Helical spring balances

Helical spring balances first used by McBain and Bakr have been extensively used for adsorption measurements. The spring is suspended inside a glass tube by attachment to a hook at the top. A bucket containing the adsorbent is connected to the bottom of the spring. The bottom of the tube, containing the sample, is immersed in the coolant. The upper portion of the tube is connected to a vacuum pump, source of adsorbate, and manometer. 

For maximum accuracy, the manifold and calibrated volumes in a volumetric apparatus should be maintained at constant temperature. Thermostating is not necessary for vacuum micro balances but in helical spring balances the spring should be maintained at constant temperature. Continuous flow apparatus need not be thermostated since the signals are immediately calibrated with known volumes at the same temperature and pressure. However, ambient temperature and pressure must be known to insure accurate calibration. 

In another static method of measuring gas adsorption, the amount of gas adsorbed is measured by the increase in the weight of the adsorbent. For this, a very useful balance was developed by McBain and Bakr (Figure 13 2), the essential part of which is a helical spring... 

Two weighing systems need to be noted, namely, deflection and the null-point balances. There is a variety of deflection balances that can be constructed— beam type, helical spring, cantilevered beam, torsion wire, etc.—but they suffer in that the sample under observation will not remain in a fixed position in the furnace. For this reason most units employ a null-point balance. 

If one or another of these [two dumbbell-shaped bar balances and four helical balance springs, each of which helped to compensate for the motions of the ship] were to have broken, the clock would no longer have fulfilled its specific function aboard ship, but it could still have performed a... 

The weight of gas adsorbed is sometimes measured, and for this purpose various kinds of microbalance have been used. McBain s sorption balance consists of a delicate helical quartz spring,5 suspending the adsorbing solid the upper end of the spring is fixed, and the lower end observed with a cathetometer. Bradley6 has used a balance with a beam, supported in the centre by a fine horizontal quartz fibre and references to other types of balance are given by McBain.7... 

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