The following results were obtained when an uncalibrated bourdon tube pressure gage was compared to a test pressure gage. Which adjustment(s) is/are necessary? True Pressure Indicated Pressure 20 35 30 45 40 55 50 65 set point adjustment proportional band (range) adjustment
• Offset vs. span error when comparing a pressure gage to a test standard • What a set point adjustment changes in an instrument’s indication • What a proportional band (range) adjustment changes in an instrument’s indication
• Look at the difference between true pressure and indicated pressure at each test point. Is that difference the same or does it change with pressure? • If ALL points are off by about the same amount, which adjustment usually fixes that kind of error? • If the error gets bigger or smaller as pressure increases, which adjustment usually fixes the slope/range of the indication?
• Calculate the error at each point: Indicated – True, for all four readings • Decide if this error is constant (same offset) or changing with pressure (span/slope issue) • Match a constant error to the proper adjustment and a changing error to the other adjustment before choosing your answer
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