by James Fesmire, senior principal investigator, Cryogenic Test Laboratory, NASA Kennedy Space Center, CSA Past-President, james.e.fesmire@nasa.gov

Part 1: Short History of Heat Measurement in Cryogenics

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The use of boiloff calorimetry has become a practical and useful tool to measure, in a direct way, the thermal insulating performances of materials and systems of materials.

The use of boiloff calorimetry to measure the effects of thermal energy (or heat) dates back to the early 1900s [1, 2]. Gas flow rates measured in evaporation—or boiloff—calorimetry enable direct calculation of quantities such as heat flux and thermal conductivity. A particularly useful approach is to use nitrogen for the heat measurement fluid as it is readily available, inert and generally safe to use. The temperature range from normal boiling point (77.4 K) to ambient (approximately 300 K) represents a wide range of particular needs in construction, transportation, food and beverage, pharmaceuticals, electrical power, electronics, medical imaging, aerospace, industrial processes and so forth, touching on virtually all aspects of modern life. The lowest temperature boiloff liquid is liquid helium with a normal boiling point of 4.2 K.

Because heat does not flow through a material as a function of temperature but according to a temperature difference, the use of a cryogen such as liquid nitrogen also provides a convenient way to establish the sub-ambient test conditions represented in the wide range of end-use applications.

Origins of Heat Measurement

Considerations and use of the cold have led to the understanding and application of the hot and have fundamentally paved the way for the development of the thermal sciences and related engineering fields. By 1761, Joseph Black had devised an experimental calorimeter using ice and came up the idea of hidden heat through his experimental studies [3]. In another example, an early ice-bath calorimeter was used in 1782–83 by Antoine Lavoisier and Pierre-Simon Laplace to determine the heat evolved in various chemical changes [4].

From these hidden heat (or latent heat) concepts of the mid-1700s we now have the terms heat of fusion and heat of vaporization. The notions of heat and absence of heat (now known as cold), first advanced by the great philosophers, remained in a highly formative stage for the next 100 years. A primary theme through all of these developments was chemistry and experimentation. Leading scientists of the day often considered heat to be a substance (caloric) that moved about in the world.

Joseph Fourier, however, went beyond the notion of heat as a substance with his famous Analytic Theory of Heat, published in 1822, emphasizing mathematical analysis applied to systematic observation [5]. He writes: “Heat, like gravity, penetrates every substance of the universe; its rays occupy all parts of space. The object of our work is to set forth the mathematical laws which this element obeys. The theory of heat will hereafter form one of the most important branches of general physics…No considerable progress can hereafter be made which is not founded on experiments such as these; for mathematical analysis can deduce from general and simple phenomena the expression of the laws of nature; but the special application of these laws to very complex effects demands a long series of exact observations.”

Nearly 200 years later, Fourier’s basic observation remains: experimental studies, testing methodologies and thermal apparatus are central to understanding heat energy. We still do not really know what heat is, though the words calorie and calorimeter remain, but we now have the terms and equations we call thermodynamics.

J. Willard Gibbs rigorously advanced the ideas of modern thermodynamics, and hence the concept of latent heat of evaporation, through his analytical work, The Scientific Papers of J. Willard Gibbs, published in 1906. Low temperature studies ensued in the following three decades, culminating in the liquefaction of helium and discovery of superconductivity by H. Kamerlingh Onnes. And it is Onnes who is credited with coining the term enthalpy that is so extensively used today [6]. The word enthalpy is a combination of the Greek prefix en- (“to put into”) and verb thalpein (“to heat”), serving as a reminder of the hard-to-pin-down nature of thermal energy that persists to this day.

Advent of Heat Measurement in Cryogenics

Cryogenics came about as both a word and technical field in the first half of the 20th century. In the US, the National Bureau of Standards (NBS) started a laboratory for cryogenics work, and by the 1940s demand arose for industrial use of liquid oxygen and other liquefied gases. These demands in turn drove the need for higher performance, larger scale thermal insulation systems for cryogenic tanks. The performance demands sharply increased again in the 1950s to support the development of the hydrogen bomb, while the space race, touched off by the launch of Sputnik in 1957, led to further extensive development in materials, testing and large-scale applications in the 1960s [7, 8].

