Liquids dispensing pumps

Direct metering of liquids/solids, followed by immediate vaporization in a vessel, also can be used. For metering of liquids (either neat, or as solutions of solids), flowmeters and various dispensing pumps are available (Figs. l-2a,b and 1-3, Tables 1-1 and 1-2). The final vaporization takes place in, for instance, a hot box - a vessel containing objects held at high temperature. This process should be differentiated from aerosol CVD (see Sect. 1.3.1.5). For some compositions, direct solid feed systems have been proven as a delivery mode (Fig. 1-4, Table 1-3). 

The first step in using a dispensing pump is to adjust the pump so that it dispenses the desired volume of liquid. Normally, the instructor will make this adjustment. Once the pump is adjusted correctly, it is a simple matter to dispense a liquid. Simply lift the head of the pump as far as it will travel. When you release the head, it will fall, and the liquid will issue from the spout. With viscous liquids, the head of the pump may not fall by itself. In such an instance, gently guide the head downward. After the liquid has been dispensed, you should touch the tip of the dispensing tube to an interior wall of the container in order to remove the last drop of liquid. 

As with automatic pipettes, dispensing pumps are designed to deliver aqueous solutions with an accuracy of within a few percentage points. The amount of liquid actually dispensed will vary, however, depending on... 

Dispensing pumps may be used in place of automatic pipettes when larger amounts (more than 0.1 mL) of liquids are being dispensed in the laboratory. The pumps... 

As you pull up the plunger, look to see if fhe liquid is being drawn up into the pump unit. Some volatile liquids may not be drawn up in the expected maimer, and you will observe an air bubble. Air bubbles are commonly observed when the pump has not been used for a while. The air bubble can be removed from the pump by dispensing and discarding several volumes of liquid to "reprime" the dispensing pump. Also check to see if the spout is filled completely with liquid. An accurate volume will not be dispensed unless the spout is filled with liquid before you lift up the plunger. 

Dispensing pumps and automatic pipets are devices used for accurately and quickly dispensing liquids. Because they are expensive, these devices are normally used to transfer liquids from containers, such as reagent bottles, that are being used by everyone in a laboratory. 

Bottle-top dispensers, often called Repipets , are quite popular. These are devices that fit on the top of reagent bottles threaded to receive screw caps. The dispensers themselves have screw caps that screw onto the bottles. The caps, however, are fitted with a hand pump with a plunger that draws liquid from the bottle in the upstroke and then dispenses a calibrated volume on the downstroke through a glass tip. Such devices are convenient and help prevent contamination of the reagent from the various pipets that the analyst might use for a transfer. 

J. P. Bare, B. Johl, and T. A. Lemke, Comparison of Vacuum-Pressure vs. Pump Dispense Engines for CMP Slurry Distribution, Proceedings of SEMlCONtWesl Contamination in Liquid Chemical Distribution Systems Workshop (July, 1998). 

Currently used dispensing systems include simple pumps, timers, visual volumetric displacement systems and weight batching. Liquid admixture... 

Hydrogen dispensers will likely make use of programmable logic controllers or PC-based control systems. Both liquid hydrogen and compressed hydrogen dispensers will need to be able to monitor pressures and temperatures, open and close valves, turn on pumps, and interface with fire and emergency shut-down systems. 

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