GENERATION, CONTROL, AND MEASUREMENT 



195 



Company, 1951; Harrison et al., 1948; Jena Glass Works; Jones, 1930; 

 Kasha, 1948; Withrow and Price, 1953). 



1. Water and inorganic salt solutions. Water (Curcio and Petty, 

 1951) and the inorganic solutions of ferrous ammonium sulfate and 

 copper sulfate (Kasha, 1948; Pfund, 1939; Withrow and Price, 1953) 

 are the most useful filters for absorbing the infrared, especially with 

 high-power sources. One centimeter of water removes practically all 

 the infrared beyond 1400 m^ (Fig. 3-18, Table 3-15), or about 75 per cent 



100 



600 



700 



800 



1200 



1300 



1400 



900 1000 1100 



WAVE LENGTH, rwfji 



Fig. 3-18. Spectral transmission of liquid water. The numbers on the curves refer 

 to the path length in centimeters. {Data from Curcio and Petty, 1951.) 



of the total energy radiated by an incandescent lamp; a 10-cm layer 

 absorbs all the energy beyond 1150 m/x. Ferrous ammonium sulfate in 

 high concentrations and 10-cm layers (Fig. 3-19) removes the red and 

 infrared. Unfortunately, ferrous ammonium sulfate is unstable and oxi- 

 dizes readily to the ferric form on exposure to air. Addition of 1 per cent 

 free sulfuric acid and a small amount of iron wire greatly retards the rate 

 of oxidation. Copper sulfate solutions (Fig. 3-20) tend to hydrolyze and 

 become slightly cloudy unless a small amount (0.1-0.3 per cent) of free 

 sulfuric acid is added. 



Water-filtered high-power incandescent irradiation assemblies present 

 a difficult problem of cooling, especially for systems that are designed to 

 irradiate a horizontal plane, as for the growing of plants. When the 

 lamp source is mounted above a water filter, most of the energy is 

 absorbed in the upper layers, there is little convection, and surface 

 evaporation is rapid. Cooling coils in the body of the water do not aid 

 in cooling the surface because of the lack of convection currents. The 

 most effective means of cooling is to place coils of flowing tap water in 

 the air space above the water surface so that the water vapor can be con- 

 densed back into the tanks (Withrow and Elstad, 1953). 



