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Subject: Vacuum Measurement The atmosphere in which we live is air. This air is affected by the earths gravity which causes it to be at a pressure. Around sea level this pressure is known as 1 atmosphere, often called 1 Bar, i.e., 1,000 millibars (where 1 millibar is 1 thousandth part of a bar). This pressure of our atmosphere decreases as we go further from the surface of the earth and varies with weather conditions. We define a vacuum as air (or gas) at a pressure below that of the atmosphere. Consider the idea of a perfect vacuum in a vessel in which there is no atmosphere, not even 1 atom. (This cannot occur in practice, there is always something present). We now consider what is the actual pressure inside any vessel. As we have said above, if it is the atmosphere at sea level then our pressure is 1,000 millibar. If we removed some of this air, then we may only have 10 millibar or 1 millibar or less. We express our vacuum in terms of how many millibars of pressure are present. By using various types of vacuum pump, we can remove atmosphere from a suitable vessel down to a tiny fraction of 1 millibar even as low as 1 part in 1,000,000,000. Such a figure is written mathematically as 10-9 millibar. In most industrial applications we are using vacuum levels down to 1 millibar. Some examples of useable vacuum levels are as follows. Very approximate levels are indicated. Vacuum cleaner: about 800 millibar Vacuum forming of plastics: 200-500 millibar Water boils: about 12 millibar at room temperature Resins and RTV Degassing: about 5 millibar at room temperature Dry out a refrigeration system: about 1 Millibar or better Freeze drying: 10-1 or better Evaporate aluminium in Decorative metalising processes: 10-4 to 10-5 millibar Vacuum Furnace processing of metals: 10-5 millibar Inside of a TV tube: 10-7 millibar or better Vacuum bag veneering: 100-500 millibar Specialised research in Ultra High Vacuum: 10-9 or better Even our atmospheric pressure varies at different places on the earth=s surface, e.g., top of Snowdon, top of Everest. Other Units of Vacuum Measurement When vacuum was first investigated, a glass tube and a bowl of mercury was used. The vacuum was measured as the height to which the mercury could be lifted up the tube. A Barometer had a closed tube and a complete vacuum. This measured 30 inches at about sea level. This practical level of vacuum would be up to 30" Hg (mercury), in metric units this would be 760mms of Hg. The measure was how high the mercury column could be lifted. A vessel or container under vacuum will have a difference in pressure across its surface. If the container is for example a flexible bag as used in veneering, the user is interested to know what thrust is being applied to the veneers. Thus another useful unit of vacuum measurement can be 1bs per square inch,. Our normal atmosphere is at 14.7 PSI. Perfect vacuum would be 0 PSI. Thus the pressure on the surface of a vessel under vacuum equals the difference in pressure between inside and atmosphere. Suppose we have a vacuum of say 500 millibar, i.e., half an atmosphere then the pressure on every square inch of surface is 7.35lbs. If a vacuum of 250 millibar then the pressure on the surface increases to about 10lbs for every square inch. Simple calculation shows that the thrust, for example an Island Scientific Chamber 12" nominal diameter is about 3/4 of a ton. This is why the lids need to be so thick and strong. Vacuum Measuring Methods 1) Dial Gauge 2) Glycerine Filled Gauge 3) Barometrically Compensated Vacuum Gauges 4) Baratron Gauges 5) Pirani Gauge 6) Penning and Ionisation Gauges 7) Mercury/Tube Gauges A chart showing the comparison of different vacuum measuring units is shown at the end. Maintenance of Vacuum Gauges Gauges should be mounted where they are free from vibration, rapid fluctuations in pressure, at room temperature, dry and visible to the user. Regular calibration checks should be made if the process depends upon accurate pressure measurement in the vacuum system. The simple dial gauge is cheap enough to consider a throw away item if it goes wrong or is suspect. Barometrically compensated gauges. Some models are sealed and cannot be repaired. Other models can be opened up to be cleaned and re-set. Electronic gauges repair is specialised and a task for the manufacturers or a gauge repair/calibration specialist. Vacuum Switches Most types of vacuum gauge mechanism can be obtained to operate an electrical switch at the pre-determined level of vacuum. Some will read as well as switch. The actual switch mechanism will only be light duty and needs to be connected to an electronic relay to operate other equipment. Fail Safe 1) If your vacuum system is ever likely to be pressurised by accident or as a process procedure then the vacuum gauges must be protected. This an be done by an isolation valve plus a pressure relief valve (leak tight under vacuum) in case leakage past the shut off valve occurs. 2) If your process is likely to create blockages in the pipes leading to vacuum gauges and controls then extra precautions need to be taken depending upon the nature of the process and layout of the equipment. 3) If part of your system could possibly crack or break causing a large leak, this will cause air to be drawn through the rotary pump. The change in sound may not be noticeable. The gauges may fail, but a technique used in Cathode Ray tube production was to fit a whistle, yes, the type used on a kettle, to the rotary pump exhaust. You could hear that. Copyright 1993 NB: The information in this pamphlet is offered in good faith and is based on our general experience. The method of use of Island Scientific Ltd equipment and materials are outside our control, the responsibility to ensure that the equipment is properly used and suitable for intended application rests with the user. No responsibility will be accepted by this company nor any person or other company associated with this leaflet for loss or consequential losses as a result of them using this information. For advice, information, general literature, prices or a discussion of your needs, telephone 01983 855822, fax 01983 852146 or A full list of Technical Notes is available free on request. |