Pressure gauges have different units because they are used to measure different types of pressure. Different types of pressure require different units of measure because of the different scales used to measure them. For example, atmospheric pressure is measured in millibars (mbar) while gauge pressure is measured in bars (bar). Additionally, pressure measured in a vacuum is measured in pascals (Pa). Standardizing all gauges to one unit would make it difficult to accurately measure different types of pressure.
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