Airlocks are essential tools in the vast majority of homebrewers' fermenting setups, and most of the airlocks out there use water or some other liquid in one configuration or another to allow CO2 to escape from a fermenter but prevent airborne contaminates from getting in. In essence, these water-based airlocks are simple hydraulic check valves. They work great, but do have some downsides. For example, the liquid can evaporate over time or be ejected during vigorous fermentation, thus rendering the airlock more or less useless.
One way to avoid these downsides is to use a non-hydraulic check valve in place of a standard S-shaped or three-piece airlock. For this project, you can use either of two small check valves outlined below and available from US Plastic in conjunction with a drilled rubber stopper or plastic carboy cap. I find the carboy cap generally easier to deal with, and it works on all types of fermenters.
Use a plastic carboy cap with a 3/8" polyethylene check valve (US Plastic part No. 64001, $1.69). Insert the check valve into the thinner, taller stem. Again, I find the fit to be snug as is, but you can caulk this connection if need be.
If you're fermenting in buckets, the above mentioned check valves will fit snugly in the airlock bung in bucket lid. Just slip it in the bung and you're ready to ferment your next batch of delicious homebrewed beer!
How it Works
The nature of check valves is to allow air to pass through in only one direction, so make sure you have the "out" end of the valve facing the outside of the stopper (look for a small arrow on the valve that indicates the direction of flow). Also note that there are several check valves available in the size useful for this project, but most of them have a cracking pressure (the minimum pressure need to open the valve and let out the CO2) that is too high for our purposes. Please use only the two check valves listed above (or do extensive testing on other models before use), or there is a possibility that you could end up with beer on the floor and ceiling in the best case and serious physical harm from exploding glass in the worst case. Do not underestimate the power of fermenting beer.
Although dry airlocks do not exhibit the familiar and delightful *kerplunk* sound as bubbles escape during fermentation, you can tell it is doing its job by the soft *pfft* sound it makes as gas is pushed through it. Also, for you airlock sniffers out there, dry airlocks expel CO2 that has not been passed through the sanitizer/water that is in a standard airlock, thus affording an unadulterated olfactory experience.
Please note that a dry airlock should be used in exactly the same manner a liquid airlock. It is not a substitute for a good blowoff hose should you require one.