From NEPA Crossroads Knowledge Base
A manometer is a gauge to measure the draft present in a stove / chimney installation. The manometer components consist of the indicator box, a fitting installed usually on the flue pipe and tubing connecting them. A fitting for the manometer tube is usually placed through the flue pipe after the exhaust port of the stove and before a barometric damper if present. A tube is installed between this fitting and the manometer device. The tube used at the flue pipe must be able to handle the temperatures present at this location, copper tubing is an example. This high temperature tubing is usually transitioned to the flexible plastic tubing used to connect to the port on the manometer. It can be used to measure the draft and to calibrate / set the counter balance weight on a barometric damper. The draft measurement scale is usually in inches of water. Different stoves recommend different ranges, but generally they are around .05" H2O.
The reading can also be taken at the firebox by having a fitting installed through the sidewall or front wall of the stove. Some stove's have a threaded port that the fitting can be threaded into. And when not in use there is a threaded plug that seals the port.