During this time, the NBS Boulder laboratories and the newly formed National Aeronautics and Space Administration (NASA) played a key role, in concert with university and industry partners, in the development of thermal insulation systems for cryogenic applications. The team of Peter Glaser of the Arthur D. Little Company, for example, developed a boiloff calorimeter apparatus for measuring thermal conductivity [9] while under contract to NASA. This apparatus became the basis for the (now withdrawn) standard ASTM C745 [10].

Several other cryogenic boiloff apparatuses were built and used to produce the substantial body of technical literature through the early 1970s [11]. And thermal measurements of cryogenic insulation materials were summarized by the ASTM Symposium proceedings of 1967 [12]. In the 1980s, the NBS again led the way in operating a productive liquid nitrogen boiloff calorimeter for cryogenic materials testing [13]. More recently, liquid nitrogen boiloff calorimeters have been developed for the thermal performance testing of cryogenic pipelines [14, 15].

Latest Technology in Cryogenic Boiloff Calorimetry

Today, cryogenic boiloff calorimetry for the performance testing of thermal insulation systems is addressed in a standard guide published by ASTM International [16]. This technical guide, ASTM C1774, includes boiloff test calorimeters (or cryostats) in both flat plate and cylindrical configurations.

As an example of the absolute flat plate apparatus, the Cryostat-500 instrument developed by the Cryogenics Test Laboratory at NASA Kennedy Space Center is shown in the laboratory setting on the previous page. The heat measurement approaches include both absolute and comparative methods. In all configurations, cryogenic boiloff can provide direct measurement of heat energy for a very wide range of thermal performance [17].

Part 2: Theory of Cryogenic Boiloff Calorimetry

There are a number of advantages to using boiloff calorimetry to test thermal insulation materials and systems. These advantages become clear for cryogenic and other below-ambient temperature applications but can also be extended to higher temperature applications. For any need in science or industry, the only practical way to directly measure heat flow is by a phase change technique and its associated energy change.

The main advantage in using boiloff calorimetry is its ultimate simplicity and provision of a direct energy measurement. The liquid provides a stable cold boundary temperature and serves as the power meter. The approach also lends itself to testing under representative conditions (i.e., those that reflect the actual-use or field-installed conditions) [1].

Even a single homogeneous material becomes a complex system when it is mostly empty space (low density) and subjected to a sub-ambient cold boundary temperature. Boiloff calorimetry can provide sensitivity to test high thermal resistance materials and systems. High thermal resistance is defined herein as a range of R-value from approximately 10 to 10,000 (or heat flux values from approximately 0.1 to 100 W/m2 for typical 25-mm thick test specimens) [2].

Boiloff calorimetry is the inherently wide range of temperature involved. A given test condition can be arranged to achieve any set of cold and warm boundary temperatures within the maximum range. Multiple thermal conductivity values as a function of mean temperature can be obtained by using intermediate temperature sensors.

Finally, a strong advantage of boiloff calorimetry also provides the capability to test novel materials, composite combinations and other non-isotropic or non-homogeneous materials [3]. These novel composites, multifunctional materials and thermal management systems are sometimes composed of both high thermal resistance and high thermal conducting materials.

Most industry standard thermal test methods such as the widely used Heat Flux Meter are limited to testing isotropic, homogeneous single materials at small temperature differences at approximately ambient environment conditions. Cryogenic boiloff methods, on the other hand, provide the means to reliably test thermal insulation systems under representative conditions for a given engineering application. The cryogenic boiloff method, because it is a direct measure of the flow of heat, enables the testing of complex materials and systems over a very wide range of conditions.

The gas flow rates measured in evaporation, or boiloff, calorimetry enable direct calculation of quantities such as heat flux and thermal conductivity. Because heat does not flow through a material as a function of temperature but according to a temperature difference, the use of a cryogen provides a convenient way to establish the sub-ambient test conditions that are representative for a wide range of end-use applications.

Energy comes in many forms, two of which are work and heat. Heat, like work, is a special case in that it is not a property of a system, but is instead a property of the process of transferring energy. In general, we cannot measure how much heat is present in an object but rather only how much energy is transferred between two objects, one hotter and one colder, during the process. Enthalpy can be described as the heat content of a system, but the enthalpy of a system cannot be directly measured. Instead, only a change in enthalpy can be measured. The phase change from liquid to vapor, or enthalpy of vaporization, is a direct way to measure heat flow between objects.

