P3D Heat Study part 3 of 7 (Testing)

[Tom Hubin <thubin@xxxxxxxxx>]

Hello,

Here is part 3 of 7.

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design

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        Testing

        Probing the air near the film plane measures the ambient 
        temperature. That is the temperature of the air flowing by the 
        film. This is not a good indicator of film temperature because 
        the infrared energy is not absorbed by the thermal probe. The 
        film temperature must be measured in order to asses the effect 
        of infrared energy absorbed by film. 

        I first mounted a piece of film leader with the black surface 
        facing the light source so as to absorb all incident energy 
        (visible, ultraviolet and infrared). I then glued a thermistor 
        onto the center of the back side of the film so that I could 
        monitor the temperature of the film. 

        A thermistor is a special kind of resistor. The resistance value 
        is related to the temperature. For some thermistors the 
        resistance value decreases as the temperature increases. For 
        others the resistance value increases as the temperature 
        increases. Either way you can use an ohmmeter to measure the 
        thermistor resistance and look up or calculate the corresponding 
        temperature. 

        Since I used a 50 cent thermistor with no lookup table I first 
        had to make my own lookup table. I put the film and thermistor 
        in a plastic bag along with two thermometers. One was a 
        household thermometer that only went as high as 130F and the 
        other was a meat thermometer that covered from 140F to 180F. I 
        then put the bag in a pot of water and warmed it slowly on the 
        stove. I recorded the thermistor resistance and the temperature 
        at 5 degree Fahrenheit intervals. I also recorded the data again 
        as the water cooled from 180F to about 110F. The results were 
        similar enough to make a reasonably accurate table of data.



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