1.1 本指南为实验室测量低温条件下绝热系统的稳态热传递特性和热通量提供了信息。隔热系统可以由一种或多种材料组成，这些材料可以是均匀的或非均匀的；平坦的、圆柱形的或球形的；在从接近绝对零或4K到400K的边界条件下；以及在从高真空到空气或残余气体的环境压力的环境中。 作为本指南一部分介绍的测试方法与其他ASTM热测试方法不同，但又是补充，包括 C177 , C518 和 C335 。本指南的一个关键方面是绝缘系统的概念，而不是绝缘材料。在大多数低温应用的实际使用环境下，即使是单个材料系统也可能是复杂的隔热系统 ( 1. 3. ) . 2. 为了确定绝缘材料的固有热性能，应参考本指南中引用的标准试验方法。 1.2 在这些应用中使用的大多数低温绝热系统的功能是保持大的温差，从而提供高水平的绝热性能。暖边界温度和冷边界温度的组合可以是在接近0K到400K的范围内的任何两个温度。冷边界温度通常在4K到100K的范围，但是可以高得多，例如300K。暖边界温度通常从250K到400K，但是可以低得多，例如40K。 典型的情况是高达300 K的大温差。在低温下用一个边界进行大温差下的热性能测试是典型的，也是大多数应用的代表。作为温度函数的热性能也可以根据实践进行评估或计算 C1058 或 C1045 当关于温度分布和物理建模的足够信息可用时。 1.3 本指南的残余气体压力范围为10 7. 托至10 3. 托（1.33 5. Pa至133kPa），并根据需要使用不同的吹扫气体。与低温系统中的应用相对应，还定义了三个子真空范围:＜10的高真空（HV） 6. 托至10 3. 托（1.333 4. Pa至0.133Pa）[自由分子状态]，软真空（SV）为10 2. 托至10托（从1.33 Pa至1333 Pa）[过渡状态]，从100托至1000托（13.3 kPa至133 kPa）的无真空（NV）[连续状态]。 1.4 在整个真空压力范围内，热性能可以变化四个数量级。 有效热导率可以在0.010mW/m-K到100mW/m-K的范围内。热性能的主要控制因素是测试环境的压力。高真空绝热系统通常在0.05mW/m-K到2mW/m-K的范围内，而非真空系统通常在10mW/m-K和30mW/m-K之间。软真空系统通常介于这两个极端之间 ( 4. ) 特别需要的是在- 环境温度环境。例如，作为许多低温绝缘系统的正常工程应用，需要仔细描绘0.01mW/m-K至1mW/m-K（从R值14400到R值144）范围内的测试结果 ( 5. 7. ) .将有效导热系数应用于多层隔热（MLI）系统和其他不同材料的组合，因为它们具有高度各向异性和专业性，必须谨慎行事，并充分提供支持性技术信息 ( 8. ) 热通量的使用（W/m 2. )一般来说，更适合报告MLI系统的热性能 ( 9- 11 ) . 1.5 本指南涵盖了在亚环境温度环境中测量热性能的不同方法。测试仪器（仪器）分为两类:沸腾量热法和电力。包括绝对仪器和比较仪器。 1.6 本指南规定了建造和操作令人满意的测试设备所需的一般设计要求。 涵盖了各种各样的仪器结构、测试条件和操作条件。未给出详细设计，但必须在一般要求的限制范围内进行开发。文献中有不同低温试验装置的例子 ( 12 ) 这些设备包括锅炉类型 ( 13- 17 ) 以及电气类型 ( 18- 21 ) . 1.7 这些测试方法适用于各种样品的测量，从不透明固体到多孔或透明材料，以及各种环境条件，包括在极端温度、各种气体和一定压力范围内进行的测量。 特别重要的是测试高度各向异性材料和系统（如多层绝缘（MLI）系统）的能力 ( 22- 25 ) 其他测试方法在这方面是有限的，并且不包括MLI和其他分层系统在这些系统典型的极端低温和真空条件下的测试。 1.8 为了确保预期的精度和准确性，使用本标准的用户必须具备热测量和测试实践要求的工作知识，以及与隔热材料和系统相关的传热理论的实际应用知识。 应为每个设备提供详细的操作程序，包括设计原理图和电气图纸，以确保测试符合本指南的要求。此外，必须验证连接到设备的自动数据收集和处理系统的准确性。可以通过使用计算机模型校准和比较具有已知结果的数据集来进行验证。 1.9 制定隔热测试设备设计和施工的所有细节，并提供涵盖与热流测量、极其精细的热平衡、高真空、温度测量和一般测试实践相关的所有意外事件的程序，是不切实际的。 用户还可能发现，在修理或修改设备时，有必要成为设计者或建设者，或两者兼而有之，对基本理解和仔细的实验技术的要求甚至更高。这里给出的测试方法用于实际使用和调整，并使未来能够开发改进的设备或程序。 1.10 本指南并未详细说明设备操作所需的所有细节。取样、试样选择、预处理、试样安装和定位、试验条件选择以及试验数据评估的决定应遵循适用的ASTM试验方法、指南、实践或产品规范或政府法规。 如果不存在适用的标准，则必须使用反映公认传热原理的合理工程判断并形成文件。 1.11 本指南允许使用广泛的仪器设计和设计精度，以满足特定测量问题的要求。遵守进一步规定的测试方法应包括一份报告，其中讨论了所涉及的重大误差因素以及每个报告变量的不确定性。 1.12 以国际单位制表示的数值应视为标准。括号中给出的值仅供参考。除非另有规定，否则报告中可以使用国际单位制或英制单位。 1.13 安全预防措施，包括使用冷冻剂的正常操作和使用规程。在使用任何潜在危险的冷冻剂或流体操作设备之前，应对所有系统的设计、施工和安装进行全面审查。 经过多年的使用，有关危险液体处理的安全实践和程序已得到广泛发展和验证。对于含有氢气的系统，应特别注意确保采取以下预防措施: 1. 测试区域的充分通风， 2. 防止泄漏， 3. 消除点火源， 4. 故障安全设计，以及 5. 流体填充和排气管线的冗余规定。 本标准并不旨在解决与其使用相关的所有安全问题（如有）。 本标准的使用者有责任在使用前制定适当的安全、健康和环境实践，并确定监管限制的适用性。 1.14 本标准中的主要章节安排如下: 部分 范围 1. 参考文件 2. 术语 3. 试验方法总结 4. 意义和用途 5. 仪器 6. 试样和制备 7. 程序 8. 结果的计算 9 汇报 10 关键词 11 附件 圆柱形蒸发量热计（绝对值） 附件A1 圆柱形蒸发量热计（比较） 附件A2 平板蒸发热量计（绝对值） 附件A3 平板蒸发热量计（比较） 附件A4 电力低温恒温器（冷冻剂） 附件A5 电力低温恒温器（低温冷却器） 附件A6 附录 根本原因 附录X1 工具书类 1.15 本国际标准是根据世界贸易组织技术性贸易壁垒委员会发布的《关于制定国际标准、指南和建议的原则的决定》中确立的国际公认的标准化原则制定的。 ====意义和用途====== 5.1 了解低温隔热系统热性能的一个关键方面是在具有代表性和可重复性的条件下进行测试，模拟材料实际组装和使用的方式。因此，通常需要绝缘层上的大温差和特定压力下的残余气体环境。除了这些要求之外，还增加了热收缩后测试条件下厚度测量的复杂性、冷却后表面接触和/或机械载荷的验证以及材料内高真空水平的测量。 考虑表面接触电阻可能是一个特别的挑战，尤其是对于刚性材料 ( 32 ) 在通常低密度、高表面积的材料中施加大的温差意味着间隙物质的组成和状态可以通过系统的厚度而发生剧烈变化。即使对于单组分系统，如主要闭孔泡沫的片材，该系统的组成也通常包括空气、湿气和发泡剂，它们在整个材料中具有不同的浓度和物理状态和形态。 在给定的一组下测试的系统 WBT , CBT 和 CVP 条件包括所有这些成分（不仅仅是泡沫材料）。这个 CVP 可以通过设计施加，或者可以响应于边界温度的变化以及绝缘材料的表面效应而变化。