In general, boiling is associated with higher heat transfer rates below the surface and evaporation with lower heat transfer rates at the surface. The thermophysics of boiling liquids and boiling point temperatures can be quite complex and variable (and is a subject of its own) [4]. Fortunately for the subject of thermal insulation testing, however, the properties are well within the evaporation-only regime with peak heat flux levels only up to several hundred watts per square meter at most. But as the common usage goes, the evaporation calorimetry described here will be referred to as boiloff calorimetry for simplicity.

Measurement of the Flow of Heat

Boiloff testing is accomplished by filling a vessel with a liquid that evaporates or boils below ambient temperature. Although the most commonly used liquid is liquid nitrogen (LN2), other cryogens such as liquid helium, liquid methane, liquid hydrogen, refrigerants, or water may be used. A test specimen is affixed to the vessel in a prescribed geometry and placed in a suitable environmental chamber to achieve the desired test conditions. The vessel is then filled with the test fluid such as a cryogenic liquid. The test apparatus can be of the absolute or comparative type. An absolute test apparatus always includes one or more guard vessels as part of its cold mass assembly.

The heat transmission through the test material to the liquid in the vessel is directly proportional to the gaseous boiloff flow rate from the vessel. This energy-going is the heat flow rate (Q), or power, in units of joules per second (W). The method is calorimetric in that the heat of vaporization of the liquid is used to determine the heat. Under steady-state conditions, the rate of heat passing through the test specimen to the vessel containing the liquid is constant at all points through the thickness of the test specimen.

The boiloff mass flow rate (m) is directly proportional to the heat flow rate by the enthalpy of vaporization (hfg) as shown below in equation 1. For a test specimen under a set environment and prescribed warm and cold boundary temperatures, the heat flux (q) and effective thermal conductivity (ke) are calculated from this direct measurement of the steady-state flow of heat as given in equations 2 and 3.
Q — Heat flow rate — quantity of heat energy transferred to a system in a unit of time
m — Mass flow rate (g/s)
hfg — Heat of vaporization (J/g)
q — Heat flux (W/m2) — heat flow rate, under steady-state conditions, through a unit area, in a direction perpendicular to the plane of the thermal insulation system
Ae — Effective heat transfer area (m2)
ke — Effective thermal conductivity (mW/m-K) — the effective thermal conductivity through the total thickness of the insulation test specimen between the reported boundary temperatures and in a specified environment. The insulation test specimen may be one material, homogeneous or non-homogeneous, or a combination of materials.
x — Thickness of insulation specimen (m)
ΔT —Temperature difference (K), warm boundary temperature (WBT) – cold boundary temperature (CBT)

Heat of Vaporization

The heat of evaporation is the energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure (such as atmospheric pressure). The hfg can be viewed as the energy required to overcome the intermolecular interactions within the liquid. For example, helium has a particularly low heat of vaporization (0.0845 kJ/mole) as the van der Waals forces between atoms are particularly weak. On the other hand, the molecules in liquid water are held together by relatively strong hydrogen bonds and its enthalpy of vaporization (40.65 kJ/mol).

Physical models as depicted in Figure 1 (page 34) for the liquid-gas phase transformation suggest that the energy required to liberate a molecule from the liquid is equivalent to the energy needed to overcome the surface resistance of the liquid [5].

Boiling point is the temperature at which the pressure exerted by the surroundings upon a liquid equals the pressure exerted by the vapor of the liquid. Under this condition, addition of heat works to transform the liquid into its vapor without raising the temperature. The boiling point of a liquid varies according to the applied pressure; the normal boiling point is the temperature at which the vapor pressure is equal to the standard sea-level atmospheric pressure (1.013 bar or 760 torr).

properties of several liquids are given in Table 1 [6]. Cryogens for boiloff testing include liquefied natural gas (112 K), liquid nitrogen (77 K), liquid hydrogen (20 K) or liquid helium (4 K). In principle, higher temperature liquids such as R134a (264 K), methanol (338 K) or water (373 K) could also be used for different applications.

