Time-pressure pump

Time-pressure pump method. Time-pressure, the simplest and oldest of the methods, is still in use today. According to this method, pulsating high-pressure air actuates a plunger that forces the adhesive through a syringe. 

Time-pressure pump dispensing is a method to deposit adhesive or solder paste by applying a pressure pulse for a specific time duration on a reservoir of material (see Rg. 40.17). A precisely controlled quantity of adhesive or solder paste emerges from the selected orifice size, which is deposited on the circuit board. Often, the material comes prepackaged in a syringe that is inserted into the machine. 

FIGURE 40.17 Time-pressure pump dispensing technique (a) The dispenser is positioned at the circuit board site (b) the dispenser is lowered to near the surface at which time, a pulse of air pushes the piston a distance Ax to dispense a set quantity of adhesive (c) the dispenser is raised from the site, leaving the adhesive deposit. 

Capillary Electrochromatography Another approach to separating neutral species is capillary electrochromatography (CEC). In this technique the capillary tubing is packed with 1.5-3-pm silica particles coated with a bonded, nonpolar stationary phase. Neutral species separate based on their ability to partition between the stationary phase and the buffer solution (which, due to electroosmotic flow, is the mobile phase). Separations are similar to the analogous HPLC separation, but without the need for high-pressure pumps, furthermore, efficiency in CEC is better than in HPLC, with shorter analysis times. 

The hexapole cannot act as a mass filter by applying a DC field and is used only in its all-RF mode, in which it allows all ions in a beam to pass through, whatever their m/z values. In doing so, the ion beam is constrained, so it leaves the hexapole as a narrow beam. This constraint is important because the ion beam from the inlet system tends to spread due to mutual ion repulsion and collision with residual air and solvent molecules. By injecting this divergent beam into a hexapole unit, it can be refocused. At the same time, vacuum pumps reduce the background pressure to about 10 mbar (Figure 22.1). The pressure needed in the TOF analyzer is about 10 ... 

On many occasions, new equipment is installed or process workers have to work in other similar plant units in order to substitute for one of their colleagues. Despite the overall similarity of the new equipment, there might be some differences in their operation which may sometimes become very critical. We caimot always rely on the operator to discover these potentially critical differences in equipment design, especially under time pressure and excessive workload. If multiskill training in a range of plant equipment is not feasible, then training should be provided for the specific new equipment. The incident below was due to lack of training for a caimed pump. 

Unfortunately, neither the computer nor the potentiometric recorder measures the primary variable, volume of mobile phase, but does measure the secondary variable, time. This places stringent demands on the LC pump as the necessary accurate and proportional relationship between time and volume flow depends on a constant flow rate. Thus, peak area measurements should never be made unless a good quality pump is used to control the mobile phase flow rate. Furthermore, the pump must be a constant flow pump and not a constant pressure pump. 

The dependence of the factor from the desired pressure is shown in Fig. 2.75. It should be noted that the pumping speed of single-stage rotary vane and rotary piston pumps decreases below 10 mbar with gas ballast and below 1 mbar without gas ballast. This fundamental behavior is different for pumps of various sizes and types but should not be ignored in the determination of the dependence of the pump-down time on pump size. It must be pointed out that the equations (2.32 to 2.36) as well Fig. 2.75 only apply when the ultimate pressure attained with the pump used is by several... 

It is seen from Table (3) that an open tubular column with an I 0 of about 86 micron and 20 meters long, operating at an inlet pressure of only 1 ps.i. can complete the very difficult separation in about a week. A very long analysis time, perhaps, but not much longer than would be required by an optimized packed column and in this case the separation is earned out with a simple tube to coat and with no high pressure pump required. 

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