为了使自由分子气体传导发生，气体分子的平均自由程必须大于两个传热表面之间的间距。平均自由程与表面之间距离的比值为克努森数（见指南 C740 以供进一步讨论）。大于1.0的克努森数被称为分子流动条件，而小于0.01的克努逊数被视为连续或粘性流动条件。低温真空绝热系统的测试可以涵盖许多不同的中间或混合模式传热条件。 5.2 热性能水平可能非常高:热通量值远低于0.5 W/m 2. 测量。例如，这种性能级别可以对应于 k e 对于300K和77K的边界温度以及25mm的厚度，低于0.05mW/m-K（R值=2900或更高）。在这些非常低的热传递率下，对于平均尺寸的测试设备，大约为几十毫瓦，必须仔细考虑方法、设计、安装和执行的所有细节，以获得有意义的结果。例如，用于温度传感器的引线可以是更小的直径、更长的长度，并且小心地安装，以尽可能降低对冷物质的热传导。 在沸腾试验的情况下，还应考虑大气压力效应、冷冻剂的启动条件以及来自周围设施的任何振动力。如果要设计一个绝对试验装置，则应通过装置和试验方法的集成设计基本上消除寄生热泄漏。性能水平越高（通常真空水平越高），总热负荷越低，因此寄生部分应接近零。 对于比较设备，寄生热泄漏必须降低到所测量的总热负荷的可接受部分的水平。最重要的是，对于比较仪器，在给定的测试条件下，热量的寄生部分应一致且可重复。 5.3 蒸发测试-- 蒸发试验可以使用许多冷冻剂或正常沸点低于环境温度的冷冻剂进行 ( 29 ) 冷边界温度通常是固定的，但可以通过在冷物质和样品之间插入热阻层（例如聚合物复合材料或任何合适的材料）而容易地调节得更高。 然而，应充分了解热接触电阻，并获得 具体的 冷侧温度可能是困难的。液氮（LN 2. )是一种常用的冷冻剂，并且可以相对容易和经济地处理和采购。它在1个大气压下的77K沸点在代表包括液氧（LO 2. )、液态空气（LAIR）和液化天然气（LNG）。利用液氮系统的低水平的空燃蒸汽加热意味着蒸汽校正是最小的，甚至可以忽略不计。 液氢（LH 2. )，具有20K的正常沸点，可以在使用易燃流体时采取适当的额外安全预防措施。液氦（LHE）的正常沸点为4K，也可以有效使用，但费用和复杂性显著增加。热性能或热流率（W）与液体蒸发热（J/g）产生的沸腾质量流速（g/s）直接相关。因此，对于计算 k e 或热通量。 5.4 电力测试-- 在某些情况下，沸腾法可能不是热性能测试的最佳选择。在没有额外的安全约束（液氢）或不合理的费用（液氦）的情况下获得低于77K的冷边界温度通常是主要原因。电力方法的使用提供了广泛的可能方法，而不受液体-蒸汽界面和液体管理的限制。电力设备可以被设计为仅使用低温冷却器、与冷冻剂或蒸汽屏蔽物结合的低温冷却器、提供制冷以维持期望的冷边界温度的冷冻剂，或者这些的任何组合。 关键的实验元件是电加热器系统，但关键的挑战是低温下的温度传感器校准。温度传感器通常是硅二极管或铂电阻温度计。因此，这些方法在计算有效热导率或热通量方面是间接的。 5.5 MLI-- 多层绝缘系统通常是抽空的（为真空环境设计）。MLI系统中使用的材料本质上是高度各向异性的。 MLI系统的热通量值比真空条件下的最佳可用粉末、纤维或泡沫绝缘材料低一到两个数量级。由于材料特性的差异，如反射屏蔽的发射率，以及结构的差异，例如层密度和接缝或接头的制作方式，多层绝缘的热性能将因样品而异。MLI系统可能因环境条件和诸如氧气或水蒸气等异物的存在而变化。 MLI系统可能由于老化、沉降或暴露于过大的机械压力而变化，这可能会起皱或以其他方式影响层的表面纹理。出于这些原因，必须仔细选择试样材料，以获得具有代表性的试样。建议对任何一个MLI系统的几个样本进行测试，每个样本至少进行三次测试。指南中提供了更多信息，包括安装方法和典型热性能数据 C740 . 5.6 高性能绝缘系统-- 从环境压力下的气凝胶到真空粉末再到高真空条件下的MLI，高性能隔热系统是低温设备和工艺中要求更高的应用的典型选择。高性能的要求意味着低的热能传递率（在毫瓦的范围内），以及对精确测量这些小的热泄漏率的更高要求。实现这样的测量需要良好的实验方法和设计，专业的真空设备- out方法，以及对数据的仔细执行和处理。 注1: 目前缺乏在低温真空条件下表征的认证参考材料，甚至实验室内部参考材料，这突出了循环测试、实验室间研究和基于这些实验结果开发强大分析工具的必要性。

1.1 This guide provides information for the laboratory measurement of the steady-state thermal transmission properties and heat flux of thermal insulation systems under cryogenic conditions. Thermal insulation systems may be composed of one or more materials that may be homogeneous or non-homogeneous; flat, cylindrical, or spherical; at boundary conditions from near absolute zero or 4 K up to 400 K; and in environments from high vacuum to an ambient pressure of air or residual gas. The testing approaches presented as part of this guide are distinct from, and yet complementary to, other ASTM thermal test methods including C177 , C518 , and C335 . A key aspect of this guide is the notion of an insulation system, not an insulation material. Under the practical use environment of most cryogenic applications even a single-material system can still be a complex insulation system ( 1- 3 ) . 2 To determine the inherent thermal properties of insulation materials, the standard test methods as cited in this guide should be consulted. 