A Historical Perspective of "force"

The term "high pressure" is relative, as, in fact, are all pressure measurements. What the term actually means depends greatly on the particular industry one is talking about. In synthetic diamond manufacturing, for example, normal reaction pressure is around 100,000 psig (6,900 bars) or more, while some fiber and plastic extruders operate at 10,000 psig (690 bars). Yet, in the average plant, pressures exceeding 1,000 psig (69 bars) are considered high.

In extruder applications, high pressures are accompanied by high temperatures, and sticky materials are likely to plug all cavities they might enter. Therefore, extruder pressure sensors are inserted flush with the inner diameter of the pipe and are usually continuously cooled.

High Pressure Designs

In the case of the button repeater (Figure 4-1A), the diaphragm can detect extruder pressures up to 10,000 psig and can operate at temperatures up to 8000¡F (4300¡C) because of its self-cooling design. It operates on direct force balance between the process pressure (P1) acting on the sensing diaphragm and the pressure of the output air signal (P2) acting on the balancing diaphragm. The pressure of the output air signal follows the process pressure in inverse ratio to the areas of the two diaphragms. If the diaphragm area ratio is 200:1, a 1,000-psig increase in process pressure will raise the air output signal by 5 psig.

The button repeater can be screwed into a H-in. coupling in the extruder discharge pipe in such a way that its 316 stainless steel diaphragm is inserted flush with the inside of the pipe. Self-cooling is provided by the continuous flow of instrument air.

Another mechanical high pressure sensor uses a helical Bourdon element (Figure 4-1B). This device may include as many as twenty coils and can measure pressures well in excess of 10,000 psig. The standard element material is heavy-duty stainless steel, and the measurement error is around 1% of span. Helical Bourdon tube sensors provide high overrange protection and are suitable for fluctuating pressure service, but must be protected from plugging. This protection can be provided by high-pressure, button diaphragm-type chemical seal elements that also are rated for 10,000-psig service.

An improvement on the design shown in Figure 4-1B detects tip motion optically, without requiring any mechanical linkage. This is desirable because of errors introduced by linkage friction. In such units, a reference diode also is provided to compensate for the aging of the light source, for temperature variations, and for dirt build-up on the optics. Because the sensor movement is usually small (0.02 in.), both hysteresis and repeatability errors typically are negligible. Such units are available for measuring pressures up to 60,000 psig.

Deadweight testers also are used as primary standards in calibrating high-pressure sensors (Figure 4-1C). The tester generates a test reference pressure when an NIST-certified weight is placed on a known piston area, which imposes a corresponding pressure on the filling fluid. (For more details, see Chapter 3 of this volume.) NIST has found that at pressures exceeding 40,000 psig, the precision of their test is about 1.5 parts in 10,000. Typical inaccuracy of an industrial deadweight tester is 1 part in 1,000 or 0.1%.

In the area of electronic sensors for high-pressure measurement, the strain gage is without equal (see Chapter 2 for more details on strain gage operation). Strain gage sensors can detect pressures in excess of 100,000 psig and can provide measurement precision of 0.1% of span or 0.25% of full scale. Temperature compensation and periodic recalibration are desirable because a 1000¡F temperature change or six months of drift can also produce an additional 0.25% error. Other electronic sensors (capacitance, potentiometric, inductive, reluctive) are also capable of detecting pressures up to 10,000 psig, but none can go as high as the strain gage.

Very High Pressures

The bulk modulus cell consists of a hollow cylindrical steel probe closed at the inner end with a projecting stem on the outer end (Figure 4-2). When exposed to a process pressure, the probe is compressed, the probe tip is moved to the right by the isotropic contraction, and the stem moves further outward. This stem motion is then converted into a pressure reading. The hysteresis and temperature sensitivity of this unit is similar to that of other elastic element pressure sensors. The main advantages of this sensor are its fast response and safety: in effect, the unit is not subject to failure. The bulk modulus cell can detect pressures up to 200,000 psig with 1% to 2% full span error.

In another high-pressure design, Manganin, gold-chromium, platinum, or lead wire sensors are wound helically on a core. The electrical resistance of these wire materials will change in proportion to the pressure experienced on their surfaces. They are reasonably insensitive to temperature variations. The pressure-resistance relationship of Manganin is positive, linear, and substantial. Manganin cells can be obtained for pressure ranges up to 400,000 psig and can provide 0.1% to 0.5% of full scale measurement precision. The main limitation of the Manganin cell is its delicate nature, making it vulnerable to damage from pressure pulsations or viscosity effects.

Some solids liquefy under high pressures. This change-of-state phenomenon also can be used as an indication of process pressure. Bismuth, for example, liquefies at between 365,000 and 375,000 psig and, when it does, it also contracts in volume. Other materials such as mercury have similar characteristics, and can be used to signal that the pressure has reached a particular value.

Vacuum Measurement

Engineers first became interested in vacuum measurements in the 1600s, when they noted the inability of pumps to raise water more than about 30 ft. The Duke of Tuscany in Italy commissioned Galileo to investigate the "problem." Galileo, among others, also devised a number of experiments to investigate the properties of air. Among the tools used for these experiments were pistons to measure force and a water barometer (about 34 ft. tall) to measure vacuum pressure.



