Announcing-The Building Science Series The "Building Science Series" disseminates technical information developed at the Bureau on building materials, components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental functions and the durability and safety characteristics of building elements and systems. These publications, similar in style and content to the NBS Building Materials and Structure Reports (1938-59), are directed toward the manufacturing, design, and construction segments of the building industry, standards organizations, officials responsible for building codes, and scientists and engineers concerned with the properties of building materials. The material for this series originates principally in the Building Research Division of the NBS Institute for Applied Technology. Published or in preparation are: BSS1. Building Research at the National Bureau of Standards. (In preparation) BSS2. Interrelations Between Cement and Concrete Properties: Part 1, Materials and Techniques, Water Requirements and Trace Elements. 35 cents BSS3. Doors as Barriers to Fire and Smoke. 15 cents BSS4. Weather Resistance of Porcelain Enamels: Effect of Exposure Site and Other Variables BSS5. Interrelations Between Cement and Concrete Properties: Part 2, Sulfate Expansion, Heat of Hydration, and Autoclave Expansion. 35 cents BSS6. BSS7. Some Properties of the Calcium Aluminoferrite Hydrates. 20 cents Organic Coatings. Properties, Selection, and Use. $2.50 BSS8. Interrelations Between Cement and Concrete Properties: Part 3, Compressive Strengths of BSS9. BSS10. Field Burnout Tests of Apartment Dwelling Units. 25 cents BSS12. Performance of Square-Edged Orifices and Orifice-Target Combinations as Air Mixers. 15 cents BSS13. Shrinkage and Creep in Prestressed Concrete. 15 cents BSS14. Experimental Determination of Eccentricity of Floor Loads Applied to a Bearing Wall. 15 cents BSS15. Interrelations Between Cement and Concrete Properties: Part 4, Shrinkage of Hardened Portland Cement Pastes. 75 cents BSS16. Techniques for the Survey and Evaluation of Live Floor Loads and Fire Loads in Modern Office Buildings. 40 cents BSS17. Causes of Variation in Chemical Analyses and Physical Tests of Portland Cement. 40 cents BSS19. A Study of the Variables Involved in the Saturating of Roofing Felts. 30 cents BSS22. Investigation of Performance Characteristics for Sanitary Plumbing Fixtures. (In press) BSS24. Natural Weathering of Mineral Stabilized Asphalt Coatings on Organic Felt. (In press) BSS25. Structural Performance Test of a Building System. (In preparation) Send orders with remittance to: Superintendent of Documents, U.S. Government [See mailing list announcement on last page.] UNITED STATES DEPARTMENT OF COMMERCE Maurice H. Stans, Secretary NATIONAL BUREAU OF STANDARDS Lewis M. Branscomb, Director Radiation Errors in Air Ducts Under Nonisothermal Conditions Using Thermocouples, Thermistors, and A Resistance Thermometer Radiation Errors in Air Ducts Under Nonisothermal Conditions Using Thermocouples, Thermistors, and a Resistance Thermometer Joseph C. Davis* Studies were made to determine the radiation error in temperature measurements made with thermocouples, thermistors, and a resistance thermometer in moving air at velocities ranging from 300 to 1300 fpm when the temperature of the duct wall surrounding the air stream was from 0 to 50 deg F higher than that of the air in the center of the duct. To eliminate all but the variable under study, conduction errors were minimized to a point where they were almost nonexistent by using Chromel P-constantan thermocouple wire and by employing other techniques. Radiation effects were studied when the probe housing the three types of temperature sensors was unshielded and again when it was shielded. The studies showed that when the sensors were unshielded and the temperature difference between the duct wall and the air was 50 deg F (28 K, approximately), the error in the sensors was about 3.8 deg F (2.1 K) for an air velocity of 300 fpm (1.5 m/s) and 1.0 deg F (0.6 K) for an air velocity of 1300 fpm (6.6 m/s). When the sensors were shielded, the error was about 0.2 deg F (0.1 K) for 300 and 500 fpm velocities and the same duct wall air-temperature difference. Tests were not performed at 1300 fpm with the sensors shielded because theory indicated that radiation error would be negligible at this velocity. Under the test conditions that prevail in the testing of air conditioners and heat pumps in laboratories, it should be possible to reduce the error in temperature measurement of the moving air to about 0.2 deg F (0.1 K) by a suitable combination of air mixers, duct insulation, radiation shields, and calibration techniques. Key words: Conduction error; radiation error; resistance thermometer; temperature measurement; thermistor; thermocouple. *Present address: 4534-47th St., N.W., Washington, D.C.20016. 1. Introduction Accuracy of measurement of the temperature of moving air depends, among other things, on the effectiveness of precautions taken to minimize conduction and radiation errors. It is known that in the determination of the thermodynamic properties of moving air, these errors can be significant at air velocities below 1000 fpm and when the temperature of the surroundings, such as a duct wall bordering a stream of moving air, is 20 deg F different from that of the air at the position of the sensor. However, the literature does not show much information on the magnitude of error at these velocities and temperature differences. The error in determining the capacity of an air conditioner or a heat pump in a laboratory can be as high as 5 percent if the error in the temperature measurement from these sources is neglected, even though the temperature difference between the air and the duct wall may not exceed 6 deg F. Similarly, the measurement of the moisture generation capacity of a humidifier can be as much as 10 percent in error if corrections are not made to the observed temperature. In a previous paper by Davis, Faison, and Achenbach [1] showing the results of the study of the errors of thermocouples, thermistors, and mercury-in-glass thermometers used in moving air, and where the temperature of the duct wall was essentially the same as that of the sensors, it was shown that the principal errors under those conditions were due to change in performance of the sensors between calibrations, and to false readings due to thermal lag of the sensors. Other smaller sources of error found in the study were self-heating of the thermistors, parallax difficulties in reading the mercury-in-glass thermometers, orientation of the thermometers, and impact error due to the energy of motion of the air stream. Shielded and unshielded temperature sensors are widely recommended in various standard test pro 1 Figures in brackets indicate the literature references on page 12. |