ASTM E377-08(R2020) pdf free download

ASTM E377-08(R2020) pdf free download.Standard Practice for Internal Temperature Measurements in Low-Conductivity Materials
1. Scope
1.1 This practice covers methods for instrumenting low- conductivity specimens for testing in an environment subject to rapid thermal changes such as produced by rocket motors, atmospheric re-entry, electric-arc plasma heaters, and so forth. Specifically, practices for bare-wire thermocouple instrumen- tation applicable to sheath-type thermocouples are discussed. 1.2 The values stated in inch-pound units are to be regarded as the standard. The metric equivalents of inch-pound units may be approximate. 1.3 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 appro- priate safety, health, and environmental practices and deter- mine the applicability ofregulatory limitations prior to use. 1.4 This international standard was developed in accor- dance with internationally recognized principles on standard- ization established in the Decision on Principles for the Development of International Standards, Guides and Recom- mendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Significance and Use
2.1 Internal temperature measurements are made on both in-flight vehicles and on-ground test specimens; and, because of the importance of the temperature measurements to the design of various missile and spacecraft heat shields, it is essential that care be taken to minimize the sources of error in obtaining these measurements. 2.2 Over the past several years, the problems of using thermocouples to obtain accurate temperature measurements in low-conductivity specimens have been studied by various people to isolate the sources of error and to establish improved temperature measurement techniques. The major sources of error are listed in this document and recommended solutions to the problems are given.
3. General
3.1 Before proceeding to the major sources of error, it is assumed that the reader is familiar with basic methods of forming and using thermocouples 2 , that is (1) electric welding to form junctions, (2) maintaining cleanliness of junction area and lead wires, (3) proper selection of thermocouple type and size, corresponding to both the temperature range to be measured and the chemical compatibility with the environment, and (4) proper use of instrumentation for readout of thermocouple emf. N OTE 1—Reader is referred to ASTM MNL 12 (1), and STP 492 (2), as well as Kinzie, P.A., Thermocouple Temperature Measurement (3), for needed information. 3.2 The most important sources of error beyond the above basic areas are the following: 3.2.1 The thermal disturbance produced in the low- conductivity material at the vicinity ofthe thermocouple sensor hot junction due to the sensor size, configuration, and instal- lation method. 3.2.2 Electrical shorting of lead wires due to the electrical conductivity of the virgin or charred ablation material, and 3.2.3 Thermocouple sensor hot junction location accuracy.
4. Thermal Disturbance at Vicinity of Thermocouple Sensor Hot Junction
4.1 General—Ideally, to measure the true internal tempera- ture of a solid body, it would be desirable not to have any foreign substance present that would create a disturbance affecting the natural flow of heat in the body. Since it is physically impossible to exclude the temperature sensor from the internal confines ofthe body, it is necessary that the thermal disturbance introduced by the sensor be minimized for accurate temperature measurements (See Refs (4-10)). 4.2 Thermocouple Junction Bead Diameter: 4.2.1 General—Excessively large junction beads result in lower than true temperature measurements in low-conductivity materials (conductivity of material less than conductivity of thermocouple wire) because of the heat sink effect of the bead. 4.2.2 Recommendations—To minimize this effect, the junc- tion bead diameter should be no larger than 1.5 wire diameters for butt-welded junctions and 2 wire diameters for other types of welds. 4.3 Thermocouple Wire in Isothermal Surface ofHot Junc- tion: 4.3.1 General—Because many materials have low thermal conductivity compared with those ofthermocouple assemblies, it has been found that certain methods of installing sensors can produce significant errors in internal temperature measurement (1-4). 3 Errors of several hundred degrees are possible unless heat conduction away from the sensor hot junction, by the sensor materials, is minimized. Test results show that a thermocouple having a sufficient length of bare wire in the isothermal surface that includes the junction will minimize these errors.