Resources Vacuum Heat Treating with Dan Herring Vacuum Measurement Systems – Part 1
| Vacuum Measurement Systems – Part 1 |
|
By Dan Herring
This is the fifth in a series of articles in our Vacuum Heat-Treatment Series. Here we begin a discussion of the types and characteristics of vacuum gauges and offer insights into which gauge should be used when working in a specific vacuum range .
One, two, three … Counting molecules is a job for vacuum gauges. Depending on the type of vacuum systems and the required operating vacuum level, different vacuum gauges are required – often in combination with one another – to accurately determine and/or control the vacuum level of the chamber at any given moment in time. The criteria for selecting a vacuum gauge are dependent on various conditions, such as:
Vacuum gauges are divided into three basic categories based on their working pressure (Fig. 1). These include:
What is a Vacuum Gauge?
A vacuum gauge is an instrument for measuring pressures below atmospheric pressure. There are many types, each designed for a specific function. Some of the more common types of vacuum gauges are shown below, listed in order of descending pressure range: Manometer A manometer is a relatively simple device that usually consists of a tube or column filled with a liquid. The pressure is found by measuring the column height or the difference in heights of several columns. The U-tube manometer is the most common type of manometer today because the difference in height between the two columns is always a true indication of the pressure regardless of variations in the internal diameter of the tube. With both ends of the tube open, the liquid is at the same height in each leg (Fig. 2a). When positive pressure is applied to one leg (Fig. 2b), the liquid is forced down in that leg and up in the other. The difference in height, "h," which is the sum of the readings above and below zero, indicates the pressure. When a vacuum is applied to one leg (Fig. 2c), the liquid rises in that leg and falls in the other. The difference in height, "h," which is the sum of the readings above and below zero, indicates the amount (or degree) of vacuum. Thermal Conductivity Gauge
These devices operate on the principle that heat transported by gas molecules can be related to gas pressure. A heat source causes changes in surface temperature (or in the heating power required to maintain constant temperature), and this is related to the pressure of the system. Various types of thermal-conductivity gauges are distinguished according to the method of indicating the surface temperature (that is, the way in which the wire temperature is measured). The most common types with a typical operational range of 1 torr to 10-3 torr include hermocouple gauges and Pirani gauges. Operation of thermocouple gauges depends on the fact that the conduction of heat away from a heated filament is dependent upon the pressure, with the cooling being greater at high pressures and less at low pressures. The system typically consists of a heated filament with thermocouples attached to an external circuit in such a manner that the temperature of the filament can be read in terms of the pressure of the gases surrounding it. In the readout devices, a special circuit is provided for standardization of the current passing through the filament. Thermocouple gauges are simple, inexpensive and rugged. They can be contaminated by material from the vessel getting into the tube, but upstream filters are available, and, in some cases, they can be cleaned. Most operating systems make use of several of these devices in the roughing line, in the foreline of a diffusion pump and in the vessel itself. Pirani gauges (Fig. 3), like thermocouple gauges, depend on the thermal conductivity of the gas surrounding the heated filament. However, the actual change in resistance of the heated platinum or tungsten filament wire is measured and used as a calibration means. These gauges have an extra element sealed away from the vacuum, employed in a bridge circuit, in order to compensate for ambient temperature (otherwise, large errors would ensue). Pirani gauges do not suffer as much from inadvertent exposure to air with the filament heated and a simple gauge readout circuit. The principal limitations include contamination from vessel vapors being pumped or the deposition of pump oil on the filament (which changes its emissivity and, hence, its temperature and pressure) and the fact that these gauges are nearly impossible to clean. Also, thermal conductivity will not be a function of pressure in the range of 1 mbar (750 microns) to atmospheric pressure (laminar flow range). In the simplest case, a Pirani gauge consists of a thin wire (diameter 2ri) that is mounted in the axis of a cylindrical tube (diameter 2ro). The thin wire is heated by a constant electrical power source with heat transported to the walls of the tube.
Knudsen Gauges Knudsen (radiometer) gauges (Fig. 4) are typically accurate to 10-6 torr. These devices measure pressure in terms of the net rate of transfer of momentum by molecules between two surfaces maintained at different temperatures and separated by a distance smaller than the mean free path of the gas molecule. Two parallel plates of the same area are held at different temperatures. Gas molecules traveling from the higher- to the lower-temperature plate have higher impact energy than gas particles coming from the surroundings to the other side of the plate. These different pressures are measured.
McLeod Gauge The McLeod gauge is typically accurate to 10-6 torr. The principle of operation involves measuring the pressure of a gas by measuring its volume twice, once at the unknown low pressure and again at a higher reference pressure. A liquid (mercury) piston is created with which a gas of a well-defined volume, V, is compressed into a known small volume, VM. By comparison, the pressure in the closed volume VM is increased from the unknown pressure to the (higher) measurable pressure that can be calculated. The use of a McLeod gauge enables accurate readings to be obtained that are independent of any external calibration device. In other words, they provide absolute measurements. All other types of gauges must be externally calibrated and, therefore, are subject to drift in use. The principle disadvantage is that the device is not a continuous readout and is not automatic. The gauge is also easily contaminated.
