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Understanding Vacuum Measurement Units

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Vacuum gauges all measure the pressure readings in the range from atmospheric pressure down to some lower pressure approaching absolute zero pressure, which is not attainable. Some gauges read the complete range and others can only read a portion of the range, usually used for very low pressures.

If you have a typical vacuum furnace it is normal to have at least three electronic vacuum gauge heads mounted on the system to monitor the level of vacuum at selected positions. These gauge heads send signals back to the controls system and the vacuum readings are used to ensure that the vacuum pumps are working correctly and that the process chamber is at the correct low pressure (vacuum) for the specific process. To many casual observers the readings and names of the measuring units being used are like a foreign language, and they may well be because many names were derived in Europe. Let’s take a look at the different vacuum measurement units in use around the world and where the names came from.

Measuring the Grain Size of Specimens with Deformed Grains

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If a specimen has been cold worked, or it did not recrystallize after hot working, the grains will not be equiaxed and extra care must be taken when assessing the specimen’s grain size. Always test a longitudinally oriented plane first to determine if the grains are, or are not, equiaxed.

A low-carbon sheet steel was tested in the as-received condition (reportedly annealed), and after cold reductions in thickness of 12, 30 and 70%. Above 70% reduction, it can be quite difficult to reveal the ferrite grain boundaries well enough to get a precise measurement. Measurements were made on the three principal planes using the Jeffries planimetric method, the Abrams three-circle intercept method and the intercept method using directed parallel test lines.

Grain Size Measurement: The Saltykov Rectangle

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The two previous articles covered methods for measuring grain size that have been incorporated into ASTM E112 for many years. The Jeffries planimetric method was introduced into standard E2 in 1917 – Committee E-4’s first standard. Zay Jeffries was a founding member of the committee and had published several articles about the method, which he learned from his PhD advisor, Albert Sauveur, the dean of American metallographers.

This method is precise, but a bit slow for production work because the grains must be marked off as they are counted manually. The method, however, can be modified for image analysis work. The second method was the Heyn intercept method, which was developed in Germany in 1903 and was mentioned briefly in ASTM E2, but not described in detail, when published in 1917. The intercept method was later modified by John Hilliard and then by Halle Abrams. The Abrams three-circle intercept method is used in production work as the intercepts (or intersections) do not need to be marked off on a template when counted. But, the writer recently has introduced the Saltykov rectangle to E112 as it can yield accurate grain size measurements down to fewer counts per field than the other two methods. As with the Jeffries method, the Saltykov method does require marking of the grains for accurate counting, although it, too, can be used by image analysis.

Backstreaming

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In simplest terms, backstreaming is the movement of pumping fluid back into the vacuum furnace chamber, that is, oil vapor molecules attempt to reverse course and move up and back toward the vacuum vessel, opposite to the direction of the desired gas flow. Backstreaming is not limited to the pumps themselves, but encompass the entire pumping system (e.g. plumbing, valves, baffles, and traps). The oil type and characteristics play a role as well. In all cases, the result of backstreaming, namely the contamination of the work chamber or workload, is totally unacceptable and often catastrophic.

Backstreaming is often due to; incorrect start-up or shutdown procedures – the far most common operator mistake as far as the writer is concerned, exceeding maximum pump throughput capacity for long periods of time and exceeding the critical discharge pressure in the foreline. Users of vacuum furnaces should be sure that the vacuum system is equipped with all the appropriate interlocks to prohibit vacuum valve cycling above specified pressures that can cause these effects to occur, which will help protect your system, especially whenever it is left unattended.

Saving Money by Maximizing Furnace Uptime Productivity

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“It was only a tiny drop of water, now and then,” lamented the homeowner. “How was I to know that all those little drops would add up to a huge water bill?” The same can be said of a heat treat furnace that is always down for this reason or that. Avoiding the hidden costs associated with equipment downtime is the key to saving money. Maximizing furnace productivity requires a proactive approach, which must continue throughout a unit’s operational lifetime. This requires careful planning and anticipation of problems. The process should begin even before the purchase of a piece of equipment by matching equipment and supplier capabilities with production and process needs. Buying good, well-built, high-quality equipment and operating and maintaining it properly will avoid most hidden costs.

For example, suppose a work center is scheduled to run for a 435-minute shift. However, the work center experiences 30 minutes of unscheduled downtime. The available time equals 435 minutes (scheduled time) minus 30 minutes (downtime), or 405 minutes. The availability is 405 minutes divided by 435 minutes or 93%. Not bad, or so you think. Now, let’s look at performance. Performance represents the speed at which the work center runs as a percentage of its designed speed. In other words, parts produced times ideal cycle time divided by available time. Continuing our example, if the available time is 405 minutes and the standard rate for the part being produced is 40 units per hour (or 1.5 minutes per unit), then the work center produces 242 total units during the shift. If the time to produce the parts (242 units times 1.5 minutes per unit) is 363 minutes, then the performance is 363 minutes divided by 405 minutes or 90%. Again, not bad, or so you think.

Biography – Dan Herring

Daniel H. Herring, “The Heat Treat Doctor”® is an active member of the engineering, materials and metallurgical community. His undergraduate work was done at the University of Illinois (1971) and graduate work at the Illinois Institute of Technology (1974). Credentials include a Research Associate Professorship at the Illinois Institute of Technology/Thermal Processing Technology Center and […]

Biography – Howard Tring

Howard Tring is the owner of Vacuum and Low Pressure Consulting, a company that supplies vacuum pump accessories such as reconditioned inlet traps and exhaust filters and new replacement elements for exhaust filters. Vacuum and Low Pressure Consulting also offers on-site vacuum technology and oil sealed vacuum pump repair training and consulting services, customized to […]