These instruments are designed to save space in cockpit panels. They utilize an internal diaphragm to read directly from the source for accuracy and reliability.

Dial Markings:

P/N 3-310-20: GA 4.25-6.25
P/N 3-310-21: GA 4.25-5.2, RR 4.25 & 5.2
P/N 3-310-22: GA 4.5-5.2, YA 3.8-4.5, RR 3.8 & 5.2