Gauges Using Thermal Conductivity for the Measurement of Pressure :
Applications: Thermal conductivity gauges are relativelyinexpensive. Many operate in a range of ~1 X 10¯3
to 20 torr. This range has been extended to atmospheric pressure in some modifications of the ‘‘traditional’’ gauge geometry. They are valuable for monitoring and control, for example, during the processes of roughing down from atmospheric pressure and for the cross-over from roughing pump to high-vacuum pump. Some are subject to drift over time, for example, as a result of contamination from mechanical pump oil, but others remain surprising stable under common system conditions.
Operating Principles: In most gauges, a ribbon or filament serves as the heated element. Heat loss from this element to the wall is measured either by the change in element temperature, in the thermocouple gauge, or as a change in electrical resistance, in the Pirani gauge.
Heat is lost from a heated surface in a vacuum system by energy transfer to individual gas molecules at low pressures (Peacock, 1998). This process has been used in the ‘‘traditional’’ types of gauges. At pressures well above 20 torr, convection currents develop. Heat loss in this mode has recently been used to extend the pressure measurement range up to atmospheric. Thermal radiation heat loss from the heated element is independent of the presence of gas, setting a lower limit to the measurement of pressure. For most practical gauges this limit is in the mid- to upper-10¯4 torr range.
Two common sources of drift in the pressure indication are changes in ambient temperature and contamination of the heated element. The first is minimized by operating the heated element at 300°C or higher. However, this increases chemical interactions at the element, such as the decomposition of organic vapors into deposits of tars or carbon; such deposits change the thermal accommodation coefficient of gases on the element, and hence the gauge sensitivity. More satisfactory solutions to drift in the ambient temperature include a thermostatically controlled envelope temperature or a temperature-sensing element that compensates for ambient temperature changes. The problem of changes in the accommodation coefficient is reduced by using chemically stable heating elements, such as the noble metals or gold-plated tungsten.Thermal conductivity gauges are commonly calibrated for air, and it is important to note that this changes significantly
with the gas. The gauge sensitivity is higher for hydrogen and lower for argon. Thus, if the gas composition
is unknown, the gauge reading may be in error by a factor of two or more.
Thermocouple Gauge:
In this gauge, the element is heated at constant power, and its change in temperature, as the pressure changes, is directly measured using a thermocouple.In many geometries the thermocouple is spot welded directly at the center of the element; the additional thermal mass of the couple reduces the response time to
pressure changes. In an ingenious modification, the thermocouple itself (Benson, 1957) becomes the heated element,and the response time is improved.
Pirani Gauge:
In this gauge, the element is heated electrically,but the temperature is sensed by measuring its resistance. The absence of a thermocouple permits a faster time constant. A further improvement in response results if the element is maintained at constant temperature, and the power required becomes the measure of pressure.
Gauges capable of measurement over a range extending to atmospheric pressure use the Pirani principle. Those relying on convection are sensitive to gauge orientation,and the recommendation of the manufacturer must be observed if calibration is to be maintained. A second point, of great importance for safe operation, arises from the difference in gauge calibration with different gases. Such gauges have been used to control the flow of argon into a sputtering system measuring the pressure on the highpressure side of a flow restriction. If pressure is set close to atmospheric, it is crucial to use a gauge calibrated for argon, or to apply the appropriate correction; using a gauge reading calibrated for air to adjust the argon to atmospheric
results in an actual argon pressure well above one atmosphere, and the danger of explosion becomes significant.
A second technique that extends the measurement range to atmospheric pressure is drastic reduction of
gauge dimensions so that the spacing between the heated element and the room temperature gauge wall is only 5 mm (Alvesteffer et al., 1995).
Applications: Thermal conductivity gauges are relativelyinexpensive. Many operate in a range of ~1 X 10¯3
to 20 torr. This range has been extended to atmospheric pressure in some modifications of the ‘‘traditional’’ gauge geometry. They are valuable for monitoring and control, for example, during the processes of roughing down from atmospheric pressure and for the cross-over from roughing pump to high-vacuum pump. Some are subject to drift over time, for example, as a result of contamination from mechanical pump oil, but others remain surprising stable under common system conditions.
Operating Principles: In most gauges, a ribbon or filament serves as the heated element. Heat loss from this element to the wall is measured either by the change in element temperature, in the thermocouple gauge, or as a change in electrical resistance, in the Pirani gauge.
Heat is lost from a heated surface in a vacuum system by energy transfer to individual gas molecules at low pressures (Peacock, 1998). This process has been used in the ‘‘traditional’’ types of gauges. At pressures well above 20 torr, convection currents develop. Heat loss in this mode has recently been used to extend the pressure measurement range up to atmospheric. Thermal radiation heat loss from the heated element is independent of the presence of gas, setting a lower limit to the measurement of pressure. For most practical gauges this limit is in the mid- to upper-10¯4 torr range.
Two common sources of drift in the pressure indication are changes in ambient temperature and contamination of the heated element. The first is minimized by operating the heated element at 300°C or higher. However, this increases chemical interactions at the element, such as the decomposition of organic vapors into deposits of tars or carbon; such deposits change the thermal accommodation coefficient of gases on the element, and hence the gauge sensitivity. More satisfactory solutions to drift in the ambient temperature include a thermostatically controlled envelope temperature or a temperature-sensing element that compensates for ambient temperature changes. The problem of changes in the accommodation coefficient is reduced by using chemically stable heating elements, such as the noble metals or gold-plated tungsten.Thermal conductivity gauges are commonly calibrated for air, and it is important to note that this changes significantly
with the gas. The gauge sensitivity is higher for hydrogen and lower for argon. Thus, if the gas composition
is unknown, the gauge reading may be in error by a factor of two or more.
Thermocouple Gauge:
In this gauge, the element is heated at constant power, and its change in temperature, as the pressure changes, is directly measured using a thermocouple.In many geometries the thermocouple is spot welded directly at the center of the element; the additional thermal mass of the couple reduces the response time to
pressure changes. In an ingenious modification, the thermocouple itself (Benson, 1957) becomes the heated element,and the response time is improved.
Pirani Gauge:
In this gauge, the element is heated electrically,but the temperature is sensed by measuring its resistance. The absence of a thermocouple permits a faster time constant. A further improvement in response results if the element is maintained at constant temperature, and the power required becomes the measure of pressure.
Gauges capable of measurement over a range extending to atmospheric pressure use the Pirani principle. Those relying on convection are sensitive to gauge orientation,and the recommendation of the manufacturer must be observed if calibration is to be maintained. A second point, of great importance for safe operation, arises from the difference in gauge calibration with different gases. Such gauges have been used to control the flow of argon into a sputtering system measuring the pressure on the highpressure side of a flow restriction. If pressure is set close to atmospheric, it is crucial to use a gauge calibrated for argon, or to apply the appropriate correction; using a gauge reading calibrated for air to adjust the argon to atmospheric
results in an actual argon pressure well above one atmosphere, and the danger of explosion becomes significant.
A second technique that extends the measurement range to atmospheric pressure is drastic reduction of
gauge dimensions so that the spacing between the heated element and the room temperature gauge wall is only 5 mm (Alvesteffer et al., 1995).
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