1.2 The function of most cryogenic thermal insulation systems used in these applications is to maintain large temperature differences thereby providing high levels of thermal insulating performance. The combination of warm and cold boundary temperatures can be any two temperatures in the range of near 0 K to 400 K. Cold boundary temperatures typically range from 4 K to 100 K, but can be much higher such as 300 K. Warm boundary temperatures typically range from 250 K to 400 K, but can be much lower such as 40 K. Large temperature differences up to 300 K are typical. Testing for thermal performance at large temperature differences with one boundary at cryogenic temperature is typical and representative of most applications. Thermal performance as a function of temperature can also be evaluated or calculated in accordance with Practices C1058 or C1045 when sufficient information on the temperature profile and physical modeling are available. 1.3 The range of residual gas pressures for this Guide is from 10 -7 torr to 10 +3 torr (1.33 -5 Pa to 133 kPa) with different purge gases as required. Corresponding to the applications in cryogenic systems, three sub-ranges of vacuum are also defined: High Vacuum (HV) from <10 -6 torr to 10 -3 torr (1.333 -4 Pa to 0.133 Pa) [free molecular regime], Soft Vacuum (SV) from 10 -2 torr to 10 torr (from 1.33 Pa to 1,333 Pa) [transition regime], No Vacuum (NV) from 100 torr to 1000 torr (13.3 kPa to 133 kPa) [continuum regime]. 1.4 Thermal performance can vary by four orders of magnitude over the entire vacuum pressure range. Effective thermal conductivities can range from 0.010 mW/m-K to 100 mW/m-K. The primary governing factor in thermal performance is the pressure of the test environment. High vacuum insulation systems are often in the range from 0.05 mW/m-K to 2 mW/m-K while non-vacuum systems are typically in the range from 10 mW/m-K to 30 mW/m-K. Soft vacuum systems are generally between these two extremes ( 4 ) . Of particular demand is the very low thermal conductivity (very high thermal resistance) range in sub-ambient temperature environments. For example, careful delineation of test results in the range of 0.01 mW/m-K to 1 mW/m-K (from R-value 14,400 to R-value 144) is required as a matter of normal engineering applications for many cryogenic insulation systems ( 5- 7 ) . The application of effective thermal conductivity values to multilayer insulation (MLI) systems and other combinations of diverse materials, because they are highly anisotropic and specialized, must be done with due caution and full provision of supporting technical information ( 8 ) . The use of heat flux (W/m 2 ) is, in general, more suitable for reporting the thermal performance of MLI systems ( 9- 11 ) . 1.5 This guide covers different approaches for thermal performance measurement in sub-ambient temperature environments. The test apparatuses (apparatus) are divided into two categories: boiloff calorimetry and electrical power. Both absolute and comparative apparatuses are included. 1.6 This guide sets forth the general design requirements necessary to construct and operate a satisfactory test apparatus. A wide variety of apparatus constructions, test conditions, and operating conditions are covered. Detailed designs are not given but must be developed within the constraints of the general requirements. Examples of different cryogenic test apparatuses are found in the literature ( 12 ) . These apparatuses include boiloff types ( 13- 17 ) as well as electrical types ( 18- 21 ) . 