Phosphine (PH3) is one of the most widely used fumigants for disinfestation of agricultural commodities such as cereal grains, tobacco, spices, animal feed, cotton, wood chips and other stored products. Fumigation is carried out generally by producing Phosphine (PH3) in situ by placing calculated amounts of solid aluminum phosphide in the fumigation enclosure. The solid reacts with the atmospheric and grain moisture to produce gaseous PH3. When this method is followed, the PH3 concentration starts building from zero and attains a maximum and then starts falling as the PH3 generation stops, while the leaks from the enclosure and the absorption of PH3 in the grain and other losses continue to slowly bring down the PH3 concentration.
The fumigation procedure requires a certain concentration profile to be maintained during the entire period of fumigation to ensure total insect mortality. The insect mortality is governed by an entity called the ‘fumigation dose’, or PH3 dose, which is defined as the concentration-time product expressed in ppm-hr, and also known as the “CT”. Hence in order to determine the adequacy of the fumigation, it is necessary to measure the total PH3 dose (CT) received by the commodity:
PH3 dose = PH3 conc.(ppm) x time (hrs) = ppm-hrs
For example, if your fumigation process uses 600 ppm PH3 for 5 days, the dose would be:
600 ppm x 5 days x 24 hrs/day = 72,000 ppm-hrs
The concentration time product is equivalent to the area under the curve obtained by plotting the concentration vs. time. If the concentration were constant over the entire time, the calculation of the area would be a simple multiplication as in the equation above, as shown in Figure 1. However, if this quantity is to be measured when the concentration is not constant but varies over a wide range starting from zero, it is necessary to make several, on-the-spot measurements of gas concentration at regular intervals over the entire period of fumigation to generate the concentration vs. time curve to find out the area under curve (Figure 2). This involves several measurements and is time consuming.
Uniphos PH3 Dosimeter tube Description
The dosimeter tube described here saves time and money by measuring the total dose received by the commodity during the entire course of fumigation with a single measurement. The length of stain produced on the dosimeter is calibrated to read directly the phosphine dose received. The tube consists of a narrow glass tubing of about 4 mm OD and 3 mm ID with a length of about 15 cm filled with PH3 sensing chemical. Both ends of the tube are sealed with round tips. The calibration markings of dose in terms of ppm-hr are printed on the tube. When the dosimeter tube is opened and placed in the environment containing PH3, the phosphine gas diffuses in and reacts with the sensing chemical, producing a colored stain. The length of stain produced depends on the concentration-time product, i.e dose, which is expressed in ppm-hr.