The authors describe a new design of support system in which the central suspension rod is under tensile stress while the concentric tubular support is subjected to compressive stress due to acceleration loads. The design not only provides a long thermal conduction path, but also eliminates thermal contraction stresses, while allowing marginal assembly misalignments.

The development of a pilot plant for cryogrinding of spices is discussed in this paper. The unit has an indigenously designed and developed cryogenic screw cooler for optimal utilization of liquid nitrogen along with a digital signal processing system (DSP) for liquid nitrogen spray and automatic temperature control. In the first part, the advantages of cryogrinding over conventional grinding are described. The latter part discusses the details of the pilot plant, along with preliminary experimental studies using pepper as the sample. Experimental results clearly indicate the quality improvement of the cryoground spice in terms of increased volatile oil contents and flavour components.

In large cryogenic storage systems, the level of cryogenic fluids such as liquid nitrogen, liquid oxygen, etc., are usually monitored using capacitance type indicators. They are both heavy and inaccurate. We report here the development of discrete array type liquid level indicators. A special class of analog multiplexer IC, which operates at cryogenic temperatures, is used in the unit. Operating it at liquid helium temperature and switching the channels one million times without any failure tested the reliable operation of the IC at cryogenic temperature. The system is capable of detecting the liquid level of any cryogen. Only a software tuning is required for operation with each fluid. The developed unit has an active length of 400 mm and six regions with sensitivities ranging from ± 5 mm to ± 0.5 mm. Redundancy of level measurement is achieved through software. The interaction of the system that has 138 sensors to ambient electronics is through only 15 wires. The integrated unit has been cycled several times between room temperature and 77 K.

The flexure bearing is one of the key components of linear motor. Due to its frictionless, wearless, high radial and low axial stiffness; it is a very critical part of any linear actuator. The flexure bearing has increased the reliability and life of operation of linear actuators used in cryocoolers. This paper discusses the failure aspects of C-Type flexure with 3 mm bending radius in comparison with flexure bearing of different bending radii. Experimental results of 3 mm and 8 mm bending radii flexures are also discussed.

This paper reports design, fabrication and testing of one of the recuperative heat exchangers for a small capacity helium recondensation system. A tube in tube heat exchanger, with multiple tubes in a coiled outer tube, has been designed to operate in the 300 K-100 K range. The key parameters of the heat exchanger are optimized by numerical analysis to obtain highest heat transfer coefficients and lowest pressure drops to meet the design goals. The pressure drop performance of the heat exchanger was tested and it is found that the annular pressure drop increases due to spacer string beyond the design toleration. Pressure drop was also measured after removing the spacer string and the results agree with the correlations available in literature. Spacer pressure drop should be considered in heat exchanger design.

This paper presents a study of the effect of vacuum level on the mass flow characteristics of a liquid nitrogen transfer line held in the horizontal orientation. A demouniable transfer line has been designed and fabricated allowing experiments under varying jacket vacuum conditions. The experiments made were \bullet $LN_2$, carrying pipe exposed to atmosphere \bullet $LN_2$, carrying pipe enclosed by jacket at varying vacuum levels \bullet $LN_2$, carrying pipe enclosed by jacket filled with carbon dioxide at atmospheric pressure. When the liquid line was enclosed by an outer jacket with air at atmospheric pressure, the interspace pressure fell \approx 100 mbar below the atmospheric pressure, as the line cooled to 77K. Under these conditions, the liquid mass flow rate was higher (and the cool-down rime less) than that of a liquid line directly exposed to atmosphere. Experiments have been performed wirh coarse vacuum (800-1 mbar), medium and high vacuum $(10^{-1}-10^{-5} mbar)$ in the jacket for different $LN_2$, supply dewar pressures. For coarse vacuum conditions, liquid mass flow rates were high as compared to high vacuum conditions for 2OO-300 s after the start of cool-down, but the flow rare decreased on reaching the steady stare. Improvement in jacket vacuum resulted in increase of the steady stare liquid mass flow rates. Thus, it is necessary to optimize the level of jacket vacuum for a particular application. Experiments were made with the interspace filled with $CO_2$, at atmospheric pressure. During $LN_2$, transfer, $CO_2$, condensed to a vacuum of about $1O^{-3}$ mbar. The liquid fraction mass flow rare was initially very low before the $CO_2$, condensed. After $CO_2$ condensation the liquid fraction mass flow rare increased becoming comparable to that of a vacuum insulated transfer line.

