This is one for the people using inches of mercury and need to be able to adjust the sea level pressure depending on airport readings.
Full guide and Code:
I've always loved the idea of making this to take up with me while gliding. The sailplanes do have a regular altimeter but it's nice to know this is in my pocket.
Lots of challenges with this kind of thing though as the DIY I've built here... doesn't connect to the pitot static line. Which is good and bad.
The good is that you don't have to pay a mechanic to install it legally. The bad is that you have to know how to use it. The pressure in the cabin of the aircraft will change with speed and sending your DIY altimeter into inaccurate numbers.
As you speed up the pressure in the cabin will be reduced. At 100 knots you'll be about 80-100 feet higher on the DIY project. This also depends on what aircraft you're flying and if it's open cockpit or not.
When you're entering a new pressure area at the airport is giving you a sea level of 29.92 you will have to know what to adjust the DIY altimeter to for it to be accurate. For a speed of about 100 knots you'll need to enter in 29.82 and you'll have your actual altitude. Thankfully this error keeps the pilot higher in the air than normal and not lower.
If you don't do the adjustment, as you approach the new airport you'll be coming into the circuit at 1100 feet instead of 1000.