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The Ultrasonic Interferometer Manometer (UIM), a primary pressure standard, has been established since 1982 at CSIR-National Physical Laboratory, India. The main component of the UIM is a piezoelectric transducer and related electronics. The transducers are situated at the bottom of the mercury manometer columns, which send short pulse signal (15 μs) with a carrier frequency of 10 MHz. These pulses reflect from the top surface of the mercury column and provide very precise column distance and hence pressure. Recently we have upgraded the UIM with window compatible software and re-evaluated the uncertainty with thirteen variable parameters and influencing factors. The expanded uncertainty is Q (0.0092 Pa, 7.2 ppm of reading) at k = 2 as per ISO guideline on the expression of uncertainty. The first part belongs to the resolution of UIM which dominates over a percentage of reading of pressure until 1 kPa range. For higher pressure range i.e., above 1 kPa till 130 kPa, the expanded uncertainty increases almost linearly with an increase in pressure. These results suggest that at higher pressure the uncertainty becomes pressure dependent which dominates the resolution part. We have re-estimated the uncertainty using 44 measurements at different frequencies for a particular pressure point to minimize the systematic phase error. In this method, the uncertainty has calculated in the measurement of fractional fringe counting and ultrasonic phase shift. A detail experimental budget has been prepared along with average uncertainty, the standard deviation of the mean, combined uncertainty, total uncertainty in differential lengths and total uncertainty in the pressure range of 1 Pa to 130 kPa. These exercises have strengthened the UIM in the measurement accuracy.
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