Vacuum chamber made of aluminum flask

What can you find lying around in the bins of an old garage: gramophone records and wax candles, bicycle wheels, laundry soap, a punching bag, an engine from IZH Planet-4. During the next general cleaning, an aluminum flask of 25 liters was discovered, once a popular item in the household. It’s a shame to throw it away, it’s cheap to sell for scrap metal, and it takes up space on the rack. So the idea arose to make a vacuum chamber out of a flask, necessary for degassing silicone, plastic or when pouring epoxy resin.

Of course, it may be more profitable to buy a ready-made vacuum chamber, taking into account the cost of effort and time, but only if you have to work with such equipment constantly. Then the camera itself can be selected competently, and you won’t mind forking out the cash. I had the opportunity to pour plastic a couple of times, exclusively on an amateur basis. Therefore, the choice was made in favor of a simpler and more artisanal, but creative solution. Thanks to an unexpected discovery, it was necessary to expand the vocabulary with such concepts as: pipe thread, bar, mercury column, pipe, bending radius and some others.

Physical concepts

The basic idea is clear: using a pump, a certain volume of air is pumped out from a tightly closed container. Air is a mixture of gases, mainly consisting of nitrogen (78.08%) and oxygen (20.95%), as well as argon, carbon dioxide and some other chemical elements. By removing some molecules from the container, we thereby create a vacuum. Vacuum is the state of a gas or vapor at a pressure below atmospheric. Atmospheric pressure is often denoted by a non-systemic unit of measurement – millimeters of mercury. Figure 1 shows Torricelli’s experiment, where the concept itself, a column of mercury, came from.

Air pressure and the weight of mercury balance each other. Mercury is a liquid metal with a density of 13.6 g/cm³; as shown in the figure on the right, the height of the mercury column is 760 mm. A similar experiment can be carried out with any liquid, for example water. If the atmospheric pressure is 760 mmHg, then in millimeters of water this value will be approximately 760 * 13.6 = 10336, that is, more than 10 meters. This is why mercury was chosen to measure pressure; denser liquids do not exist.

In the International System of Units (SI), the unit of pressure is the Pascal. Atmospheric pressure equal to the pressure of a column of mercury 760 mm high at a temperature of 0 °C is called normal atmospheric pressure (101,325 Pa). Another widely used non-systemic unit of pressure measurement is the bar. One bar is equal to 10⁵ Pa, i.e. approximately one atmosphere. Probably due to their convenience, bars are widely used in industry, for example for calibrating barometers. Depending on the pressure, vacuum is divided into several types.

Having understood the basic physical concepts, you can move on to an activity related to technology – choosing a vacuum pump.

Vacuum pump selection

A low vacuum can be created even with a large medical syringe. This is exactly the method used by the hero of one videocame across while studying this topic. A vacuum pump will help save time and effort; almost any one on the market will be suitable for creating a low vacuum. And yet I wanted to find some optimal option in terms of price and quality. And since the camera is intended to be used in a residential environment, characteristics such as the level of noise generated or the presence of oil mist take on special weight.

The main characteristics of several budget vacuum pumps, differing in operating principles, are shown in the table:

Unfortunately, it was not possible to find the volume levels for a particular vacuum pump, so the table shows approximate characteristics based on information from the network. It seems logical that the deeper the vacuum a pump can create, the noisier it will be. The maximum residual pressure is the maximum achievable vacuum in a closed volume, i.e., having reached such a vacuum, the pump continues to work only to maintain pressure. The refrigerator compressor was not originally designed as a vacuum pump; the value indicated in the table is experimental.

Weighing the advantages and disadvantages, I chose the refrigerator compressor. On the one hand, a low vacuum is sufficient to degas silicone, so there is no need to spend money on a professional solution from Zensen. On the other hand, the use of a Chinese diaphragm mini pump raises reliability concerns. Considering the abundance of old refrigerators on the secondary market, you can find a D91C18RAW5 compressor for a modest 2 thousand rubles. it wasn't difficult.

Vacuum chamber design

Before starting assembly, several sketches of the future vacuum chamber were made in the Solid Works program. The horizontal arrangement of the aluminum flask seemed to me the most logical. But, in this case, vertical wooden supports will be needed to hold it. And then the idea came to combine a vacuum chamber with a tabletop, because in the process of work you need to place auxiliary accessories somewhere: scales, syringes, jars and bottles, paper cups.

