The Molecule Counters - Part Two: More About Vacuum Gauges
Counting molecules is a job for vacuum gauges, and it’s now time to understand the differences between these devices and when to use them. Let’s learn more.
Recall first that the vacuum level in a vessel is determined by the pressure differential between the evacuated volume and the surrounding atmosphere (Table 1). The two basic reference points in all these measurements are standard atmospheric pressure (760 torr) and perfect vacuum (0 torr), so calculating changes in volume in vacuum systems requires conversions to negative pressure (psig) or absolute pressure (psia).
Mechanical Gauge DesignsMechanical gauges measure pressure or vacuum by making use of the mechanical deformation of tubes or diaphragms when exposed to a difference in pressure. For this reason, they are classified as differential pressure gauges. Typically, one side of the element is exposed to a reference vacuum, and the instrument measures the mechanical deformation that occurs when an unknown vacuum pressure is exposed to the other side.
These gauges work on the principle of the Bourdon tube, which consists of a tube with an elliptical cross section formed in an arc. The tube is rigidly fixed at one end and closed at the other end. When the pressure in the tube increases, the radius of the arc increases (in other words, the tube tries to straighten itself out). When the pressure decreases, the radius decreases (thus, the free end of the tube moves in response to a change in pressure). A system of mechanical linkages attached to the free end moves a pointer over a calibrated scale.
A quartz Bourdon tube uses a quartz helix element, and instead of moving linkages, the deformation rotates a mirror. When used for vacuum detection, two quartz Bourdon elements are formed into a helix. The reference side contains a sealed vacuum and the measurement side is connected to the unknown process vacuum. The pressure difference between the two sides causes an angular deflection that is detected optically. The optical readout has a high resolution, about one part in 100,000. Advantages of this sensor are its precision and the corrosion resistance of quartz. Its main limitation is high price.
The basic manometer consists of a reservoir filled with a liquid. When detecting vacuums, the top of the column is evacuated and sealed. A relatively small change in vacuum pressure will cause a relatively large movement of the liquid. Manometers are simple, low cost and can detect vacuums down to about 10-3 torr. The accuracy of the gauge is determined by how closely the difference in height of the two arms can be measured. Today, digital-readout manometers have greatly improved accuracy. The sensitivity of the gauge depends on the density of the fluid used.
Capacitance Vacuum Manometer
A capacitance sensor operates by measuring the change in electrical capacitance that results from the movement of a sensing diaphragm relative to some fixed capacitance electrodes (Fig 1). The higher the process vacuum, the farther it will pull the measuring diaphragm away from the fixed capacitance plates. In some designs, the diaphragm is also allowed to move. In others, a variable dc voltage is applied to keep the sensor’s Wheatstone bridge in a balanced condition. The amount of voltage required is directly related to the pressure.
The great advantage of a capacitance gauge is its ability to detect extremely small diaphragm movements. Accuracy is typically 0.25 to 0.5% of the reading. Thin diaphragms can measure down to 10-5 torr, while thicker diaphragms can measure in the low vacuum to atmospheric range. To cover a wide vacuum range, one can connect two or more capacitance sensing heads into a multi-range package.
The McLeod gauge uses the principle of Boyle’s law (P1V1 = P2V2 ; that is, if the temperature is held constant, the increase in pressure is exactly proportional to the decrease in volume) to amplify and measure pressures that are too small to be measured with a manometer. To do this, a sample of gas from the system is isolated and reduced in volume by a known amount. This reduction in volume causes a proportional increase in the pressure of the gas. Originally invented in 1878, the McLeod gauge is still used today, mainly for calibrating other gauges (although other techniques are making this obsolete). McLeod gauges can cover vacuum ranges between 1 and 10-6 torr.
Molecular Momentum Gauges
This vacuum gauge is operated with a rotor that spins at a constant speed. Gas molecules in the process sample come in contact with the rotor and are propelled into the restrained cylinder. The force of impact drives the cylinder to a distance proportional to the energy transferred, which is a measure of the number of gas molecules in that space. The full scale of the instrument depends on the gas being measured. The detector has to be calibrated for each application.
Viscous Friction Gauges
At high-vacuum levels, viscosity and friction both depend on pressure. This instrument measures vacuum down to 10-7 torr by detecting the deceleration caused by molecular friction on a ball that is spinning in a magnetic field. Vacuum is determined by measuring the length of time it takes for the ball to drop from 425 to 405 revolutions per second after drive power is turned off. The higher the vacuum, the lower the friction and, therefore, the more time it will take. This design is accurate to 1.5% of indicated reading, is resistant to corrosion and can operate at temperatures up to 7500°F (4150°C).
Thermal Gauge DesignsBelow 1 torr, a change in the pressure of a gas will cause a change in its thermal conductivity (the ability of a gas to conduct heat). If an element heated by a constant power source is placed in a gas, the resulting surface temperature of the element will be a function of the surrounding vacuum. Because the sensor is an electrically heated wire, thermal vacuum sensors are often called hot-wire gauges. Because the characteristics of all gases are different, the response of a thermal-conductivity gauge will vary for each gas. To read accurately, the gauge must be calibrated for the gas being measured.
