Here the Y- axis of the graph is actual pressure measured by a capacitance manometer. The X-axis is the pressure reading indicated by a particular convection gauge. Note this data does not necessarily apply to other convection gauges. Ionization Gauges. There are two types of cold-cathode ionization gauges: the Penning gauge and the inverted magnetron. The Penning gauge is the style most often used in vacuum furnaces.
Hot cathode ionization gauges can also be used and will be the subject of a future discussion. Ionization gauges are relatively accurate at high vacuum and are an indirect-acting type of gauge. They sense pressure indirectly by measuring the electrical ions produced when the gas is bombarded with electrons. Fewer ions will be produced by lower density gases. In an ionization gauge, thermionic emission generates electrons, which collide with gas atoms and generate positive ions.
The ions are attracted to a suitably biased electrode known as the collector. The current in the collector is proportional to the rate of ionization, which is a directly related to the pressure in the system. By measuring the collector current, the gas pressure is determined. The calibration of an ion gauge is dependent on the properties and proportion of the different gases in the vacuum being measured. Penning gauge calibration is also sensitive to physical geometry, corrosion and surface deposits.
Further their calibration can be invalidated by activation at atmospheric pressure or low vacuum. Cold-cathode gauges such as Penning gauges may not start at very low pressures due to the fact that the near-absence of a gas makes it difficult to establish an electrode current. Penning gauges use an axially symmetric magnetic field to create path lengths for electrons that are of the order of meters.
In ambient air, suitable ion pairs are formed by cosmic radiation. In a Penning gauge, design features are used to ease the set-up of a discharge path.
For example, the electrode of a Penning gauge is usually finely tapered to facilitate the field emission of electrons. Maintenance cycles of Penning gauges are, in general, measured in years, depending on the gas type and pressure that they are operated in. Using a cold cathode gauge in gases with substantial organic components, such as pump oil fractions, can result in the growth of delicate carbon films and shards within the gauge that eventually either short circuit the electrodes of the gauge or impede the generation of a discharge path.
To help resist against failure due to buildup or debris, these gauges are typically mounted in the vertical down orientation. Penning gauges are accurate in the range of 10 -3 to 10 Torr. We now look at the purpose of each specific gauge in a typical vacuum system. The first step of a vacuum furnace pumping cycle is generally to evacuate the oil diffusion pump to a low enough pressure that the heaters can be switched on. To monitor that pressure, a Pirani, thermocouple or convection gauge is located in the vacuum piping near the exhaust connection of the diffusion pump.
This pipeline is called the foreline of the mechanical pump or the discharge line of the diffusion pump. Later in the furnace process cycle, when the diffusion pump is up to operating temperature, the vacuum furnace exits the low vacuum phase and enters high vacuum. As the crossover is made from roughing to high vacuum pumping, the foreline gauge is used to monitor the diffusion pump discharge pressure.
Oil diffusion pumps work with hot oil vapor jets and the discharge pressure must be kept below the critical discharge pressure of the pump and fluid used for the vapor jets to work correctly. The foreline gauge is used to close the high vacuum valve if the discharge line pressure approaches the critical discharge pressure. The chamber roughing i. It is mounted on the vacuum chamber to measure the pressure attained during the low vacuum phase of the process.
This is the second step of the evacuation, when the mechanical pumps are directly connected to the chamber through the roughing line. Once the diffusion pump has heated up and is pumping against the closed vacuum chamber, the foreline valve is closed and the roughing valve opened to allow the mechanical vacuum pump to bring the furnace down to the crossover pressure. In small systems, this may take only a few minutes and with clean oil and minimal outgassing, the pressure in the discharge line will stay below the critical discharge pressure.
For higher volume furnaces, the roughing cycle will take longer. To ensure that the critical discharge pressure is not exceeded while the main mechanical pump is on the roughing cycle, a small 0. Typically, one side of the element is exposed to reference vacuum, and the instrument measures the mechanical deformation that occurs when unknown vacuum pressure is exposed to the other side. Types of mechanical gauges:. Many vacuum furnace manufacturers supply absolute pressure gauges on their equipment so that users have an indication of pressure over the full vacuum range.
Absolute pressure is measured relative to a full vacuum, whereas gauge pressure aka barometric pressure is measured against atmospheric pressure. A full vacuum has an absolute pressure reading of 0 psia or Absolute pressure remains precise and accurate regardless of changes in ambient or process temperature. When measuring gauge pressure, current atmospheric pressure is the baseline and is a reading of 0 psig.
The principle of operation is one in which a low-pressure fluid is compressed to a higher pressure that is then read by manometer techniques. Its design consists of a system of glass tubes and a glass bulb that are interconnected, somewhat similar to a mercury-column manometer.
A volume of gas from a vacuum chamber is pumped into the gauge and the gauge is rotated, trapping the gas inside. When rotated, the mercury stored in a reservoir pours toward a tube with measurement marks, where the mercury is displaced to a mark depending on the pressure of the gas trapped within the system.
