This 1967 Naval Air Systems Command CP-165A/UM Psychrometric Computer is made of plastic and was made by the Felsenthal Instruments Company.  It is 11.25 inches in diameter.  When the air temperature and a humidity value are known, it allows calculation of the other humidity values, among which are wet bulb temperature, dew point temperature, and relative humidity.  It has scales for calculations at 30, 29, 28, 27, 25, and 23 inHg barometric pressure.  The front side of the device is for calculations at high air temperatures while the back side is for low air temperatures.  The scales were designed by the US Coast and Geodetic Survey.

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