We will conduct our experiments in the range of 10 Torr to 10 mTorr, which is easy to achieve using a low-cost, mechanical vacuum pump. Very good results have been obtained by amateur experimenters and educators using vacuum pumps made for servicing air conditioning and refrigeration units. These vacuum pumps are not part of the refrigeration system itself, but rather are used by service technicians to reduce the internal system pressure of a refrigeration/air conditioning system so moisture and other contaminants can be removed.
A robust, relatively low-cost unit that is popular with experimenters is the model 15600 refrigeration service pump made by Robinair. It is rated to reach a vacuum of 20 mTorr (refrigeration vacuum pumps are commonly specified in terms of their base pressure in “microns,” where 1 micron = 0.001 Torr = 1 mTorr). In practice, we reach pressures down to 1 mTorr by using Kurt J. Lesker TKO 19 Ultra vacuum oil instead of the Robinair oil recommended for this pump.
The vacuum intake port of the Robinair pump is terminated with a 45° flared fitting (a standard of the Society of Automotive Engineers, or SAE). SAE flared fittings seal by the mating of two beveled metal surfaces. The function of the threads is simply to draw these two surfaces together. These couplings are usually used for low-pressure applications, such as refrigerant and fuel lines in conjunction with copper tubing, which flares easily to 45°. This is not the most convenient connection for our purposes, so visit the hardware store and put together an adapter that will take you from the 1/2-in. SAE flare to a hose barb that you can connect to 1/2-in. ID steel spring—reinforced PVC hose, or better yet, to a vacuum-service rubber hose from the Kurt J. Lesker company. Remember to buy any copper flare gaskets and O-rings that you may need to make vacuum-tight connections.
The other side of the hose will connect to your vacuum chamber. The best way is through the type of vacuum flange that conforms to one of the industry standards to connect vacuum chambers, tubing, and vacuum pumps to each other. We prefer to use a standard quick-release flange known as a quick flange (QF) or Klein flange (KF). KF/QF flanges* are made with a chamfered back surface that attaches with a circular clamp and an elastomeric O-ring mounted in a metal centering ring. These come in various sizes that are indicated by their inner diameter in millimeters. We use mostly KF16 (16-mm ID) flanges, some KF25 flanges, and occasionally KF40 flanges.
As such, you will need to terminate the hose with a KF-to-hose adapter to fit your hose. For example, you could use a KF16 to 1/2-in. hose adapter made by the Kurt J. Lesker Company (catalog number QF16-050-H). Your vacuum components would then attach to the KF16 flange using another KF16 flange, a centering ring (e.g., Kurt J. Lesker catalog number QF16-075-ARB) and a clamp (e.g., Kurt J. Lesker catalog number QF16-075-C). Use some vacuum grease (e.g., Fomblin VAC3 grease, or Dow Corning high-vacuum grease) on all joints and rubber O-rings.
As an alternative, Steve Hansen, editor of the amateur vacuum experimenter’s magazine The Bell Jar, sells a ready-made inlet manifold for the Robinair pump. A vacuum gauge and vacuum chamber can be attached directly to this manifold. The bottom connector is a 1/2-in. SAE flare adapter that mates directly to the pump’s vacuum port. The manifold has one side port fitted with a needle valve that can be used for pressure control and inert gas inlet. A second side port connects to a standard thermocouple (T/C) gauge tube (more on those in the next section). As shown in Figure 35, the main port of the manifold is a KF40 flange that attaches to the vacuum chamber used for an experiment. The pump does vibrate quite a bit, so you will need to rest the pump on some vibration-absorbing material if your vacuum chamber is connected to the pump through a rigid coupling. We use a mat made of Sorbothane®, but any other shock-absorbing material will do. For example, you could use the gel pads from insoles sold at pharmacies to reduce impact in walking shoes.
Figure 35 Our simple vacuum setup, which reaches 10 mTorr, comprises a refrigeration service vacuum pump, a vacuum manifold handcrafted by Steve Hansen, and a T/C vacuum gauge. Our experimental chambers connect to the manifold through a KF-40 flange. A pinch valve connected to the manifold allows us to introduce small amounts of gas into the system.
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