Ideally, a boiling fluid maintained at constant saturation conditions intercepts all of the energy crossing the cold boundary in a direction normal to the plane of the insulation system. This energy is absorbed by the vaporization of the calorimetric fluid (cryogen) that is subsequently vented. For absolute boiloff methods and lower fill levels (wetted surface area less than 75 percent for liquid nitrogen and less than 90 percent for liquid hydrogen or liquid helium), the temperature of the gas exiting the test measurement tank should be measured and the change in sensible heat added to the energy from boiloff flow.

This analysis assumes that all heat flow to the calorimeter goes to vaporizing the liquid and none of it sensibly heats the vapor. The vapor heating effect can be neglected for liquid nitrogen calorimeters with small ullage spaces (less than approximately 75 percent of the total volume). The error due to vapor heating is estimated to be less than 0.1 percent when applying the results of the study by Jacobs [7]. For liquid hydrogen calorimeters the ullage vapor heating error is a manageable 5 percent while for liquid helium it becomes about 20 percent depending on the geometric factors. The heat of vaporization of the cryogen is the largest source of uncertainty and is typically taken to be a two percent uncertainty error for liquid nitrogen.

The boiloff flow rates are measured by a weight scale, mass flow meters or volumetric flow meters. Industry standard flow meters as well as volumetric techniques can be used to reliably measure gas flow rates down to around one standard cubic centimeter per minute (1 sccm) and up to 100,000 sccm or more in the laboratory setting. For nitrogen, the corresponding power levels are 0.004 W and 414 W respectively, showing both a very small lower limit and a wide range of heat flow measurement capability.

Part 3: Cylindrical Boiloff Testing of Insulation Systems

Evaporation or “boiloff” calorimetry has been a valuable technique for testing the thermal insulating performance of materials for more than 50 years [1]. As discussed in Part 2 (Cold Facts Vol 32 No 5), the heat or enthalpy of vaporization (hfg) is the energy required to transform a given quantity of a liquid into a gas at a given pressure. The use of liquid nitrogen (LN2) or another cryogen provides a direct measure of the heat flow rate.
Cryogenic insulation systems encompass a wide range of material combinations. An insulation test specimen is a system composed of one or more materials (homogeneous or not, isotropic or not and with or without inclusion of a gas).

Boiloff calorimetry is the measurement principle for determining the effective thermal conductivity (ke) and heat flux (q) of a test specimen at a fixed environmental condition (boundary temperatures, cold vacuum pressure and residual gas composition).

Cylindrical cryostat instruments provide an effective and reliable way to characterize the thermal performance of materials and systems under subambient conditions. Future multilayer insulation systems are envisioned that will challenge the theoretical limit in thermal insulation performance (ke <0.01 mW/m-K and/or q <0.1 W/m2 for typical boundary conditions of 300 K / 77 K in high vacuum). Boiloff technology to measure ultralow heat flow is at the heart of efforts to develop and prove such advancements. Ultralow heat flow systems are needed for superconducting power devices, long-duration storage of cryofuels, science instruments, space exploration craft, medical imaging equipment and other performance-driven applications.

Engineers have developed and standardized several cryostat instruments for laboratory testing of thermal insulation systems in a cylindrical configuration. Insulation test cryostats are either absolute (guarded) or comparative (unguarded or partially guarded). Cold mass designs follow either a stratified or destratifed (mixed) liquid approach to thermal stability. Stratified designs use stainless steel construction techniques while the latter rely on high thermal conductivity materials such as copper. Convection currents are the mechanism of heat transport through the insulation thickness and into the liquid, carrying heat up to the surface where the evaporation occurs [2].

Although both cylindrical and flat-plate cryostats have been standardized for laboratory operation [3, 4], cylindrical configurations are better at minimizing (or even eliminating) unwanted lateral heat transfer or “end effects.” Cylinders are representative of many end-use applications such as tanks and piping. The vertical cold mass assemblies of cylindrical cryostats can cause some convection problems for ambient pressure tests but otherwise provide a stable platform for testing over a wide range of heat flows.
For example, the Cryostat-100 apparatus—following the guidelines of ASTM C1774, Annex A1—is guarded on top and bottom for absolute thermal performance measurement [5, 6]. Each of the three chambers is filled and vented through a single feedthrough (also connected from the lid) for easy operation and minimum heat leakage by conduction, as shown in the basic schematic of Figure 1. Liquid within each chamber is allowed to stabilize in a stratified state to provide stable and consistent heat flow measurement.

