Until confidence is built up in the instrumental set-up, it is a good idea to carry a 

 pail of water with an immersion thermometer along on the trip. This pail of water can be 

 held in front of the optical head of the Infrared Thermometer at periodic intervals to as- 

 certain correct performance of the Infrared Thermometer. The water must be agitated 

 vigorously before and during these calibration checks, since if permitted to stand even for 

 a few moments, the surface will cool by evaporation; vigorous agitation insures adequate 

 mixing of the cooler surface water with the sub-surface water as measured by the im- 

 mersion thermometer. 



The IT-2 contains an internal response-time switch which provides a fast response 

 when required, as, for example, if "rapid" temperature variations within bays or inlets are 

 to be studied, or a slow response, providing less noise and greater resolution. For our 

 purposes, the slow position is usually of more value. 



The current production run of the IT-2's should be exhausted within a few weeks, and 

 the next production run, ready for August delivery, will incorporate numerous changes as a 

 result of suggestions made by users, in order to eliminate some of the aforementioned 

 problems. These changes will now be discussed. 



A major change is a different recorder-drive circuit, one terminal of which is 

 grounded, permitting it to drive any electronic recorder whether or not the recorder in- 

 put is floating. This circuit is designed to drive all standard high- impedance electronic 

 recorders and has an adjustable output from to 50 millivolts. 



The most significant change is a more elaborate electrical signal filter which will 

 permit the instrument to maintain specified accuracy for line frequency variations from 

 58.7 to 61.3 cycles, permitting the instrument to be utilized with inexpensive, inaccurate, 

 unstable vibrator-type inverters. 



The microphony of the sensing head has been reduced by an improved optical and 

 mechanical design. It will be less susceptible to noise and vibration and is capable of 

 operating without additional sources of heat in ambient temperatures down to F. An 

 improved open-sighting arrangement has been employed for locating the field of view. 



The main electronics unit has a more reliable means of containirfg the printed cir- 

 cuit boards, and contains numerous external cosmetic changes. These include a com- 

 fortable carrying handle, a tilt-up bail for easier meter viewing, and a larger readout 

 meter simultaneously calibrated in both °C and °F. The power line fuse and the response 

 time-constant switch have been moved to the front panel. A more reliable method of hold- 

 ing the chassis in the carrying case has been developed. The meter face will be silk- 

 screened for greater accuracy and more professional appearance. The new model, although 

 generally quite similar to the IT-2, will be designated the IT-3. 



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