1.7 These testing approaches are applicable to the measurement of a wide variety of specimens, ranging from opaque solids to porous or transparent materials, and a wide range of environmental conditions including measurements conducted at extremes of temperature and with various gases and over a range of pressures. Of particular importance is the ability to test highly anisotropic materials and systems such as multilayer insulation (MLI) systems ( 22- 25 ) . Other test methods are limited in this regard and do not cover the testing of MLI and other layered systems under the extreme cryogenic and vacuum conditions that are typical for these systems. 1.8 In order to ensure the level of precision and accuracy expected, users applying this standard must possess a working knowledge of the requirements of thermal measurements and testing practice and of the practical application of heat transfer theory relating to thermal insulation materials and systems. Detailed operating procedures, including design schematics and electrical drawings, should be available for each apparatus to ensure that tests are in accordance with this Guide. In addition, automated data collecting and handling systems connected to the apparatus must be verified as to their accuracy. Verification can be done by calibration and comparing data sets, which have known results associated with them, using computer models. 1.9 It is impractical to establish all details of design and construction of thermal insulation test equipment and to provide procedures covering all contingencies associated with the measurement of heat flow, extremely delicate thermal balances, high vacuum, temperature measurements, and general testing practices. The user may also find it necessary, when repairing or modifying the apparatus, to become a designer or builder, or both, on whom the demands for fundamental understanding and careful experimental technique are even greater. The test methodologies given here are for practical use and adaptation as well as to enable future development of improved equipment or procedures. 1.10 This guide does not specify all details necessary for the operation of the apparatus. Decisions on sampling, specimen selection, preconditioning, specimen mounting and positioning, the choice of test conditions, and the evaluation of test data shall follow applicable ASTM Test Methods, Guides, Practices or Product Specifications or governmental regulations. If no applicable standard exists, sound engineering judgment that reflects accepted heat transfer principles must be used and documented. 1.11 This guide allows a wide range of apparatus design and design accuracy to be used in order to satisfy the requirements of specific measurement problems. Compliance with a further specified test method should include a report with a discussion of the significant error factors involved as well the uncertainty of each reported variable. 1.12 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only. Either SI or Imperial units may be used in the report, unless otherwise specified. 