Two beam balances

Figure 3.1. Experimental setup for gravimetric measurements of pure gas adsorption equilibria using a two beam balance. For commercial instruments cp. Tab. 3.2.

To elucidate the physics of gas adsorption measurements using a two beam balance we consider the scheme sketched in Fig. 3.3 below. It shows the two beams of the balance with an electromagnetic coil in its center and equipped with two baskets containing a sample of the sorbent material (m ) and a ballast or tare of approximately the same mass (m m ) respectively. 

Figure 3.3. Schematics of a two beam balance with electromagnetic compensation (M )...

As has been outlined in Chap. 2 uncertainties or errors of measured data constitute an important part of any kind of experimental work and hence always should be considered [2.18], However, for sake of brevity we here provide the reader only with the formulae allowing one to calculate uncertainties or mean square deviations (MSD) (cj ge) of th Gibbs excess mass (m g) of an adsorbate which has been measured gravimetrically by using a two beam balance. This mass can be calculated from eq. (3.14) combined with eqs. (3.10) and (3.13). By using Gauss law ofpropagation of uncertainties we have... 

The procedure for pure gas adsorption measurement using the installation of Figs. 3.4, 3.5 is basically the same as with two beam balances which already has been described in Sect. 2.1.1. Nevertheless some additional remarks reflecting more than 10 years of practical experience with magnetic suspension balances (MSBs) seems to be appropriate ... 

Calibration measurement of the empty balance parameters as for example the so-called asymmetry of beams in a two beam balance or the mass mo of the permanent magnet and auxiliary equipment inside a magnetic suspension balance i2o = mo-... 

Figure 22. Returned flux at 330 nm as a function of the power balance between the two beams at 589 nm and 569 nm. Circles spectral width at L>2 = 1 GHz. Squares spectral width at L>2 = 3 GHz. Filled symbols pulse repetition rate = 4.3 kHz open symbols rep rate= 12.9 kHz.

As mentioned above, mainly two types of balances are used beam balances and spring or torsion balances. Spring or torsion balances show a special relation between... 

This specific type of the double-beam optical-null recording spectrophotometer is termed so because it critically balances out by the help of optical means the differential between the two beams. 

To be strictly correct we should use the word mass instead of weight because gravitational acceleration is not constant at all places on earth. But electronic balances record weights, unlike two-pan or triple-beam balances, which record masses. 

A triple-beam balance with a sensitivity less than that of a typical top-loading auxiliary balance is also useful. This is a single-pan balance with three decades of masses that slide along individual calibrated scales. The precision of a triple-beam balance may be one or two orders of magnitude less than that of a top-loading instrument but is adequate for many weighing operations. This type of balance offers the advantages of simplicity, durability, and low cost. 

Two instruments used to determine mass are the double-pan balance and a triple-beam balance. How each functions is illustrated in Figure 2. You can read how an electronic balance functions in How it Works in Chapter 12. 

A pointer is fixed to the middle of the beam, and when the beam is swinging, the end of this pointed moves over a white graduated scale. When the two pans balance, the pointer will move over, the same number of divisions on each side of the zero position. 

The final case is that of critical damping, in which the quantity inside the square root in Eq. (8.41) exactly vanishes. This case is not likely to happen by chance, but it is possible to construct an oscillating object such as a galvanometer mirror or a two-pan balance beam that is critically damped by a magnetic field. The condition for critical damping is... 

The specific gravity balance may be used to monitor change within a settling suspension (7, 8). Such a balance comprises two bobs, one in clear fluid and the other in the suspension being studied. The bobs are connected to the two arms of a beam balance. The depth of immersion of the bobs is adjustable. The change in buoyancy is counterbalanced by means of solenoids that are connected to a pen recorder. From the settling behavior as monitored by the trace of pen recorder, the particle size distribution can be calculated. 

Owing to light attenuation in such systems, the light source of the spectrophotometer may have to be modified to provide higher intensities. With double-beam spectrophotometers equipped with photomultipliers, normally the detection system should be sensitive enough for such applications, however, the reference beam will have to be attenuated by a suitable means to balance the two beams. 

Weigh 0.25 g of powdered zinc, Zn, onto a piece of filter paper on a beam balance. Insert a rolled 4 x 6 piece of paper almost to the CuBr sample in the test tube used in part A, and add the powdered Zn to the test tube. Tap the test tube gently on the desktop so that all the Zn falls to the bottom of the test tube then remove the rolled paper. Swirl the test tube to mix the two powders, but not so vigorously as to get powder up the sides of the test tube. Clamp the test tube to a ring stand at the same angle and the same height as shown in FIGURE 40.1. 

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