Shrinking the Pirani Vacuum Gauge

January 1, 2005 By: Paul Dozoretz, Chris Stone, Ole Wenzel Sensors

Micron-scale fabrication techniques fostered by the semiconductor and IC industries have produced an enormous variety of sensing devices that are both smaller and smarter than their precursors. Microelectromechanical systems (MEMS), as these devices are called, have become common in the past 20 years [1-3]. For example, the accelerometers and gyroscopes designed for automotive applications are now nearly universally configured as MEMS [4]. The technology is becoming ubiquitous in areas as diverse as consumer electronics (optical projection) [5], communications (optical multiplexing) [6], surface analysis (scanning tunneling microscopy) [7], and biotechnology (DNA amplification and identification) [8].

Gas flow, temperature, and pressure, as well as a variety of special properties, are routinely measured with solid-state MEMS devices [1]. Vacuum sensing has followed this trend [9], with both the sensing element and its associated electronics miniaturized to the point where multiple sensing functionalities, along with control and signal conditioning electronics, can be integrated into a single device.

The MicroPirani

Pirani gauges have been widely used for vacuum measurement since their invention in 1906 [10]. A heated wire with a high temperature coefficient of resistance is placed in a vacuum environment. The wire, whose resistance is proportional to its temperature, forms one leg of a balanced Wheatstone bridge. Gas molecules collide with the wire, transferring heat away from it and unbalancing the bridge relative to a reference state. Since the frequency of molecular collisions is proportional to the gas pressure, the voltage required to maintain the bridge in balance is proportional to the pressure. Pirani gauges have become an industry standard, owing to their reliability, low cost, and relatively wide pressure range [11].

Conventional Pirani gauges have certain drawbacks, though. In the large older gauges, the element's high temperature (~100°C to 150°C above ambient) presents a problem in process environments where the gases are thermally reactive. Reaction products can coat the wire element and modify its resistive response to pressure changes. The sensing mechanism is affected by other factors as well. Heat dissipation within the gauge occurs via heat transfer from the heated element to the ambient gas, followed by heat transfer between the gas and the cool instrument wall. This works well at relatively low pressures because the mean free path of the gas molecules in the system is greater than the element-to-wall distance. At higher pressures, however, the mean free path is reduced to values significantly less than this distance; nonlinearities enter into the pressure-voltage relationship and reduce the gauge's sensitivity. For conventional Pirani gauges to operate at pressures up to 760 torr, they must be constructed with wire-to-wall distances great enough to allow heat transfer to occur by convective flow at higher pressures. Unfortunately, this convective flow introduces sensitivity to the mounting position because gravity affects convective currents within the gauge. A difference in mounting position thus results in a difference in the electrical output–pressure relationship. The heat transfer mechanism in Pirani transducers also makes them sensitive to differences in ambient temperature since these cause variations in the instrument wall temperature. In the conventional Pirani gauge, the wire is thermally anchored at two points, causing pressure-dependent temperature gradients at the end of the wire [12]. Finally, the heat transfer mechanism dictates that gauge response will depend on the composition of the gas being measured, since the heat capacity of the gas affects heat transfer.

 Figure 1. The MKS MicroPirani vacuum gauge solves many of the performance problems associated with conventional Pirani gauges—and has a much smaller footprint as well.
Figure 1. The MKS MicroPirani vacuum gauge solves many of the performance problems associated with conventional Pirani gauges—and has a much smaller footprint as well.

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