Cryostat-100 includes an external heating system for bakeout and an internal heater system for fine control of the warm boundary temperature (WBT). Funnel filling tubes interface with its three LN2 feedthroughs for cooldown, filling and replenishment by pouring from a small open dewar. Vacuum instrumentation may include two capacitance manometers, an ion gage and a full-range transducer. The vacuum pumping system includes a directly connected turbopump and a separately plumbed mechanical pump. A gaseous nitrogen (GN2) system provides purging and residual gas supply for controlling vacuum levels as low at 5 × 10−5 torr.

Materials can be blanket, clamshell, molded or bulk-fill. Following the guidance of ASTM C740, engineers can apply MLI systems in individual layers or in various layering combinations as desired, with temperature sensors placed between the blanket layers [7]. Upon establishing the desired WBT and Cold Vacuum Pressure (CVP), the test specimen is cooled down, stabilized and tested according to standard procedure. The steady-state condition is reached when the boiloff flow rates from all three chambers are stabilized, the temperature profile through the thickness is stabilized and the liquid level in the test chamber is at least 90 percent full. A stable state of the system is indicated by slight oscillation of the temperature sensors with no overall trend in the average value [9]. The total test duration may be hours to days, depending on the level of heat flow involved.

The liquid within the chambers can be stratified, mixed or in transition. All liquid masses must be either stratified or mixed for steady-state measurement to be achieved, a condition reached only by the inherent design of the cold mass assembly. Other important factors in boiloff flow rate stability are the regional variations and twice-daily fluctuations in atmospheric pressure that correlate to the atmospheric tides. Without systematic controls to counteract this effect these fluctuations can influence the results by up to 20 percent at very low rates of heat flux. These fluctuations can be minimized by applying a slight back pressure to chambers of approximately 3 torr above the prevailing mean at atmospheric pressure and by controlling the back pressures of all three chambers within ±0.1 torr.

The rate of heat transfer through the insulation test specimen and into the side wall of the test chamber of the cold mass assembly (Q) is directly proportional to the LN2 boiloff flow rate (V), as given by Formula 1, where STP = Standard Temperature and Pressure (0°C and 760 torr) and the right hand term is the density correction, if any, between the liquid and the saturated liquid conditions.

The value of ke is determined from Fourier’s law of heat conduction through a cylindrical wall. For cylindrical geometries, the effective heat transfer area (Ae) is the mean area between the two concentric cylinders. The heat flux (q) is calculated by dividing the total heat transfer rate by the effective heat transfer area. Full details are given in ASTM C1774 [3].

The total uncertainty in ke is calculated to be 3.3 percent for the Cryostat-100 [5]. Measurement of the boiloff flow rate is made using a mass flow meter that automatically compensates for gas densities. The heat of vaporization of the cryogen is the largest source of uncertainty, typically 2 percent for LN2, and should be adjusted according to the saturated vapor pressure inside the test chamber. Physical measurement of the test specimen is robust because diameters and not thickness are part of the calculation. In most cases, for a given series of tests, the repeatability is demonstrated to be within 2 percent.

If the temperature of the vapor leaving the test chamber is the same temperature as the saturated vapor at the liquid surface, then all heat flow to the calorimeter is assumed to go into vaporizing the liquid and none into sensible heating of the vapor. The vapor heating effect can be neglected for LN2 calorimeters with small ullage spaces (less than 20 percent of the total volume). The error attributable to vapor heating in nitrogen is estimated to be <0.1% when the results of the study by Jacobs [8] are applied. But tests in the range of very low heat flows (such as MLI systems below 1 W/m2) are routinely performed at liquid fill levels above 98 percent in which case there is essentially no ullage space.

