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TEMPERATURE and LIQUID-LEVEL SENSOR for LIQUID-HYDROGEN PRESSURIZATION and EXPULSION STUDIES by Robert J

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TEMPERATURE and LIQUID-LEVEL SENSOR for LIQUID-HYDROGEN PRESSURIZATION and EXPULSION STUDIES by Robert J NASA TECHNICAL NOTE ---NASA TN D-4339 c./ o* m m d I m L TEMPERATURE AND LIQUID-LEVEL SENSOR FOR LIQUID-HYDROGEN PRESSURIZATION AND EXPULSION STUDIES by Robert J. Stochl und Richard L. DeWitt Lewis Reseurch Center CZeveZund, Ohio NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. FEBRUARY 1968 TECH LIBRARY KAFB, NM __ . 0333238 TEMPERATURE AND LIQUID-LEVEL SENSOR FOR LIQUID- HYDROGEN PRESSURIZATION AND EXPULSION STUDIES By Robert J. Stochl and Richard L. DeWitt Lewis Research Center Cleveland, Ohio NATIONAL AERONAUTICS AND SPACE ADMINISTRATION ~~ For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 - CFSTI price $3.00 TEMPERATURE AND LIQU I D-LEVEL SENSOR FOR LlQU ID-HY D ROGEN PRESSURIZATION AND EXPULSION STUDIES by Robert J. Stochl and Richard L. DeWitt Lewis Research Center SUMMARY In experimental studies of the pressurization and expulsion of liquid hydrogen, temperature-measuring systems are needed to understand the effectiveness of a parti- cular pressurization system. Existing state-of -the-art temperature-measuring systems are considered inadequate to meet the rather stringent design requirements necessary for accuracy and precision in the measurement of ullage gas temperatures. A temperature-measurement technique that uses thermopiles (i. e. , thermocouples in series) as sensors was therefore developed. The results of this development indicate (1) that an instrument rake using measurement stations constructed of thermopiles can measure temperatures to within *l.65' K (for gas temperatures between 20' and 300' K) and (2) that, in addition to their use as temperature sensors, thermopile units can be used as point liquid-level sensors for subcooled liquid during liquid outflow. Tests indi- cate that thermopiles can detect liquid level to within *O. 453 centimeter. INTRODUCTION In experimental studies of the pressurization and expulsion of liquid hydrogen, mea- surements of wall, liquid, and gas temperatures are needed to understand the effective- ness of a particular pressurization system. Several commercial sensors are capable of measuring tank-wall and liquid temperatures. The ullage gas temperatures are used to determine the amount of gas in the tank ullage. To achieve the accuracy and precision required for research on hydrogen ex- pulsions, temperature-measuring systems are needed which generally meet the fol- lowing design requirements: (1) The sensors must be accurate over a large range of temperatures (usually be- tween liquid-hydrogen (approx. 20' K) and inlet-gas (165' to 390' K) temperatures). -.- (2) The sensors and their support structure must withstand temperature cycling and (where it might affect sensor calibration) pressure cycling inherent in tank-pressurization experimental work. (3) The measurement system must be of minimum mass to be a negligible heat sink when compared to the tank walls and other necessary internal tank hardware. A reason- able goal would be a measurement-system heat capacity that is less than 1 percent of the heat capacity of the tank walls and internal hardware. (4) The measurement system must also be of minimum cross-sectional area. Other- wise, it tends to disturb the normal environmental conditions in the ullage. (5) The sensors must have a sufficient change in output signal for 1' K change in tem- perature to keep the readout equipment within the limitations of ordinary test-cell appar- atus. (6) The sensors must have a fast response to changes in temperature. In past experimental studies at Lewis, most of the temperature-measuring-system design requirements were realized. However, the poor response characteristics of temperature sensors has been a major difficulty in the accurate measurement of rapid changes in temperature, particularly near the moving liquid-gas interface. During an expulsion, a film of liquid adheres to a sensor as it passes from the liquid propellant into the ullage gas. The sensor indicates true temperature only after this film of liquid has evaporated. In view of this difficulty, Lewis undertook to develop a ullage gas temperature- measurement system which would satisfy the above mentioned range, structural, and output-signal requirements. Moreover, the goal is a system capable of measuring tem- peratures to within 1 percent of the absolute values and that has a time constant of less than 1 second going from saturated liquid to saturated vapor. A survey was made to evaluate the characteristics of various temperature sensors. A transducer was chosen and built into a measurement system. This system was then used in a 0.82-cubic-meter cylindrical tank during liquid-hydrogen expulsion studies. This report discusses the considerations which led to the choice of the particular sensor and the construction of the measurement system. It also discusses the accuracies ob- tained with this system during the expulsion studies. The original function of the measuring system was to determine ullage gas temper- ature profiles. During the testing, however, it was found that the system could also be used to determine liquid level during expulsion. A photographic calibration of the system was made to verify one of the system output characteristics with the actual location of the liquid surface. This calibration, as well as the resulting expected accuracy of liquid- level indication, is discussed herein. 