On Lyra, our Reliance 44 ketch, we can throw a valve to switch between dual Racor filters, an expensive option that is wonderful insurance. However, even if the filters are switched on the run, there can still be downtime before we're back to 100 percent power. Wouldn't it be better just to glance at a gauge to see if the filter needs changing prior to using the boat for the day or making a gnarly entrance? Being able to do so would also save money by eliminating unnecessary filter changes. As it happens, Racor sells a vacuum gauge that lets you do exactly that for about $130. But here's how to build a similar system for about $30, and you can install it yourself in about an hour's time.