the batteries. Both of these units are capable of tolerating wide variations in power line 

 voltage. Both are critical, however, of the line frequency, which should be within about 

 1/2 cycle of the specified 60 cycles per second. Thus, a common problem in utilizing them 

 on inverter or vibrator -type power supplies is that the frequency of the supply may be 

 neither sufficiently accurate nor sufficiently stable. At least one user, upon finding his in- 

 expensive inverter operated at a constant 61.8 cycles per second, calibrated his unit at that 

 frequency using the actual inverter in his calibration laboratory. This gave him correct 

 readings on the inverter, but the instrument would read in error when plugged into the 60 

 cycle power line. A better solution is the purchase of an inverter whose output frequency 

 is accurately controlled. A suitable, inexpensive inverter is the Accurate Instrument Co. 

 unit, costing about $500.00. 



The power consumption of the Infrared Thermometer depends upon the temperature 

 of the air within the aircraft, but is approximately 15 watts for the IT-1, and 25 watts for 

 the IT-2. Since most recorders consume very little power, and since the smallest inverters 

 commonly available put out at least 50 or 100 watts, it is quite feasible to operate both the 

 Infrared Thermometer and the chart recorder from a common inverter, which converts the 

 12 or 28 volts dc available in the aircraft to 110 volts, 60 cycles for operation of the in- 

 struments . 



A common recorder problem ai-ises, however, with both the IT-1 and IT-2. The 

 recorder-drive circuit used in these instruments is of such a nature that it requires that 

 the input terminals of the recorder be completely floating; if either terminal is connected 

 to ground or chassis, either directly or through a resistor or capacitor, erroneous readings 

 result. It is usually possible to remove the short circuit, resistor, or capacitor by opening 

 up the recorder. 



Late models of the IT-1 and early models of the IT-2 used non- sealed detectors' 

 cavities. In environments where the acoustic noise was high, this permitted a "swish" 

 noise to be present on the signal as air currents rushed back and forth in front of the de- 

 tector flake. Current production insures an air-tight seal around the cavity to eliminate 

 this possible source of noise. The Infrared Thermometer head, however, is somewhat 

 sensitive to vibration, since it contains components which must handle extremely small 

 electrical signals. Any serious vibration of the head will introduce spurious signals and 

 produce a noisy output indication. It is generally possible to mount the head in such a way 

 so as to reduce the vibrations transmitted directly to it. It should, of course, be placed 

 well within an aircraft and not in the slip stream or in a location where severe drafts or 

 air currents are present. It must also have a clear view of the ocean; it is not feasible 

 to try to look through any windows on the aircraft. It is capable of operating m ambient 

 temperatures from 35°F to 105°F, and its stabilized internal reference cavity insures 

 that it will maintain its specified accuracy over this ambient range. If it is necessary to 

 operate in colder ambients, external heat must be applied to the unit. This may be done 

 by small heating blankets, which can be wrapped around the head and around the main 

 electronics unit. 



-34- 



