Definition of spectrometer. 1  1 : an instrument used for measuring wavelengths of light spectra. 2  2 : any of various analytical instruments in which an emission (as of particles or radiation) is dispersed according to some property (as mass or energy) of the emission and the amount of dispersion is measured nuclear magnetic resonance spectrometer.
1 bar is equal to 1000 millibars. Just like how there are 1000 millilitres in a litre, and 1000 milliseconds in a second. 1 millibar = 0.001 bars.
1 millibar (or hectopascal/hPa), is equivalent to 0.02953 inches of mercury (Hg). It is therefore only necessary to multiply a reading in millibars by the latter figure, to achieve the required conversion. E.g. for 1023 mbar, multiply by 0.02953=30.21 inches.