Ionization Gauge An ionization gauge is typically accurate to 10-9 torr and beyond. These devices have a means of ionizing the gas molecules and a means of correlating the number and type of ions produced with the pressure of the gas. Various types of ionization gauges are distinguished according to the method of producing the ionization. The common types are:
A cold cathode produces an ion current by utilizing a high-voltage discharge. The electrons emanating from the cathode are caused to spiral by a magnetic field as they move across the tube to the anode. This spiraling of electrons increases the probability of collision with any gas molecules present. Such collisions set free positively charged ions, which are attracted to a negatively charged plate. The greatest advantage of this type of gauge is that it is not harmed by inadvertent exposure to atmospheric pressure. Issues involve hazards and insulating methods of high voltage (>2,000 volts) in the vicinity of the gauge. These gauges are also readily contaminated by either hydrocarbons or metallic ions and are very difficult to clean. Hot-cathode gauges can be employed in all types of applications. In it, electrons are emitted from a heated filament and accelerated toward a positively charged cylindrical grid. Some of these electrons collide with gas molecules, causing the production of positively charged ions, which are collected at a negatively charged plate (as in the case of cold-cathode gauges). Since no magnetic field is employed, the ion currents produced are considerably lower than cold-cathode gauges. It is necessary to employ an amplifying stage in the readout device in order to increase the current sufficiently to provide a readout. The advantages of these gauges are their availability over a wide range of pressures, relatively inexpensive sensing tubes, the availability of an outgassing cycle (to ride the tube of contaminants) and a linear output down to relatively low pressures. The principle limitations are exposure to atmospheric pressure and measuring errors arising from pumping effects. Indeed, exposure to any pressure much above 0.001 mbar (1 micron) while the filament is heated will burn it out. A Little History Engineers first became interested in vacuum measurements in the 1600s, when they noted the inability of pumps to raise water more than about 30 feet (9 m). The Duke of Tuscany in Italy commissioned Galileo to investigate the "problem." Galileo, among others, devised a number of experiments to investigate the properties of air. After Galileo's death in 1642, Evangelista Torricelli continued the work that included vacuum-related investigations and the invention of the mercury barometer. He discovered that the atmosphere exerts a force of 14.7 psi (101.3 kPa) and that inside a fully evacuated tube the pressure was enough to raise a column of mercury to a height of 29.9 inches (760 mm). The height of a column of mercury is therefore a direct measure of the atmospheric pressure. The value of 1/760th of an atmosphere is called a torr, in honor of Torricelli. Pressure is simply defined as a force per unit area, and the most accurate way to measure air pressure is to balance a column of liquid of known weight against it and measure the height of the liquid column so balanced. The units of measure commonly used in the U.S. are inches of mercury (Hg), using mercury as the fluid, and inches of water column (W.C.), using water or oil as the fluid. Pressure Ranges The correct choice of gauge depends on knowledge of the working principles of the gauge, the range of pressures it can measure and its accuracy over the required range. These factors have been determined by experience (Fig. 3), and there is a vacuum gauge for every pressure range.
Location Finally, a number of factors must be considered when installing the devices discussed above. In particular, selection of the location for installation needs to:
Transmitters measuring sensor output, control units and cables must be properly matched to the device. Abbreviated Dictionary of Vacuum Gauge Terms [5,6] Absolute pressure – Pressure measured above the zero value of a perfect vacuum; designated psia (pounds per square inch absolute). Atmospheric pressure – The pressure exerted by a mercury column 760 mm high at 0ºC under a standard acceleration of gravity (980.665 cm/sec2 or 14.7 psi at sea level). Boyle’s Law – One of the gas laws, Boyle’s law states that pressure and volume in a gas are inversely proportional (assuming constant temperature and mass). Gauge pressure – Pressure measured at atmospheric pressure as a reference point. Gauge pressure is designated psig (pounds per square inch gauge). Millimeter of Mercury (mm Hg) – A unit of pressure defined as that pressure that will support a column of mercury 1 millimeter high. Pressure – The force per unit area a gas exerts. Common units are torr, millibar, microns, psia or millimeters of mercury. Torr – A unit of pressure defined as 1/760th of an atmosphere. Vacuum – A space filled with gas at a pressure less than atmospheric pressure. Vacuums are classified as rough (760 torr to 1 torr), low (1 torr to 10-3 torr), high (10-3 to 10-6 torr), very high (10-6 to 10-9 torr) and ultrahigh (10-9 and above). Next time: Part six of this series concludes our discussion of the types and characteristics of vacuum gauges and offers insights into which gauge should be used when working in a specific vacuum range. Daniel H. Herring / Tel: (630) 834-3017) /E-mail:
This e-mail address is being protected from spambots. You need JavaScript enabled to view it
1. American vacuum society (www.avs.org) |
