A vacuum Gauge is a pressure measuring instrument that measures pressure in a vacuum (i.e., in a vessel operating at sub-atmospheric pressure). Depending on the type of vacuum system (Fig. 1) 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:
- The vacuum range to be detected;
- The gas composition (inert, reactive, corrosive);
- Required accuracy and repeatability;
- Environmental conditions.
Vacuum gauges are divided into three basic categories based on their working pressure (Fig. 2). These include:
- Absolute pressure gauges;
- Medium vacuum gauges – useful down to around 0.001 mbar (1 micron);
- High vacuum gauges for use below 0.001 mbar (1 micron).
Sage Advice from Howard Tring
A few years ago, in an article titled Vacuum Gauges Used on Vacuum Furnaces author Howard Tring offered these insights into the world of vacuum gauges when he told us:
“Vacuum gauges measure the pressure readings in the range from atmospheric pressure down to some lower pressure approaching absolute zero, which is not attainable. Some gauges read the complete range with low resolution and others can only read a portion of the range but with better resolution, usually used for the lower pressures.
There are three groups of vacuum gauges based on the method of operation, mechanical, thermal conductivity, and ionization. For this discussion, we will only talk about the thermal conductivity and ionization gauges because purely mechanical vacuum gauges are generally not used on vacuum furnaces.
A typical electrical vacuum gauge consists of a sensing head (gauge head) and readout. It has a power cable to supply power (115V) to the readout and an electrical cable to supply power to the gauge head and also to carry the signal from the gauge head to the readout. Gauge heads are usually mounted vertically, i.e. with the connection downwards. This is to allow the gauge heads to read correctly and also to prevent debris from entering the head.
For systems such as vacuum furnaces the high vacuum gauge most used is the Penning cold cathode gauge that indicates pressure from the 10-2 Torr decade down to around 10-9 Torr. It has a robust stainless steel gauge tube and can be dismantled and physically cleaned if it becomes contaminated is actually a Wide Range Active gauge head and incorporates a small Pirani head inside the Penning gauge tube. The Penning type gauge is bulkier than other gauge heads due to the magnets mounted around the gauge tube.
In recent years the measuring range of capacitance manometer gauges, has been improved that now models can read pressures down to about 10-5 Torr. This allows them to be useful for some high vacuum processes because the heads are also corrosion resistant and are the most accurate vacuum gauge available.
For measuring foreline pressure and chamber roughing pressure there are several options. The thermocouple gauge (TC), the Pirani gauge and the Convection gauge. The convection gauge is a hybrid design of the Pirani gauge to enable readings at higher pressures. They all read from about 20 Torr down to about 1 x 10-3 Torr (1 micron) and have a good resolution from 1 Torr to 1 x 10-3 Torr.
Often, US made TC and Convection gauge heads are supplied with a 1/4″ NPT threaded connection as standard. Threaded connections are difficult to seal for use in vacuum systems as there are often small leaks through the root and crest of the threadform. (How many of these gauge heads have you seen with the black sealing compound stuck around the connection?) The modern “NW” rubber O-ring connection is a much better option, much less prone to leak and easier to put on and take off.”
You might be interested to know that gauge development continues to advance and here is a brief look at some of the more interesting recent advances.
Types of Gauges
Passive Vacuum Gauges1
Traditional vacuum gauges (aka vacuum sensors, transducers, and/or vacuum tubes) are considered passive gauges and including such sensors as thermocouple gauges, convection gauges, diaphragm gauges (for low vacuum measurement) and cold cathode gauges or hot ion gauges (for high vacuum measurement). All passive vacuum gauges require a vacuum controller or an active vacuum gauge (see below) in order to provide vacuum.
Active Vacuum Gauges1
Active gauges (aka digital vacuum gauges, smart gauges) combine the passive vacuum gauge and vacuum controller electronics in one device. These gauges often feature thermocouple gauges, convection gauges, and cold cathode gauge technology bundled together to provide a full range vacuum measurement from atmosphere to ultra-high vacuum applications. These gauges are especially attractive when frequent calibration and/or sensor replacement is needed as they can be easily disassembled, cleaned, and calibrated or replaced. In particular dirty applications, you may consider the use of an in-line filter before the gauge.
Vacuum controllers (Fig. 3) are designed for use with passive vacuum gauges and each style has certain offers unique features and benefits. For example, rack-mountable controllers, can control multiple vacuum gauges (typically between 2 and 10) and are configurable to meet a number of specific requirements. These devices are also easily integrated into a plant network. Portable vacuum controllers often control one thermocouple gauge to provide high-precision sensing technology in a compact, self-contained package.
Adapting Gauges to Shop Floor Environments
Manufacturing environments can be problematic for sensitive instrumentation and vacuum gauges and sensors are prone to damage, drift and other issues in any shop environment. Remember that some form of contamination (e.g., oil vapors, dirt) is always present. Here are some hints on common types of vacuum gauges to extend their life:
Improving Contamination Resistance
- Use Pirani, thermocouple, convection technologies (thermal conductivity gauges) whenever possible.
- Use of macro versus micro (MEMS) sensors since the latter are more susceptible to contamination, particularly in heat treat/vacuum furnace applications and require more frequent calibration and/or replacement. MEMS sensors often include electronics, adding to replacement cost.
Use Digital Calibration Methods
- Historically, calibration methods were analog in nature, performed with potentiometers (aka run-up pots). Today, many gauges and controllers have digital displays, which allows calibration at gauge.
- Digital calibration can be done via digital communications (e.g., EtherNet/IP, RS-232, RS-485, and others). This prevents a mismatch between HMI and the display. It also allows for integration of calibration into the HMI.
Special Notes About Cold Cathode Gauges (Fig. 4)
- Consider using Penning magnetron cold cathode technology where possible as it offers the greatest contamination resistant (over other designs, for example, those using inverted/double inverted magnetron cold cathode technology).
- Select cleanable cold cathodes for extended gauge life (bead blasting or abrasive pad cleaning). Simple gauge disassembly and cleaning for extended life and lower maintenance costs.
Nadcap Accreditation Note
To maintain Nadcap accreditation and comply with the requirements outlined in AMS 2769, 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.
- For low vacuum ranges (higher pressures) between atmospheric and 10 torr, Bourdon tubes, bellows, active strain gauges, and capacitance sensors are all suitable measurement devices.
- For mid-range vacuum requirements, those in the 10+1 torr to 10-3 torr range there are several choices including the capacitance manometer, a good choice for more accurate measurements or the thermocouple or Pirani type gauges.
- For high vacuum, from 10-3 torr to 10-6 torr, either a cold cathode or Bayard-Alpert hot cathode gauges are used. There is some concern over accuracy and/or stability, and both require frequent calibration.
Finally, a number of factors must be considered when installing the devices discussed above. In particular, the selection of the location for installation needs to:
- Avoid vacuum gradients
- Negate pumping effects
- Avoid strong magnetic and electrical fields
- Avoid contamination from product evaporation or oil vapors from diffusion or mechanical pumps.
Transmitters measuring sensor output, control units and cables must be properly matched to the device.
The Bottom Line
The secret to success is having accurate and repeatable gauges so as to ensure that the process being run remains under control and that the calibration procedures are simple and routine.
1. Tring, Howard “Vacuum Gauges Used on Vacuum Furnaces”, Vac-Aero International, March 14, 2013
2. The Fredricks Company (https://www.frederickscompany.com/televac-vacuum-measurement/)