For liquid hydrogen or liquid helium, the errors for similar geometry and liquid level are about 1 percent and 4 percent respectively. However, the errors can rise appreciably—according to the specific combination of geometry, cold mass design, ullage space height, boiloff flow rate (heat flow rate), and even instabilities in the evaporation process [2]. The theoretical density ratio term in equation 1 increases as the normal boiling point of the liquid decreases. For water, this density ratio is approximately 1.00; it increases to 1.01 for nitrogen, to 1.02 for hydrogen and then increases dramatically to 1.16 for helium. As pointed out, the cold mass design and method of operation are paramount in the evaporation heat flow analysis and if in doubt, measurement of the boiloff vapor temperature exiting the test chamber is the key.

An example Cryostat-100 test result of a 60-layer insulation system at high vacuum is presented in Figure 2. In this plot, the boiloff flow rates from all three chambers are shown for the test duration. Periodic oscillation of the test chamber flow rate, induced by atmospheric tides in coastal Florida, is indicated by the regular 12-hour peaks (note that no back-pressure control was used in this case). Test results for various thermal insulation systems and materials in terms of the variation of ke with CVP are given in Figure 3. Further details of these and many other materials are given in the literature [5, 9].

Part 4: Flat Plate Boiloff Testing of Insulation Systems

In general, one cannot measure how much heat is present in an object, but rather only how much energy is transferred between objects at different temperatures, hot and cold. Early attempts at temperature measurement include the experiments of Greek physician Galen in AD 170, followed many centuries later by Fahrenheit’s description of the first modern temperature scale in 1724. A few decades later, J. Black devised an ice calorimeter based on his discovery of hidden heat. And by 1822, J. Fourier had published The Analytical Theory of Heat, a work that remains the basis of our notions of heat, thermal energy and temperature.

The technological development of large-scale liquid hydrogen in the US in the 1950s gave rise to the demand for high performance thermal insulation systems [1]. To meet this demand and enable the development of multilayer insulation (MLI) and evacuated perlite powder systems, engineers devised different apparatuses to directly measure heat flows from a few milliwatts and up using evaporation or “boiloff” calorimetry. Following the examples of cylindrical boiloff testing given in Part 3 (Cold Facts Vol. 32 No. 6), the series concludes with a look at the methods and apparatus for flat plate boiloff testing of cryogenic-vacuum thermal insulation systems.

Researchers have developed and standardized several cryostat instruments for laboratory testing of thermal insulation systems in a flat plate configuration [2]. Cryostat-500 (203-mm diameter test specimen) is thermally guarded by a separate cryogen chamber to provide absolute thermal performance data. Absolute (guarded) instruments produce the data by which comparative (unguarded) instruments can be calibrated [3]. The larger Cryostat-600 (305-mm diameter) is a similar guarded design that includes the option of attaching structural elements for testing. Cryostat-400 is a comparative version (no guard chamber) of the Cryostat-500 [4]. The Macroflash Cup Cryostat (76-mm diameter) is a comparative, benchtop-size instrument for thermal conductivity testing of materials, from aerogel insulation to carbon composites. The Macroflash is designed to test in ambient pressure environments with different purge gases and under compression loads from zero up to approximately 100 kPa.

Although cylindrical configurations are better at minimizing unwanted lateral heat transfer, flat-plate configurations offer a number of potential advantages regarding the test specimens, including the ability to test small size test specimens, compression loading capability and specialized ambient pressure testing with different purge gases. Flat-plate cryostats are easier to adjust for different cold boundary temperatures (CBT) by the placement of an intermediary material on the cold-side surface.

For example, the Cryostat-500 apparatus—following the guidelines of ASTM C1774, Annex A3—is guarded on the top and around its perimeter for absolute heat flow measurement [5]. An adjustable-edge guard ring enables calibration with a known material. With liquid nitrogen as the working cryogen, the CBT can be adjusted to temperatures between 77 K and ambient by placing a known thermal resistance layer between the cold mass and the test specimen. Multiple data points for a range of mean temperatures can be obtained from a single test by the use of intermediate temperature sensors.

The cold mass assembly, comprising the test chamber and guard chamber, is suspended from the vacuum chamber and uses a thermal break between chambers to preclude direct solid-conduction heat transfer between the two liquid volumes. This isolation is critical for thermal stability and the fine equilibrium necessary for an accurate boiloff measurement. A low thermal conductivity suspension system includes compliance rod assemblies that can be adjusted for test specimen thickness and for rigid or compressible materials. Compression loading up to 100 kPa can be applied if required.

