hysteresis effect while pressures up to several tenths of a millimeter can be applied inside the pressure cell without harmful effects on the diaphragm.
APPARATUS FOR STUDYING TEMPERATURE-SENSING DEVICES IN JET ENGINES
Efficient operation and adequate protection of the power plant in jet engines require very rapid reduction in the temperature of the gas stream whenever this temperature exceeds a limiting safe value. The rates of response of whatever temperature-sensing devices are used thus assume considerable importance in jet-engine operation.
Various methods for determining response rates have been worked out, but in most cases they have proved unsatisfactory because they failed to simulate operating conditions adequately.
Recently A. I. Dahl and E. F. Fiock of the National Bureau of Standards have developed apparatus 1 which determines the rate of response of jet-engine temperature elements under conditions very closely approximating those obtaining in the combustion stream. With this equipment, response times as short as 0.02 second have been measured for a wide variety of thermocouples, resistance thermometers, and thermistors.
The rate of response, or "characteristic time," of a specific temperature element (defined as the time required by the element to undergo 63.2 per cent of the total change in temperature to which it is subjected) depends on its mass, surface area, and heat capacity, and on the rate at which heat is transferred to the element from the gases flowing through the engine. Thus, in any test system in which heat is transferred to the temperature element mainly by radiation or natural convection, the observed values would not be applicable to jet engines. The Bureau therefore developed a system for studying characteristic times in which heat is transferred primarily by forced convection,* as actually occurs in engines.
This apparatus consists essentially of a jet-engine combustor, or burner, with provision for mounting instruments in the exhaust gas stream, and a device for producing rapid changes in the temperature of the mounted test instrument. A compressor or blower supplies air to a single Jumo 004 turbo-jet engine combustor equipped with its normal fuel injector and spark plug. Exhaust gases from the combustor pass through two 90-degree turns, through a perforated plate, and then through about 10 ft. of straight pipe before reaching the test section, which has three convenience hatches for mounting instruments. In this way, the gas stream is kept at essentially uniform temperature and velocity over the central half diameter of the test section, and