Analog & Digital Pressure / Temperature Sensors

Sensor Description

The downhole environment is very stressful to sensors. The following basic requirements were used during the development of the Analog and Digital Pressure/Temperature Sensors so that reliable data was assured:

• Wide operating temperature range - Must be capable of surface and downhole.
• Large output signal on the Transducer - Simplifies the processing and minimizes the effect of measurement circuit errors.
• Repeatability - Under pressure and temperature cycling, the sensor output for given input conditions should be the same each time the conditions are applied.
• Stability - Under constant pressure and temperature, the transducer output should not change.
• Resistant to mechanical shock and vibration - Minimal error during operation and no long term degradation.
• Resistant to gases and liquids produced or placed downhole, whether naturally occurring (CO2, brine, H2S) or from well treatment (acids, alkalis).

Transducer

The design incorporates a Piezo Resistive transducer which incorporates a Silicon on Insulator type diaphragm. A silicon chip (approx. 0.1 inch square) is micro machined with a cavity on the underside to form a pressure-sensing diaphragm. The depth of the cavity determines the pressure range. Four resistors are defused into the surface, located so that two are in compression and two are in tension. The diaphragms are fabricated in quantity on wafers using integrated circuit production methods. The sensor is made when the diaphragm is fused to a silicon substrate.

Basics of Operation

The Piezo Resistive transducer converts a pressure into a change in resistance. The strain of the applied pressure is measured across an active four wire resistive bridge and the temperature is measured from a secondary of the main bridge.

The purpose of the temperature measurement is so that the pressure transducer output can be compensated for temperature induced errors ('temperature compensated' for increased pressure accuracy over a broad temperature range, 15°C to 125°C / 59°F to 257°F).

The designs incorporate minimal electronics that decreases infant mortality commonly experienced in other designs.