Cryostat-500 includes an external heating system for bakeout and a heating plate system for control of the warm boundary temperature (WBT). Two funnel-filling tubes interface with the two LN2 feedthroughs to provide the means for cooldown, filling and replenishment. Boiloff flows, from both the test chamber and the guard chamber, are routed to their respective mass flow meters. Vacuum instrumentation includes one or more capacitance manometers and an ion gage for high vacuum. The vacuum pumping system includes a turbopump and a separately plumbed mechanical pump. A gaseous nitrogen supply system provides purging and residual gas pressure control to set any desired CVP from 760 torr to 5 x 10-5 torr.

Testing Methodology

Materials can be in the form of monolithic disks, layered slabs, composite panels, blankets or layered blankets. Bulk-fill materials are more difficult to use on flat-plate calorimeters, but have been successfully tested. From the measured thickness of a given specimen, the suspension system is precisely adjusted for the desired test thickness. Springs are used for rigid materials to provide a built-in compliance while fixed spacers are used for soft materials to establish a predetermined thickness and compression.

The steady-state condition is reached when the boiloff flow rates from both chambers are stabilized, the temperature profile through the test specimen is stabilized and the liquid level in the guard chamber is at least 50 percent full (that is, covering the top surface of the test chamber). For simplicity in operation, the test chamber liquid can be any level between zero and 100 percent, with test durations lasting from two hours to two days depending on the range of heat flow that spans nearly four decades. Both liquid masses must be stratified and stable for steady-state measurement to be achieved, a condition provided by inherent design of the cold mass assembly.

The rate of heat transfer through the insulation system and into the bottom of the test chamber of the cold mass assembly (Q) is directly proportional to the LN2 boiloff flow rate (V), as given in Part 2 of this series (Cold Facts Vol 32 No. 5). Effective thermal conductivity (ke) is determined from Fourier’s law of heat conduction through a flat plate as detailed in ASTM C1774. The heat flux (q) is calculated by dividing the total heat transfer rate by the effective area for heat transfer (Ae). The total uncertainty in ke is calculated to be 4.8 percent for Cryostat-500. Thickness is the largest source of uncertainty and must be handled carefully, particularly for specimens thinner than 10 mm. In addition, fit-up is crucial for good thermal contact to be maintained through the usual thermal cycles and shrinkage associated with testing. For testing of MLI systems, the accounting for edge effects, layer density and compressibility are especially critical [6]. In most testing situations, for a given series of tests, the overall repeatability is demonstrated to be within 2 percent.

Example Test Results

From a data library of hundreds of materials, including foams, fiberglass, polymers, aerogels, honeycombs, layered composites and MLI, some example Cryostat-500 test results are given for the boundary temperatures of 293 K / 78 K and a residual gas of nitrogen. The results for a five-run test of a carbon fiber composite panel at 760 torr CVP are given in Figure 2 (see page 31). This test specimen was a six-layer stack, with temperature sensors between layers to enable analysis of the variation of thermal conductivity with mean temperature [7]. Figure 3 presents an example test of an MLI system showing a stable boiloff flow rate for more than 30 hours. Details of these and many other materials are given in the literature [3, 8]. An advantage of flat-plate boiloff calorimetry is the capability for testing novel composites, multifunctional materials and advanced heat management systems that are composed of both very high and very low thermal resistance materials.

Different materials and varied test objectives require an appropriate combination of apparatus (cylindrical or flat plate), and method (absolute or comparative). The material type, thickness, density, flatness, compressibility, outgassing characteristics are important considerations as well as relevance to the configuration of the end-use thermal insulation system. In addition to providing a means for the direct measurement of heat flow—with sensitivity over a very wide range—another advantage of boiloff calorimetry is the large temperature difference that is readily available. If desired, the boundary temperatures can be adjusted or a given temperature difference can be subdivided to obtain additional data for thermal modeling, materials research or product development. For example, multiple data points can be obtained from a single test with the placement of intermediate temperature sensors through the thickness of the test specimen. In any case, the heat transfer principle remains the same as postulated by Fourier in 1822: heat flows not as a function of temperature but because of a difference in temperature.