The Pirani gauge utilizes the change in electrical resistance of a wire with temperature. A sensor wire is heated electrically, and the pressure of the gas is determined by measuring the current needed to keep the wire at a constant temperature by use of a Wheatstone bridge network (Fig. 2). The Pirani gauge is linear in the 10-2 to 10-4 torr range. Above these pressures, output is roughly logarithmic. Pirani gauges are inexpensive, convenient and reasonably accurate (within 2% at the calibration point and 10% over their operating range).
The thermocouple gauge relates the temperature of a filament in the process gas to its vacuum pressure (Fig. 3). The filament is heated by a constant current of 20-200 mA dc, and the thermocouple generates an output of about 20 mV dc. The heater-wire temperature increases as pressure is reduced.
Typical thermocouple gauges measure 10-3 to 2 torr. This range can be increased by use of a gauge controller with a digital/analog converter and digital processing. Using an industry-standard thermocouple sensor, such a gauge controller can extend the range of a thermocouple sensor to cover from 10-3 to 1,000 torr, thereby giving it the same range as a convection-type Pirani gauge but at a lower price.
Similar to the Pirani gauge, this sensor uses a temperature-compensated, gold-plated tungsten wire to detect the cooling effects of both conduction and convection and thereby extends the sensing range (Fig. 5). At higher vacuum, response depends on the thermal conductivity of the gas, while at lower vacuums it depends on convective cooling by the gas molecules. Measurement range is from 10-3 to 1,000 torr. With the exception of its expanded range, features and limitations of this sensor are the same as those of Pirani and most thermocouple gauges.
The convection gauge (Fig. 6) measures absolute pressures by determining the heat loss from a fine wire filament maintained at a constant temperature. The response of the sensor depends on the gas type. A pair of thermocouples is mounted at a fixed distance from each other. One thermocouple is heated to a constant temperature by a variable-current power supply. Power is pulsed, and the temperature is measured between heating pulses. The second thermocouple measures convection effects and also compensates for ambient temperature. This sensor must be mounted vertically.
To get around the range limitations of certain sensors, gauge manufacturers have devised a means for electronically linking multiple sensor heads. For example, one manufacturer offers a wide-range vacuum gauge that incorporates two pressure sensors in one housing: a fast-response diaphragm manometer for measurements between 1,500 torr and 2 torr and a Pirani gauge for measuring between 2 torr and 10-3 torr. The gauge controller automatically switches between the two sensors.
Ionization Gauge DesignsWhen fast-moving electrons pass through a gas, they can knock some of the outer electrons off of the gas molecules. The remaining part of the molecule (called an ion) then has a positive charge. The process is called ionization by bombardment. For a constant current of electrons at a given gas velocity, the rate at which these positive ions are formed is proportional to the concentration of the gas molecules (assuming the temperature remains constant). The ionization efficiency varies with the kind of gas, so these gauges must be calibrated for the gas for which they will be used. Two types are available: hot cathode and cold cathode.
Refined by Bayard-Alpert in 1950, the hot filament in the hot-cathode gauge emits electrons into the vacuum, where they collide with gas molecules to create ions (Figs. 7, 8). These positively charged ions are accelerated toward a collector, where they create a current in a conventional ion-gauge detector circuit. The amount of current formed is proportional to the gas density or pressure. Most hot-cathode sensors measure vacuum in the range of 10-2 to 10-10 torr.
Newer instruments extend this range significantly by using a modulated electron beam, synchronously detected to give two values for ion current. At pressures below 10-3 torr, there is little difference in the two values. At higher pressures, the ratio between the two readings increases monotonically, allowing the gauge to measure vacuums up to 1 torr.
Because most high-vacuum systems were made of glass in 1950, it made sense to enclose the electrode structure in glass. Today, however, a modern vacuum system may be made entirely of metal. One argument in favor of this is that glass decomposes during routine degassing, producing spurious sodium ions and other forms of contamination. Nevertheless, glass gauges remain the most popular choice for hot-cathode sensors.
This gauge (also called a Philips gauge or Penning gauge) is based on the glow discharge that occurs in gas at low pressures in the presence of a magnetic field. Electrons that originate in one of the cathodes in the gauge do not go directly to the anode (because of the magnetic field). Instead, they travel back and forth in helical paths between the cathodes multiple times before striking the anode. The increased path length provides a high probability of ionization (even at low gas pressures where a glow discharge normally does not occur). The total discharge current (negative and positive ions) is used to measure the pressure.
The major difference between hot- and cold-cathode sensors is in their methods of electron production. In a cold-cathode device, electrons are drawn from the electrode surface by a high potential field. In the Phillips design (Fig. 9 - 10), a magnetic field around the tube deflects the electrons, causing them to spiral as they move across the magnetic field to the anode. This spiraling increases the opportunity for them to encounter and ionize the molecules. Typical measuring range is from 10-10 to 10-2 torr. The main advantages of cold-cathode devices are that there are no filaments to burn out, they are unaffected by the inrush of air, and they are relatively insensitive to vibration.