1.13 Safety precautions including normal handling and usage practices for the cryogen of use. Prior to operation of the apparatus with any potentially hazardous cryogen or fluid, a complete review of the design, construction, and installation of all systems shall be conducted. Safety practices and procedures regarding handling of hazardous fluids have been extensively developed and proven through many years of use. For systems containing hydrogen, particular attention shall be given to ensure the following precautions are addressed: (1) adequate ventilation in the test area, (2) prevention of leaks, (3) elimination of ignition sources, (4) fail safe design, and (5) redundancy provisions for fluid fill and vent lines. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. 1.14 Major sections within this standard are arranged as follows: Section Scope 1 Referenced Documents 2 Terminology 3 Summary of Test Methods 4 Significance and Use 5 Apparatus 6 Test Specimens and Preparation 7 Procedure 8 Calculation of Results 9 Report 10 Keywords 11 Annexes Cylindrical Boiloff Calorimeter (Absolute) Annex A1 Cylindrical Boiloff Calorimeter (Comparative) Annex A2 Flat Plate Boiloff Calorimeter (Absolute) Annex A3 Flat Plate Boiloff Calorimeter (Comparative) Annex A4 Electrical Power Cryostat Apparatus (Cryogen) Annex A5 Electrical Power Cryostat Apparatus (Cryocooler) Annex A6 Appendix Rationale Appendix X1 References 1.15 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee. ====== Significance And Use ====== 5.1 A key aspect in understanding the thermal performance of cryogenic insulation systems is to perform tests under representative and reproducible conditions, simulating the way that the materials are actually put together and used in service. Therefore, a large temperature differential across the insulation and a residual gas environment at some specific pressure are usually required. Added to these requirements are the complexities of thickness measurement at test condition after thermal contraction, verification of surface contact and/or mechanical loading after cooldown, and measurement of high vacuum levels within the material. Accounting for the surface contact resistance can be a particular challenge, especially for rigid materials ( 32 ) . The imposition of a large differential temperature in generally low density, high surface area materials means that the composition and states of the interstitial species can have drastic changes through the thickness of the system. Even for a single component system such as a sheet of predominately closed-cell foam, the composition of the system will often include air, moisture, and blowing agents at different concentrations and physical states and morphologies throughout the material. The system, as tested under a given set of WBT , CBT , and CVP conditions, includes all of these components (not only the foam material). The CVP can be imposed by design or can vary in response to the change in boundary temperatures as well as the surface effects of the insulation materials. In order for free molecular gas conduction to occur, the mean free path of the gas molecules must be larger than the spacing between the two heat transfer surfaces. The ratio of the mean free path to the distance between surfaces is the Knudsen number (see Guide C740 for further discussion). A Knudsen number greater than 1.0 is termed the molecular flow condition while a Knudsen less than 0.01 is considered a continuum or viscous flow condition. Testing of cryogenic-vacuum insulation systems can cover a number of different intermediate or mixed mode heat transfer conditions. 