The compressor can be mounted underneath the flask, and since it adds additional weight to the entire structure, we’ll add wheels around the perimeter. The vacuum hose will be connected through a fitting to the bottom of the flask and additionally secured with clamps. A pressure gauge is needed to measure the current vacuum, and a ball valve is needed to relieve it. Additionally, a one-way valve with mounting on a hose was purchased, because it was not certain whether the system would hold pressure after the compressor was turned off. The functional diagram of the final vacuum system is presented in Figure 2.

After turning on the compressor TO air begins to be pumped out of the aluminum flask through a one-way valve F. A pressure gauge connected through a tee displays the current vacuum level. The one-way valve prevents air from flowing back into the chamber if the compressor is turned off. A ball valve is necessary to regulate the pressure during the evacuation process or to release it after completion of work.

To observe the process, you will need a window made of some transparent material. The choice was made in favor of polycarbonate, since this material, unlike plexiglass, is viscous. For a vacuum chamber, this characteristic of polycarbonate is important, because we not only have to bend it, but also subsequently subject it to a pressure difference. Monolithic polycarbonate can be cold formed, that is, it can be bent in an arc without heating. But any bend has a limit. The minimum bending radius is an indicator that reflects the amount by which it is possible to minimally bend monolithic polycarbonate without using heating for a long period of operation without compromising its characteristics. The simplest formula for determining the minimum bending radius is: Rmin = 150 x h, where h is the value corresponding to the sheet thickness in millimeters. The radius of the aluminum flask in my case is 145 mm, the minimum bending radius for the selected 4 mm thick polycarbonate sheet is 600 mm, i.e. It will not be possible to bend the material at room temperature.

It is logical to make the tabletop from wood; hardware stores are filled with all kinds of lumber for making furniture. There you will also find the necessary screws, Euroscrews, corners, etc. Initially, it was planned to use chipboard sheets, which are sold in several colors and with finished edges. However, particle boards tend to split and crumble. The strength characteristics of this material for the tabletop of a vacuum chamber may also be insufficient. Therefore, I settled on a regular planed pine board. Although the visual qualities of the latter are inferior to chipboards, in terms of processing the material turned out to be extremely convenient.

Additionally, for the vacuum chamber you will need a 1/4″, 1/4″ tee, a thread tap, a silicone gasket, nuts, bolts and some other parts. A G1/4 tap is required for cutting pipe threads in aluminum. Such threads, unlike metric ones, are measured in inches and are widely used for connecting pipeline parts and all related equipment. Using this thread, the fitting and tee can be screwed to the flask, additionally ensuring the tightness of the connection using FUM tape.

Vacuum chamber assembly

I can’t help but remember the fairy tale about the porridge made from an ax, in which the story began with an inconspicuous carpentry tool and ended with a rich stew. In my case, the aluminum flask served only as a trigger to start development. Next was a long process of cleaning the flask from oxides, designing the countertop, purchasing materials and equipment, and finally, assembly. Development costs were painstakingly recorded in a notebook, and it's always interesting to see if there were any savings by going the DIY route. Taking into account the auxiliary tools and some unused materials, the result was a modest 7,700 rubles. But, as you understand, a lot of time was spent.

The main problems arose in ensuring the tightness of the structure. The old rubber gasket became hard and was replaced with silicone. However, the new gasket was not tight enough, allowing air to pass through at the bending points. A subsequent experiment with a homemade gasket made from silicone sealant was successful. The same sealant was also used to create a thin sealing layer between the polycarbonate sheet and the flask.

Bending 4 mm thick polycarbonate, even with heating, is a very difficult technological task. By heating the sheet with an industrial hair dryer, we managed to bend it to the desired diameter. The operation was carried out “live”, the sheet was attached to the flask, locally heated, bent, attached, etc. Despite all efforts, microcracks formed on the surface of the sheet, the refraction of light by which can be seen on a sunny day. Probably, given the depth of the vacuum created, polycarbonate of thinner thickness should have been chosen, which would have significantly simplified installation and avoided the described difficulties.

The wooden parts are connected with metal corners and euroscrews, the structure turned out to be quite strong, withstanding lifting from the tabletop. The wood acquired a darker shade after additional varnishing. Inside there is a small shelf, also made of wood. And although the vacuum chamber stands out with its intricate appearance among other equipment, it fulfills its main task.

PS I am interested in the opinion of knowledgeable people on silicone vacuum technology: is it worth bringing it to a boil and how long to keep it in the chamber? Boiling of silicone in a vacuum occurs at pressures of 75…80 kPa below atmospheric; the installed compressor from the refrigerator is capable of pumping up to 85 kPa below atmospheric.