Zero point drift

Most analytical balances used today are electronic balances. The mechanical single-pan balance is still used, though, and so we will describe its operation. Both types are based on comparison of one weight against another (the electronic one for calibration) and have in common factors such as zero-point drift and air buoyancy. We really deal with masses rather than weights. The weight of an object is the force exerted on it by the gravitational attraction. This force will differ at different locations on Earth. Mass, on the other hand, is the quantity of matter of which the object is composed and is invariant. 

When drift observed in a sensor signal is attributable exclusively to zero-point drift, and there... 

A zero-point drift value should be provided for a sensor in a matrix-free solution and also with respect to real samples involving a matrix. The same applies to the stability of the slope (varying sensor sensitivity). 

For the experimental investigation of compressibility of superheated and saturated vapors, the method of the constant-volume nonballasted piezometer was employed. The fundamental variables (pressure, temperature, and mass of the substance) were measured with instruments of the same class as those used in the work of MEI. The distinguishing feature of the piezometric rig was the use of a membrane zero indicator of pressure of the electrocontact type. The sensitivity of this instrument reached 1 mm water column, but the zero point drifted by 10 mm water column. The authors in Refs. [2.19, 2.21] evaluated the scatter of experimental points on the saturation curve as 0.4% for pressure, and the repeatability of presssure during the forward and reverse tracing along the constant-volume lines was 0.15%. 

Electromagnetic flowmeters offer the advantage of no moving parts in the flow stream, possible two-phase fluid capability, direct mass flow measurement, good time response, and linear calibration. For reproducible results, the flow velocity profile must be symmetric with respect to the axis of the pipe. Problems with this flowmeter include zero point drift with temperature, complex electronics, and difficulty in maintaining a hydrogen leak-proof flow tube. 

Since the sensitivity of electrochemical CO2 sensors changes only slightly in the course of time, it is sufficient to carry out regularly one-point calibrations in order to correct the zero-point drift. Two-point calibratirHis are only necessary in longer time intervals or after the sensor had been out of use for a longer period. If the sensor is continuously in use, as a rule, one-point calibrations at least once per week and two-point calibrations at least (Mice per month are recommended. For two-point calibrations two calibration solutions with different CO2 c(Micentrati(Mis are needed. 

The term lEP has been used even quite recently for a zero point determined by drift method [18]. The principle of the method is as follows. A series of buffer solutions of equal volume and different pH (in this instance chloroacetic acid-sodium chloroacetate for acidic range and NH3-NH4NO3 for basic range) is prepared. The same amount of powder is added to each solution and the pH of the slurry is measured. The instant change in pH (negative or positive) induced by addition of powder is plotted as the function of initial pH. The pH at which this change equals zero is taken as the zero point. This method is in fact a modified potentiometric titration without correction. Consequently such results are referred to as pH in Table 3.1. Moreover, weak acids often adsorb specifically and this affects the obtained zero point, thus pristine value can be only obtained in case of fortuitous coincidence using this method. 

The zero point of a balance is not a constant that can be determined or set and forgotten. It will drift for a number of reasons, including temperature changes, humidity, and static electricity. The zero point should therefore be checked at least once every half-hour during the period of using the balance. 

Some monitors are prone to drift in their zero point and are affected by external environmental conditions. Thus, they require sophisticated maintenance by skilled personnel or even by the manufacturer. Calibrating a monitor to a known concentration of a target gas is the most accurate method. However, in general it is desirable to have a linear repeatable response over the widest possible gas concentration range. Some sensors require a stabilization period before calibration can be performed following maintenance. This results in a longer overall calibration time. The cost of materials used for calibration is another factor to consider. 

Undefined delay times and distortion of the rise time reduce the derivable information of waveform analysis but the end-tidal value and the average value can be used clinically. Figure 23-24 presents an example of a flow mismatch with distortion of the CO waveform. The inspiratory value may be used to detect system failures. During the warm-up time and the subsequent Hrst hour, (he IR sensor typically shows increased drift and reduced accuracy until temperature balance is reached. Zero-point calibration can be effected periodically by automatically aspirating ambient air. Drifts of the measuring system are calibrated manually or automatically over longer periods by use of test gases. 

Zero/minimum drift of set points, built-in auxiliary connections for data transfer... 

As the electric field always points in the direction of the electrode, the densities of the electrons and negative ions are set equal to zero at the electrode. It is assumed that the ion flux at the electrodes has only a drift component, i.e., the density gradient is set equal to zero. The conditions in the sheath, which depend on pressure, voltage drop, and sheath thickness, are generally such that secondary electrons (created at the electrodes as a result of ion impact) will ionize at most a few molecules, so no ionization avalanches will occur. Therefore, secondary electrons can be neglected. 

It is important to check the zero setting (or the setting of the lower range value) for an instrument as a zero error will cause the whole of the instrument span to be displaced. The zero setting may drift or change over a period of time (zero shift). Such drifting is frequently due to variations in ambient conditions—most commonly temperature. In addition to zero shift, point values of the measured variable in different regions of the span may drift by different amounts. 

Fe,v-Fe2+ system in the presence of a trace of dissolved oxygen. The measured zero-current potential is that value where the rate of 02 reduction at the electrode surface is equal to the rate of Fe2+ oxidation rather than the value of since at the latter point simultaneous 02 reduction produces excess cathodic current. In addition, because the net reaction of Em converts Fe2+ to Fe >>+, the measured potential exhibits a slow drift. Such mixed potentials are of little worth in determining equilibrium Eh values. 

Roberts (18J indicates that each experimental point took 3 or 4 days to attain equilibrium in this portion of Figure 2f, whereas for the other experimental data equilibrium was achieved in under six hours. He postulates that during this time there may have been a zero drift in the thermistor for measuring composition. 

Attention should be drawn to the fact that there has been a degree of inconsistency in the treatments of ionic clouds (Chapter 3) and the elementary theory of ionic drift (Section 4.4.2). When the ion atmosphere was described, the central ion was considered—from a time-averaged point of view—at rest. To the extent that one seeks to interpret the equilibrium properties of electrolytic solutions, this picture of a static central ion is quite reasonable. This is because in the absence of a spatially directed field acting on the ions, the only ionic motion to be considered is random walk, the characteristic of which is that the mean distance traveled by an ion (not the mean square distance see Section 4.2.5) is zero. The central ion can therefore be considered to remain where it is, i.e., to be at rest. 

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