INDUSTRIAL FUMIGATION AGAINST INSECTS 



37 



should be closed and the necessary valve adjustments made to allow 

 the gas to be circulated in the vacuum tank. The gas should be cir- 

 culated for 15 minutes. By holding the remaining vacuum without 

 change for the duration of the fumigation, better results are ob- 

 tained than by breaking the vacuum with air. If the instrument 

 panel is equipped with a recording device, a chart record (fig. 34) 

 can be made of each fumigation for future reference. 



Figure 32. — Portion of vacuum fumigator installed in a large candy establishment. Note 

 vacuum pump in left foreground; cylindrical accumulator on tank; gages for auto- 

 matically recording pressure, vacuum, and temperature ; and cylinder of the ethylene 

 oxide-carbon dioxide mixture in front of operator. Cylinders of methyl formate-carbon 



< dioxide and methyl bromide-carbon dioxide mixtures are attached in the same manner. 



The dosage is usually measured by- weighing the fumigant into 

 the tank or vaporizer, by measuring it volumetrically, or by dropping 

 the vacuum a given number of inches with the fumigant. The first 

 two methods are accurate. The third method cannot be used indis- 

 criminately, since with a given quantity of gas the drop in vacuum 

 will vary both with the quantity and the type of commodity being 

 fumigated and with the temperature of the gas and of the com- 

 modity in the tank. It is convenient for commercial fumigations, 

 however, and if proper allowance is made for these variations it 

 will be sufficiently accurate for ordinary work. 



At the end of the fumigation the gas is pumped out of the tank 

 and the vacuum is broken with air. If desired, the fumigated prod- 

 ucts can be "air-washed" several times by alternately drawing and 

 breaking a vacuum of about 27 inches. 



In unloading large tanks filled with commodities of an absorbent 

 nature, the residual gas is quite noticeable and may affect workmen 

 who remain within the tank too long. It is therefore a wise policy 