2 SYMBOLS E thermoelectric potential (emf) J total number of individual errors K thermopile position AL accuracy of liquid-level determination n number of thermocouples in series R readability of oscillograph T temperature, OK T slope of temperature-time curve V velocity of test configuration Ax precision in locating variable-speed shaft assembly (T standard deviation Subscripts: b bulk ref reference sat saturation SELECTION OF SENSORS FOR ULLAGE GAS TEMPERATURE MEASUREMENT The response time of a temperature sensor is directly proportional to sensor mass and inversely proportional to exposed surface area. Therefore, the survey of temperature-measurement sensors was restricted to units which have a low mass-to - surface-area ratio. The results of the survey showed that the following three major types of sensors could possibly be used: (1) carbon resistors (0. l-W, 100-52 units, which are widely used at Lewis), (2) miniature platinum resistance sensors, and (3) thermo- couples. Two techniques were considered for measuring the ullage gas temperature pro- file. The first was to obtain a series of absolute temperature measurements (sensor types (1) and (2)). The second was to obtain a series of differential temperature mea- surements relative to either a single or several known reference temperatures (sensor type (3)). 3 A bsol ut e Ye mperat u re-Meas u rement Sensors Carbon resistors. - Investigators (ref. 1) have used carbon resistors for tempera- ture measurements in a hydrogen environment. Through proper selection and periodic calibration, an accuracy within 1percent of the absolute temperature can be obtained between 20' and 55' K. However, because of the high negative temperature coefficient of carbon, large errors could result when measuring temperatures greater than 55' K. For increasing temperatures, this characteristic of carbon results in a lower resistance and, thus, in reduced sensitivity (i. e. , reduced change in signal/'K). For instance, the sensitivity of a carbon resistor, in maximum percent change in signal, is 3 percent per 0K at 20' K and only 0.025 percent per OK at 300' K. Therefore, in recording this signal with a system that has an accuracy of 1/2 percent of full scale, the temper- ature uncertainty would be *O. 17' K at 20' K and doo K at 300' K. The carbon resistor has an analytically estimated time constant between 0.2 and 2.0 seconds for a step change in temperature of less than 110' K in a single-phase fluid, but time constants between 2.0 and 10.0 seconds are estimated when going from a saturated liquid to a saturated vapor. As a result, temperature measurements near the liquid surface during a liquid discharge would not be representative of the true temperatures at the instant of time they were taken. The correction of each carbon resistor for its particular time lag would be difficult because of the varying environmental conditions existing above the liquid inter - face. The carbon-film sensor, under development by several industrial firms, shows improvement in response time over the commercial carbon resistor. However, it is still usable only over a limited temperature range. In view of these difficulties, the car- bon resistor was dropped from further consideration as a sensor for the test objectives of this program. Miniature platinum.- resistor sensors..- - This type of sensor has approximately the same physical dimensions (approx. 0.13 cm in diam., and 1.27 cm long) as the 0.1-watt carbon resistor. This sensor can be used over the entire temperature range encountered in pressurization and expulsion testing. The sensitivity of a platinum sensor is 14 per- cent per OK at 20' K and 0.38 percent per OK at 300' K. For a &1/2 percent measuring system, the expected uncertainty would be *O. 036' K at 20' K and 4.32' K at 300' K. (I'hese sensitivities were taken from a sensor which has a resistance of 1000 S-2 at 280'K.) The 14 percent per OK sensitivity at 20' K for platinum resistance sensors can be misleading. A sensor that has 1000ohms resistanceat room temperature has only 4. Oohms at 20' K. Thus, the 14 percent per OK sensitivity means a change of about 0.60 ohm per OK. Readout equipment accuracy of kO.01 percent of full scale would be necessary in order to measure temperatures to within kl percent over the entire range expected (20Oto 390' K). This readout equipment requirement can be reduced by using sensor 4 and bridge systems designed to given certain outputs over various temperature ranges. Thus, redundant sensors each with a different bridge network could be necessary to obtain a satisfactory balance between readout equipment accuracy and temperature- measurement accuracy.
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