Air change calculation: For calculation of quantities of silica gel or for the selection of proper humidity control equipment, one of the most important factors is the leak rate of the display case.
There are many ways to determine the leak rate of the display case, but the cheapest, safe and relatively simple method is the use of carbon dioxide as a trace gas. The following brief description of the procedure:
- Carbon dioxide sensor.
- Small CO2 container
- A small fan inside the case is recommended but not necessary.
- Air exchange calculator (below)
- Under normal conditions, air contains 385 ppm of CO2. However, in confined spaces, this level may be elevated so be sure to first check and record the level in the environment.
- Place the sensor inside the showcase. If you choose to use a fan, you need to turn it on and leave it on for the full duration of the test.
- Using a simple dispenser, inject into showcase, a small amount of CO2 enough to get the level between 3000 and 9000 ppm of carbon dioxide. (If you anticipate low leakage rate for that showcase, level of 3000 ppm, is enough, otherwise the concentration should be higher – 9000 ppm.)
- Wait a few minutes for the gas level to be stabilized and record the level of CO2 and time of measurement.
- Test should last at least several hours (6 – 24)
- After finishing the test, record the level of CO2.
- Using the calculator below, enter the data: the CO2 level in the environment, the initial level inside the case, the final level inside the case.
- Number Q indicates the leakage rate in multiples of the total exchange of air in the case within 24 hours. For example, Q = 0.2 means that during the day, 0.2 of the volume showcase is replaced, Q = 1 – Air inside the case is completely replaced.
Good quality showcases offered by leading manufacturers have a level of 0.1-0.2 air change daily.