After Galileo's death in 1642, Evangelista Torricelli carried on the work of vacuum-related investigation and invented the mercury barometer (Figure 4-3). He discovered that the atmosphere exerts a force of 14.7 lb. per square in. (psi) and that, inside a fully evacuated tube, the pressure was enough to raise a column of mercury to a height of 29.9 in. (760 mm). The height of a mercury column is therefore a direct measure of the atmospheric pressure.



In 1644, French mathematician Blaise Pascal asked a group of mountaineers to carry a barometer into the Alps and proved that air pressure decreases with altitude. The average barometric pressure at sea level can balance the height of a 760 mm mercury column, and this pressure is defined as a standard Atmosphere. The value for 1/760th of an atmosphere is called a torr, in honor of Torricelli.

In 1872, McLeod invented the McLeod vacuum detector gauge, which measures the pressure of a gas by measuring its volume twice, once at the unknown low pressure and again at a higher reference pressure. The pressurized new volume is then an indication of the initial absolute pressure. Versions of the McLeod Gauge continue to be used today as a standard for calibrating vacuum gauges.

Applications

Vacuum gauges in use today fall into three main categories: mechanical, thermal, and ionization. Their pressure ranges are given in Figure 4-4. In general, for high vacuum services (around 10-6 torr), either cold cathode or Bayard-Alpert hot cathode gauges are suitable. Neither is particularly accurate or stable, and both require frequent calibration.

For vacuums in the millitorr range (required for sputtering applications), one might consider a hot cathode ion gauge. For more accurate measurements in this intermediate range, the capacitance manometer is a good choice. For intermediate vacuum applications (between 10-4 and 10-2 torr), capacitance manometers are the best in terms of performance, but are also the most expensive. The lowest priced gauge is the thermocouple type, but its error is the greatest. Digital Pirani gauges can represent a good compromise solution, with accuracy between that of capacitance and thermocouple sensors.

For low vacuums (higher pressures) between atmospheric and 10-2 torr, Bourdon tubes, bellows, active strain gages, and capacitance sensors are all suitable.

Mechanical Designs

Mechanical gauges measure pressure or vacuum by making use of the mechanical deformation of tubes or diaphragms when exposed to a difference in pressure. 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.

Quartz Bourdon Tube: Similar to a standard Bourdon tube, this gauge uses a quartz helix element, but 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.



Manometer: A basic manometer can consist of a reservoir filled with a liquid and a vertical tube (Figure 4-5). When detecting vacuums, the top of the column is sealed evacuated. A manometer without a reservoir is simply a U-shaped tube, with one leg sealed and evacuated and the other connected to the unknown process pressure (Figure 4-5A). The difference in the two column heights indicates the process vacuum. An inclined manometer (Figure 4-5D) can consist of a well and transparent tube mounted at an angle. A 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 1 millitorr.

Capacitance 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 (Figure 4-6). 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 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 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 capacitance diaphragm gauge is widely used in the semiconductor industry, because its Inconel body and diaphragm are suitable for the corrosive services of this industry. They are also favored because of their high accuracy and immunity to contamination.

McLeod Gauge: Originally invented in 1878, the McLeod gauge measures the pressure of gases by compressing a known volume with a fixed pressure. The new volume is then a measure of the initial absolute pressure. Little changed since the day it was invented, the McLeod gauge has been used until recently for calibrating other gauges. It covers the vacuum range between 1 and 10-6 torr.

Molecular Momentum: 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: At high vacuums, viscosity and friction both depend on pressure. This instrument measures vacuums 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 reading, is resistant to corrosion, and can operate at temperatures up to 7500¡ F.

Thermal Designs: The thermal conductivity of a gas changes with its pressure in the vacuum range. 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. Typically, hot wire gauges can be used to measure down to 10-3 mm Hg.

Pirani: In this design, 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. The thermal conductivity of each gas is different, so the gauge has to be calibrated for the individual gas being measured. A Pirani gauge will not work to detect pressures above 1.0 torr, because, above these pressures, the thermal conductivity of the gases no longer changes with pressure. 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. They are 2% accurate at the calibration point and 10% accurate over the operating range.

Thermocouple: The thermocouple gauge relates the temperature of a filament in the process gas to its vacuum pressure. 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 1 millitorr 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.

Convection Gauge: 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. At higher vacuums, 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.

Combined Gauges: To get around the range limitations of certain sensors, gauge manufacturers have devised 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 1 millitorr. The gauge controller automatically switches between the two sensors.

Ionization Types: Ionization detectors have been available since 1916. They measure vacuum by making use of the current carried by ions formed in the gas by the impact of electrons. Two types are available: hot cathode and cold cathode.



Refined by Bayard-Alpert in 1950, the hot filament off the hot-cathode gauge emits electrons into the vacuum, where they collide with gas molecules to create ions (Figure 4-7). 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 for the time being do remain the most popular hot cathode sensors.



Cold Cathode: 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 (Figure 4-8), 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.