INTRODUCTION
(Taken from the full technote. )

Determining the differential pressure (Dp) between two fluid sections is particularly valuable in various downhole measurement and control applications. The downhole flow calculation through a flowmeter requires a Dp measurement, as does controlling the Dp between the tubing and the annulus in drilling applications. The pressure difference between two Quartzdyne absolute pressure transducers is often used for critical downhole Dp measurements.

Most of our customers currently use two of our transducers to measure Dp. Presumably, the cumulative long-term drift occurring in the two transducers limits this approach. Drift between two gauges can be offset by periodically zeroing the two gauges, which requires shutting-in the well. In late 2000, we reduced the long-term drift by four to tenfold, and we continue to dedicate significant resources to eventually eliminate drift. This stability improvement allows for long-term Dp accuracies and less frequent rezeroing of transducers. This technote addresses the feasibility of using a single Quartzdyne^® Pressure Transducer to measure a Dp, completely avoiding the drift issue altogether.

The quartz pressure-sensing crystal housed inside a Quartzdyne^® Pressure Transducer is well suited to measure a Dp as a standalone unit. In Figure 1, a venturi meter using a single pressure transducer to measure the inlet and throat pressures is depicted. Best accomplished by actuating a valve between the two fluid regions, a single Quartzdyne^® Pressure Transducer provides several benefits over using two absolute pressure transducers:

• Achievable Dp accuracy of < 0.1 psi [690 Pa] (based on £ 5 psid tests)
• Resolves Dp measurements as small as 0.005 psi [34 Pa]
• Negates the effect of transducer drift
• Improved downhole flow (less constrictive Venturi construction)