Postharvest seed loss due to insect damage is problematic for farmers as well as seed banks. One approach to control insects in a container of seeds is to lower oxygen levels using vacuum drawn with modified bicycle pumps, brake bleeder pumps, or other devices (see ECHO TN 93).
Air contains 21% oxygen by volume, meaning 100 liters of air is comprised of 21 liters of oxygen. Research has shown that insect mortality occurs when 5% (or less) of a container’s volume is occupied by oxygen (Njoroge et al., 2019). How do you know if your vacuum is strong enough to achieve such low levels of oxygen?
Part 1: Gathering required data
To calculate percent of container space occupied by oxygen, after removing some of the air by a vacuum-drawing device, we need to know a few things:
What crop are we dealing with? Let’s say we are storing maize.
What percentage of the container is occupied by seeds of that crop? Let’s say the container is 75% full of maize seed.
What percentage of the seed volume is air space? This relates to empty spaces between kernels of seed and is expressed as percent voids or porosity. Percent porosity can vary with factors such as percent moisture content of the seeds. For purposes of an estimate, we can use published values for grain types. Table 2 of a research article entitled “Porosity Rate of Some Kernel Crops” (Kocabiyik et al. 2004) gives porosity values for some crops. Let’s use a porosity value of 35% for maize at 15% moisture content. If you want to calculate porosity yourself, wikiHow provides step-by-step instructions for four different methods; method 3 seems easiest.
What is the atmospheric pressure at your elevation? For this calculation, we need barometric pressure in mm of mercury (Hg) for our unit of measure. A chart such as that on The Engineering ToolBox can tell you what the atmospheric pressure is for your altitude. *Let’s assume you are close to sea level, which has an atmospheric pressure of 760 mm Hg.
What is the absolute vacuum pressure after you have drawn as much air from your container as your pump is capable of? To know this, you need a vacuum gauge. Brake bleeder pumps come with one already attached (Figure 1). Otherwise, you could purchase one like that shown in Figure 2. They are available in stores that sell automobile parts and supplies. See ECHO TN 93 for an explanation of how to use the gauge in Figure 2 to measure vacuum pressure in a container. Gauges like those in Figures 1 and 2 display inches (in) or millimeters (mm) mercury (Hg) of gauge pressure. Gauge pressure goes from 0 mm (no vacuum) to full vacuum (760 mm Hg). For the purposes of this exercise, however, we need to determine absolute pressure, which is “zero-referenced” against a perfect vacuum (i.e., it is equal to gauge pressure plus
atmospheric pressure). Absolute pressure, at sea level, goes from 760 mm Hg (no vacuum) to 0 mm Hg (full vacuum). Let’s say your gauge dial indicates 500 mm Hg gauge pressure.
To convert gauge pressure to absolute pressure, calculate the sum of gauge and atmospheric pressures with both numbers in the same units (mm for our example) and with gauge pressure as a negative number:
Absolute pressure = -500 mm Hg + 760 mm Hg
Our final absolute pressure in the container, then, is 260 mm Hg.
Part 2: Inputting the data into calculations
Now we are ready to calculate the percent oxygen that remains in a container 75% full of maize seed under 260 mm Hg (absolute pressure) of vacuum. Start by visualizing an empty container with 100 units of container volume. Think of that volume as being occupied by 100 units of air.
Step 1: Maize seeds are added, occupying 75 units of volume. Calculate the units of air displaced by the maize seed.
Of the 75 units of volume occupied by maize seed, 35% is pore space (voids between kernels). Therefore, multiply 75 X 0.35 (porosity of maize as a number instead of percent) to get a value of 26.25 units of pore space (air).
Subtract 26.25 from 75 to get a value of 48.75. In other words, even though the seeds take up 75 units of volume, they only displace 48.75 units of air.
Step 2: Determine percent vacuum achieved with your pump. Use the following formula:
Vacuum % = 100% - (final absolute vacuum pressure/absolute atmospheric pressure) X 100
In our example, then:
Vacuum % = 100% - (260 mm Hg/760 mm Hg) X 100
Vacuum % = 100% - 0.34 X 100
Vacuum % = 100% – 34
Vacuum % = 66%
Step 3: Calculate the units of container volume comprised of air after adding seed. The seeds displaced 48.75 units of the air, meaning 51.25 units (100 – 48.75 = 51.25) of the original 100 units (when empty) remain.
Step 4: Calculate the units of air remaining after using vacuum to remove some of the air from the seed-filled container.
Vacuum removed 66% of the air left (51.25 units) after adding the seeds. Multiplying 51.25 X 0.66, we see that vacuum removed 33.82 units of air that remained after adding the seed.
Since vacuum removed 33.82 units of the 51.25 units we had left after adding the seed, we subtract 33.82 from 51.25 to get 17.43 units of air remaining after drawing a vacuum.
Step 5: Knowing there is 21% oxygen in air, even with some the air removed, calculate the remaining units of container volume occupied by oxygen. Multiply 17.43 units of air X 0.21 to get a value of 3.66 units of container volume occupied by oxygen. Since we started with 100 units of container volume, 3.66 can be thought of as 3.66% of the container’s volume occupied by oxygen.
One could run these calculations with greater or less than 75% seed volume. I think what you would find is that the more seed there is in a container, the less air would have to be removed with vacuum reach the 5% oxygen threshold. Please feel free to check these calculations and comment if there are errors.
References
Kocabiyik, H., T. Aktas., and B. Kayisoglu. 2004. Porosity Rate of Some Kernel Crops. Journal of Agronomy 3:76-80
Njoroge, A.W., R.W. Mankin, B. Smith, and D. Baributsa. 2019. Effects of Hypoxia on Acoustic Activity of Two Stored-Produce Pests, Adult Emergence, and Grain Quality. Journal of Economic Entomology 112(4):1989-1996