The difference in the mercury levels between the compressed volume and that at system pressure is a function of the pressure within the system. This method is fairly accurate for non-condensable gases, such as oxygen and nitrogen. However, condensable gases, such as water vapor, ammonia, carbon dioxide, and pump-oil vapors will condense when compressed by the McLeod gauge. The result is an erroneous reading, showing a pressure much lower than actually present. A cold trap may be used in conjunction with a McLeod gauge to condense these vapors before they enter the gauge.
Transport type gauges are designed to measure vacuum based on two different phenomena. In the case of a thermocouple or Pirani gauge, the thermal conductivity of the gas is measured and converted into a vacuum reading.
Whereas in a spinning rotor gauge, the gaseous drag on the rotating part is measured and turned into a pressure reading. Thermocouple and Pirani gauges are very widely used and belong to a family of gauges that rely on the principle of thermal conductivity for their operation. By measuring thermal conductivity, one has an indication of pressure.
These gauges rely on an electrical current to heat a wire usually tungsten. The heated wire then cools as the gas molecules collide with the wire. Higher pressure in the vacuum chamber results in a greater the number of molecules colliding with the wire resulting in a large amount of heat loss and a cooler wire temperature.
Lower pressures mean fewer molecules present in the atmosphere, yielding fewer collisions with the heated wire. Because the sensor is an electrically heated wire, these gauges are often called thermal vacuum sensors or hot-wire gauges.
Since the characteristics of all gases are different, the response of a thermal-conductivity gauge will vary for each gas. As a result, to read accurately, the gauge must be calibrated for the gas being measured.
The sensitivity of the thermocouple is dependent on the surface condition of the wire filament. If contaminants are present i. In thermocouple gauges, a thermocouple is spot welded directly to the resistance wire so that a direct temperature measurement can be taken and converted into a pressure measurement. Similarly, if the pressure decreases the temperature of the wire will subsequently increase and this change in resistance measured and converted into a reading of pressure.
At vacuum pressures below 1. Therefore, a specialty type of gauge is used for measuring very low-pressure ranges, typically from to 1 mbar. In these ionization gauges, energized electrons will ionize residual gases and the resulting positive ions are then collected at an electrode and the current is converted to a pressure reading.
Due to their extreme calibration sensitivity to the gases in which they monitor, ionization gauges are often used under laboratory conditions. In fact, they are so sensitive that the use of a mass spectrometer is often necessary to determine the composition of the gas surrounding the gauge. There are two forms of ionization gauges, hot cathode, and cold cathode. Operating ranges for cold cathode gauges are 10 -2 to 10 -7 mbar 7. While there exist several types of cold-cathode gauges Fig.
Since they have no heated filament, the electron discharge is initiated by stray field emission, that is, the magnetic field generated by the magnetization in a magnet.
Since this process of electron discharge can take minutes to achieve at lower temperatures, cold cathode gauges are often turned on at high pressures 10 -2 Torr or higher. Once activated, the magnetic field within the gauge will constrain the electrons in long path lengths that have a high probability of ionizing the residual gas.
These ions are then collected and measured to calculate the pressure of the gas. To maintain Nadcap accreditation and comply with the requirements outlined in AMS , rough vacuum gauges must be annually once every year calibrated to a NIST traceable standard, whereas high vacuum gauges cold cathodes require quarterly once every 3 months calibration.
Active gauges greatly simplify this process by allowing you to return an individual active gauge for calibration without removing any cabling or other equipment from the vacuum system. 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 and there is a vacuum gauge for every pressure range. Finally, a number of factors must be considered when installing the devices discussed above.
It can be configured to cover a wide range of vacuum pressures from rough to high vacuum. User can select units of Torr, mbar, kilopascal kPa , millitorr, or microns. If 2 valves are chosen, both can be controlled by readings from 1 sensor or each valve can be controlled by different sensors.
Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases solid or liquid at a given temperature in a closed system.
It relates to the tendency of particles to escape from the liquid or a solid. A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. As the temperature of a liquid increases, the kinetic energy of its molecules also increases.
As the kinetic energy of the molecules increases, the number of molecules transitioning into a vapor also increases, thereby increasing the vapor pressure. As a general trend, vapor pressures of liquids at ambient temperatures increase with decreasing boiling points. Accurate vapor pressure control is important in processing since a the addition of vacuum pressure control allows you to process at a lower temperature, which helps keep expensive process material from oxidizing, degrading, or being lost.
The DigiVac Vapor Pressure Controller allows you to determine set-points and maintain vacuum at that level. The pressure controller is particularly useful when you are looking to maintain pressure at a certain level as needed for botanical processing and rotary evaporation. No products in the cart. Contact Us: Understanding Vacuum Terminology. We write this to help our customers determine what type of vacuum gauge and what kind of vacuum sensor they need.
Below is a quick overview of the vacuum terminology often used when dealing with vacuum instrumentation. Terminology used by vacuum vendors The most important thing that any vendor can offer you is the ability to solve your vacuum measurement or process control challenge in a quick, efficient and cost effective manner.
What is a Vacuum Gauge? A Vacuum gauge is a device that measures the pressure, or that pressure below atmosphere. What is the Range of Accuracy for your application? What is Piezo-resistive technology? Unlike thermal sensors, Piezo sensors are NOT gas dependent.
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