A demountable transfer line has been designed and fabricated to carry out experimental studies on cool-down time and mass flow characteristics of a liquid nitrogen transfer line held in a horizontal position. The effects of varying degrees of vacuum in the interspace on the cool-down time and mass flow rates have been evaluated. It has been found that during the initial phase of cool-down, lines with coarse vacuum perform better than high-vacuum-insulated lines. The performances of multilayer-insulated lines and lines with interspace medium of carbon dioxide have been evaluated as well as that of bare line exposed to an ambient environment. An experimental correlation has been developed to predict the cool-down time as a function of vacuum level and supply dewar pressure.

Monitoring gas purity is an important aspect of gas recovery stations where air is usually one of the major impurities. Purity monitors of Katherometric type ate commercially available for this purpose. Alternatively, we discuss here a helium gas purity monitor based on acoustic resonance of a cavity at audio frequencies. It measures the purity by monitoring the resonant frequency of a cylindrical cavity filled with the gas under test and excited by conventional telephone transducers fixed at the ends. The use of the latter simplifies the design considerably. The paper discusses the details of the resonant cavity and the electronic circuit along with temperature compensation. The unit has been calibrated with helium gas of known purities. The unit has a response time of the order of 10 minutes and measures the gas purity to an accuracy of 0.02%. The unit has been installed in our helium recovery system and is found to perform satisfactorily.

Remote delivery tubes (RDTs) in Collins and similar types of helium liquefiers interconnect the liquefier with the storage dewar. In these liquefiers the Joule–Thompson (J–T) valve is separately situated in the liquefier cryostat. Therefore there is a regime of J–T heating during the initial cool down of the liquefier, since there is a time lag in reaching the inversion temperature for the J–T valve. The note describes the design, fabrication, and testing of a RDT with an integral J–T valve. A demountable copper cold seal working at a pressure of 15 bar was successfully tested. The J–T valve seat made of Cu‐Cr‐Zr alloy provided excellent sealing with the stainless‐steel valve tip at liquid‐helium temperatures. The testing of this RDT with integral J–T showed substantial reduction in cool down time for the liquefier as well as better control of the dewar pressures.

The design of a special bending tool which is used in the formation of 90° bends for the tube‐in‐tube helium flow passages of a remote delivery tube is described. The concentric thin walled stainless‐steel tube assembly is filled with water and cryofrozen before they are clamped into the bending tool. The tubes are bent by pressing the tube using the bending lever over the guiding wheel. The split bush and split cone clamping arrangement ensures that there is no movement of the tubes during the bending operation. This procedure has yielded kink‐free bends with good circularity.

He II flow driven by Fountain effect pumps (FEPs) operating in self sustained mode in test loops of different impedances have shown oscillatory behaviour for test section heat loads upto 1.2 W. The period of oscillation depends on the applied heat load, bath temperature of He II (Tbath) and the impedance of the test loop. The occurrence of oscillations is qualitatively understood due to the existence of an `onset temperature' to generate convection in the loop. The boundary between the stable and oscillatory states have been mapped. A combined normal and self sustained mode of operation of FEP eliminates these oscillations for practical applications.

The measurement of the cryogen level in a cryostage of space crafts is crucial. At the same time the weight of the sensor should be small as it affects the payload fraction of the space craft. An attempt to develop a HTS based level sensor of 400 mm for Liquid Oxygen (LOX) measurement was made. In the initial phase of testing, loss of superconductivity of HTS wire in LOX inside a cryostat was noticed. Thus, a new four wall cryostat was designed to have a stable LOX level to provide thermal stability to the HTS based LOX sensor. The calibration of the developed sensor was carried out against capacitance level sensor which was pre calibrated using diode array to verify its linearity and performance for different current excitation levels. The calibrations were carried out without heater wires. The automatic data logging was accomplished using a program developed in LabVIEW 11.0.