A barometer is a scientific instrument used to measure air pressure. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis to help find surface troughs, high-pressure systems, and frontal boundaries.
Barometers and pressure altimeters (the most basic and common type of altimeter) are essentially the same instrument but used for different purposes. An altimeter is intended to be used at different levels matching the corresponding atmospheric pressure to the altitude, while a barometer is kept at the same level and measures subtle pressure changes caused by weather.
Reference barometers are listed initially in D357-49 p. 15 (the precursor to D2700) and D908-51 p. 31 (the precursor to D2699). In both instances, the methods state that the barometric pressure should be measured by a mercurial barometer and corrected for the temperature. Another option for barometric readings is an Aneroid barometer. These can be used if they are compensated for temperature and are calibrated yearly.
In D2699, “barometer” in its various forms is referenced 76 times. The same holds for D2700 where it is referenced 79 times. There is no mention of the type, accuracy, or calibration schedule of such barometers.
A barometer is a critical variable in the engine rating procedures. It is referenced every single day to set the barometric correction value of the cylinder height offset. This critical number is utilized in each method.
Our philosophy is that if the true barometric corrected offset value is off from the very beginning of a rating due to an inaccurate barometer, the barometer should be a piece of equipment that should be serviced like any other piece of the engine.
Fortier barometer – very accurate that is considered a reference after correction. However, Fortier barometers contain mercury. Mercury is illegal in many labs and states and must be removed and disposed of properly.
Aneroid – slightly less accurate than the Fortier barometer, but with the added benefit of no mercury. These must be calibrated yearly. They are not ideal in diverse climates because hysteresis exists when large shifts of temperature or pressure occur. Some Aneroid barometers also have parallax issues which can result in imperfect readings.
Digital – range of quality is very wide. Some barometers will not cover the specified range of the test method. Some will be inaccurate to the point of being unreliable sources. Other digital barometers will be too costly to be implemented despite being the ideal technical solution.
Taking all these factors into consideration, a final designation must be made for what the proper barometer type and specifications should be for D2699 and D2700.