5.2 Levels of thermal performance can be very high: heat flux values well below 0.5 W/m 2 are measured. This level of performance could, for example, correspond to a k e below 0.05 mW/m-K (R-value = 2900 or higher) for the boundary temperatures of 300 K and 77 K and a thickness of 25 mm. At these very low rates of heat transmission, on the order of tens of milliwatts for an average size test apparatus, all details in approach, design, installation, and execution must be carefully considered to obtain a meaningful result. For example, lead wires for temperature sensors can be smaller diameter, longer length, and carefully installed for the lowest possible heat conduction to the cold mass. In the case of boiloff testing, the atmospheric pressure effects, the starting condition of the cryogen, and any vibration forces from surrounding facilities should also be considered. If an absolute test apparatus is to be devised, then the parasitic heat leaks shall be essentially eliminated by the integrated design of the apparatus and test methodology. The higher the level of performance (and usually the higher level of vacuum), the lower the total heat load and thus the parasitic portion shall be near zero. For a comparative apparatus, the parasitic heat leaks must be reduced to a level that is an acceptable fraction of the total heat load to be measured. And most importantly, for the comparative apparatus, the parasitic portion of the heat shall be consistent and repeatable for a given test condition. 5.3 Boiloff Testing— Boiloff testing can be performed with a number of cryogens or refrigerants with normal boiling points below ambient temperature ( 29 ) . The cold boundary temperature is usually fixed but can be easily adjusted higher by interposing a thermal resistance layer (such as polymer composite or any suitable material) between the cold mass and the specimen. However, the thermal contact resistance shall be fairly well understood and obtaining a specific cold-side temperature can be difficult. Liquid nitrogen (LN 2 ) is a commonly used cryogen and can be handled and procured with relative ease and economy. Its 77 K boiling point at 1 atmosphere pressure is in a temperature range representative of many applications including liquid oxygen (LO 2 ), liquid air (LAIR), and liquefied natural gas (LNG). The low level of ullage vapor heating with liquid nitrogen systems means that the vapor correction is minimal or even negligible. Liquid hydrogen (LH 2 ), with a normal boiling point of 20 K, can be used with the proper additional safety precautions for working with a flammable fluid. Liquid helium (LHE), with a normal boiling point of 4 K, can also be used effectively, but with a significant rise in expense and complexity. The thermal performance, or heat flow rate (W), is a direct relation to the boiloff mass flow rate (g/s) by the heat of vaporization (J/g) of the liquid. Boiloff methods are therefore direct with respect to calculating a k e or heat flux. 5.4 Electrical