Drawback pumps

These methods are the simplest, cheapest, and practically most accessible for airflow visualization. They are sold in the form of small glass tubes or plastic bottles through which air is pumped manually. Time for use of one unit is typically one hour or up to one day. One drawback of these two principles is the fact that the emitted smoke is strongly irritating if inhaled and also corrosive. Therefore, they must be used with some care, but this will normally not lead to any major re strictions. Another limitation is the low amount of smoke that is emitted. 

As illustrated in Fig. 10.3c, a final solution is to use the eluent pump instead of the recycling pump. This implementation may enable the setup to be simplified, but more valves are required than option a), and a drawback is that one outlet must be recycled to the eluent tank. 

The other drawback is simply one of economics. Because pumps come in a variety of shaft sizes and speeds, the use of this type of seal requires inventorying several sizes of spare springs, which ties up capital. 

Nevertheless, the simple and reliable coil spring seal has proven itself in the pumping industry and is often selected for use despite its drawbacks. In regulated industries, this type of seal design far exceeds the capabilities of a compressed packing ring seal. 

Air conditioners are frequently combined with heaters. A convenient place must be found for the compressor, generally outdoors. Heat pumps have been used a great deal in recent years with satisfactory results. In very dry climates, laboratories often install evaporative coolers because of their reasonable cost of installation and operation. Their one drawback, sometimes serious, is that the inside air tends to become uncomfortably humid. 

On the other hand, we cannot ignore drawbacks in observing fourth-order responses. The desired response is always weak due to the high optical order. The damage threshold of the interface to be analyzed is severe with intense irradiation. The difficulty has been overridden by one-photon resonant enhancement of Raman-pump efficiency. The observable range of materials is somewhat limited as a result. There is still much room for technical improvements and the author is optimistic for the future. 

The drawback of GALDI is the possible contamination of the ion source with graphite powder that can cause short circuits. The authors have found that the application of thin graphite films and low laser powers minimise the amount of sputtering in the ion source. However, modern commercial spectrometers have compact ion sources that can be easily contaminated, and care should be taken in the use of this methodology. The same is true for turbomolecular pumps if they are located directly below the ion source. 

Most mechanical pumps exhibit vibration that may represent a crucial drawback for very low-temperature refrigerators. 

Another drawback of oil-sealed pumps is the back-streaming of oil vapour into the roughing line, which may occur at low pressure. Contamination by back-streaming oil can be drastically reduced by using proper traps like molecular sieve traps with zeolite (see Section 1.6.4). 

A system of cascaded booster pumps has the characteristics of a medium-size rotary vane pump but does not present the drawback of the back-streaming. For this reason, it finds application in cryogenics, for example, for the circulation of the He mixture in dilution refrigerators. 

There are two main drawbacks in scroll pumps the production of Polytetrafluoroethy-lene (PITT) powder dust due to the friction between the fixed and the moving part moreover, since in the present commercial versions the motor is not in vacuum, the tightness is not the best. 

A drawback of oil diffusion pumps is the so-called back-steaming. It is the flow of a small quantity of oil vapour towards the inlet of the pump. A water-cooled baffle like that shown in Fig. 1.13 can be put above the inlet. Baffles are made up of arrays of optically dense fins cooled by a continuous water flow. A baffle always reduces the pumping speed. 

Resistance bridges sometimes contain a multiplexer to carry out measurement of several thermometers with the same bridge. However, the multiplexing procedure presents several drawbacks (e.g., the temperature readings are not taken at the same time). It is safe, when possible, to use one bridge for each thermometer. The reference frequencies of the bridges must differ for at least a few hertz and of course must not be at the frequency of the power line or motors of the turbomolecular pumps. 

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