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Full text of "Method of rating fumigation chambers for tightness"

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September 1945 ET-224 

United r^ates Department of Agriculture 

Agricultural Research Administration 
B>ireau of "Entomology and Plant Quarantine 


By Robert D. Chisholm and Louis Koblitsky, 
Division of Insecticide Investigations 

It is practically impossible to construct and maintain commercled 
fumigation chambers in a gastight condition (i.e., with respect to 
differential total pressure). Consequently, a portion of the fumigant 
leaks out during fumigations. Since the chambers are of various 
degrees of tightness, leakage is also variable. In extreme cases it 
may influence the insect icidal resxilts. 

yjhen a fumigant is volatilized in a chamber at atmospheric pres- 
sure, a positive pressure i^ created, which may then be continuously 
reduced by leakage of the air-fumigant mixture. The time required 
for atmospheric pressure to be reached depends upon the tightness of 
the chamber. The volume of vapor required to create a given positive 
pressure is proportionate to the volume of the chamber. Consequently, 
if the time required for this pressure to be reduced to that of the 
atmosphere in a given chamber is equal to the time required for a 
similar reduction in another chamber, the voliuaes of air-fumigant 
mixture lost by leakage would be proportionate to the volumes of the 
chambers. These chambers would also have the same degree of tightness! 
This paper describes a method for rating the tightness of fumigation 
chambers based on these principles. 

Methods and Materials 

A 500-cubic-foot chamber fitted with an open-ann manometer 
charged with deodorized kerosene was used to establish pressure-time 
relationships. The temperature of the chamber ranged between 15.6° 
and 16.1°C. (60° and 6l^F. ) . The temperature of the kerosene was 25°C. 
(77°F.), at which temperature it has a density of 0,7725 gram per 
cubic centimeter. 

2/ Acknowledgement is made to V. A. Johnson and W. L. Caskey, 
Division of Japanese Beetle Control, and W. C. Fest, Division of 
Fmlt Insect Investigations. The fumigation chamber was made avail- 
able by the Division of Fruit Insect Investigations. 

2 - 

A tube of convenient size was fitted tightly into a hole bored 
into the chamber. The outer end of the tube was connected with an air 
blower. (A vacuum cleaner contains a blower which has ideal capacity 
and characteristics for this purpose.) The blower was started and 
continued in operation until a positive pressure equal to slightly more 
than 50 mm. of kerosene was created in the' chamber. The blower tub© 
was then closed and the blower stopped. The pressure was recorded at 
10-second intervals and the rate of leakage detennlned from the time 
required for the pressure to drop from 50 mm. to 5 nnn* 

To provide a measurable leakage, pipe caps with holes of various 
sizes (1/8 to 1 inch diameter) were attached to a 2-lnch pipe entering 
the chamber. " The ends of the pipe caps had been machined to I/I6 inch 
in thickness, which is the average thickness of the metal walls of 
fvimigation chambers. Leakage from the sealed chamber was compensated 
for, and curves were drawn representing the relation between time and 
pressure for each size hole. Data from these curves are presented in 
table 1. 


Other gases or other temperatures will result in slightly different 
pressure-time relationships owing to modification of the flow character- 
istics of the fumigant-air mixture. At other temperatures slight 
differences will result from the consequent changes in density of the 
liquid in the open-arm manometer. It is believed that these variables 
need not be considered in the practical application of the method. 
Liquids other than kerosene may be used in the open-arm manometer, and 
pressure-time rolationahips established by calculation. 

The data in table 1 may be used for comparing the tightness of 
fumigation chambers, whenever a desirable rate of tightness is established. 
Either air or fumigant-air pressure may be used in the chambers to be 
rated. For example, if the time required for the pressure to drop from 
50 to 5 mm. of kerosene is 5A seconds, the chamber would be rated as 
having a tightness equal to that of a 500-cublc-foot chamber which has 
one 1/2-lnch diameter circular hole in it. The size of a single circular 
hole in the rated chamber required to cause this drop in pressure in 
this specified time is related only indirectly to the volume of the 
chamber. The flow characteristics of fluids Issuing from holes are 
affected by the size and shape of the holes, and the thickness of the 
diaphragm. For the same reasons the table cannot be used to measure 
leakage areas in chambers, as leakage usually takes place through a 
number of small holes or cracks. 

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Experimental chambers used for the establishment of dosage-time- 
tenperature schedules are generally classed as being nearly gastight. 
One of these chambers rated by this method was found to have a tight- 
ness equivalent to that of a 500-cubic-foot chamber with a circular 
hole a little over 1/L inch in diameter, < A few commercial chambers 
have been rated by this method. One was tighter than the experimental 
chamber, but most of them were less tight although largely within the 
ranf;e in table 1, Since the maximum of this range represents a leakage 
area of less than 1 square inch in a total surface of at least 5A,A30 
square inches, it may be considered that chambers within the range are 
relatively tight. 

In general, the method described is an adaptation of the airtight 
room test {2_, p. 1,55) one of the established methods of testing fans. 
AS applied by Durley (_1 ) , air is allowed to discharge from the room 
through circular orifices in thin plates. This method is now being 
used for testing the tightness of fumigation chambers employed in 
connection with the Japanese beetle quarantine. 


(1) Durley, R. J,_ I906, On the measurement of air flowing into the 
atmosphere through circular orifices in thin plates and under 
small differences of pressure. Amer, Soc, liech. Engin, Trans. 
27: 193-231. 

(2) Liddell, D. M. , Ed, 1922. Handbook of chemical engineering. 
Vol. 1, New York and London.