Balances null-type

Before an object is weighed, the balance should first be nulled, or zeroed. This zeroing procedure is done with nothing on the balance pan and by placing all of the weights (on each beam) to their zero points. If the balance does not indicate centered, rotate the balancing screw (on the pan side of the balance) as necessary to zero, or balance the balance. Balances that require you to complete (finish) the balance process with the beam in the same balanced (null) position at which you started are called null-type balances. 

One reason the deflection-type torsion balance is preferred to the null-type is the complication introduced in activating a torsion wheel or... 

Recording balances can be divided basically into two general classifications, based on their mode of operation (I) deflection-type instruments and (2) null-type instruments (2). 

The servomotor itself receives its signal from a conventional servo-amplifier, which in turn receives a chopped error signal from a null type pH electrometer. The electrodes which supply the signal to the electrometer are located in the reaction cell. Since the electrometer is of the null-balance type, a preset end point is required and this presupposes a knowledge of the titration curve. This electrometer system has been described previously [3]. 

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. 

Conductivity Measurements. Cell resistance measurements were made with a General Radio type 1650-A impedance bridge. It is equipped with an internal, 1000-cycle signal source and tuned null detector. For more sensitive balance at high resistances, a Hewlett Packard 400L vacuum tube voltmeter is used as an external null detector. 

In the different types of apparatus described in the previous section, mass changes may be measured through the amount of countermass variation required to restore the balance to the null position or through calibration of the distance of displacement from the zero position. For small movements, the displacement distance is usually directly proportional to the mass change (and always requires confirmation). Alternatively, both methods can be used in combination, the larger changes compensated by added calibrated masses and the smaller changes determined by a displacement measurement. 

Ewing (57) has reviewed electronic laboratory balances. The various types of null detectors, such as optical, inductive, and capacitance types, as well as the electronic readouts are discussed. 

Null and other types of balances are available that have the capability of measuring the mass of a sample between 100 and 0.02 g to a precision of 0.01-1%. As will be shown later, the smaller the sample, the better the results often will be. 

The inhinsic disadvantage of this method is its two-terminal nature the facts that a dc potential cannot be applied to the electrode of interest with respect to a suitable reference electrode and that the potential e, across the specimen varies during the balance procedure. Since the in-phase and quadrature null signals usually are derived from a PSD, instruments of this type are limited at low frequencies to approximately 1 Hz due to the instability of analog filters with longer time constants. 

A thermobalance is built around a highly sensitive balance module, a furnace and a controlled atmosphere cabinet. Different types of balance detectors are available but the most commonly used is based on the principle of the null position balance. An example is given in Fig. 2.29. 

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. 

The top-loading scalepan TMAs require no calibration. Instruments fitted with the voice coil type motors will require calibration. Most instruments achieve this by applying one or more known masses (which can be accurately measured on a balance cahbrated by traceable standards) and nulling the displacement that occurs. This way the force required to rebalance the applied mass is obtained independently from the displacement measurement. The instrument measuring system will adjust the voltage or current applied to the coil, and when the displacement has been successfully nulled this value is equivalent to the force due to the applied mass. The value of force to current should be linear over the range of operation of the instrument. 

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