Power Testing— In some cases a boiloff method may not be the best option for thermal performance testing. Obtaining a cold boundary temperature below 77 K without additional safety constraints (liquid hydrogen) or unreasonable expense (liquid helium) is often the main reason. The use of electrical power methods provides a wide range of possible approaches without the constraints of a liquid-vapor interface and liquid management. Electrical power apparatus can be designed to use only cryocoolers, cryocoolers in conjunction with cryogens or vapor shields, cryogens to provide the refrigeration to maintain the desired cold boundary temperature, or any combination of these. The key experimental element is the electrical heater system(s), but the key challenge is the temperature sensor calibration at the low temperatures. Temperature sensors are generally silicon diodes or platinum resistance thermometers. These methods are therefore indirect with respect to calculating effective thermal conductivity or heat flux. 5.5 MLI— Multilayer insulation systems are usually evacuated (designed for a vacuum environment). Materials used in MLI systems are highly anisotropic by nature. MLI systems exhibit heat flux values one or two orders of magnitude lower than the best available powder, fiber, or foam insulations under vacuum conditions. The thermal performance of multilayer insulations will vary from specimen to specimen due to differences in the material properties, such as the emittance of the reflective shields, and differences in construction, such as layer density and the way seams or joints are made. MLI systems can vary due to environmental conditioning and the presence of foreign matter such as oxygen or water vapor. MLI systems can vary due to aging, settling, or exposure to excessive mechanical pressures which could wrinkle or otherwise affect the surface texture of the layers. For these reasons, it is imperative that specimen materials be selected carefully to obtain representative specimens. It is recommended that several specimens of any one MLI system be tested with at least three tests performed on each specimen. Further information, including installation methods and typical thermal performance data are given in Guide C740 . 5.6 High Performance Insulation Systems— High performance insulation systems, ranging from aerogels at ambient pressure to evacuated powders to MLI under high vacuum conditions, are typical for the more-demanding applications in cryogenic equipment and processes. The requirements of high performance mean low rates of heat energy transfer (in the range of milliwatts) and even more demanding requirements for accurately measuring these small heat leakage rates. Achieving such measurements requires a sound experimental approach and design, specialized vacuum equipment, a well though-out methodology, and careful execution and handling of data. Note 1: The current lack of Certified Reference Materials (CRMs), or even internal laboratory reference materials, that are characterized under cryogenic-vacuum conditions underscores the need for round robin testing, inter-laboratory studies, and development of robust analytical tools based on these experimental results.