Parallax error

When actually taking a reading on the scale S during an experiment, a small lens is used, and the usual precautions must be taken to avoid an error by parallax. ... 

This is sometimes made of mirror glass in order to eliminate the error due to parallax, t Manufactured by Edwards High Vacuum Ltd. This is essentially a form of McLeod gauge. 

Second, when filling a pipet or volumetric flask, set the liquid s level exactly at the calibration mark. The liquid s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware s calibration mark (Figure 2.6). The meniscus should be adjusted with the calibration mark at eye level to avoid parallax errors. If your eye level is above the calibration mark the pipet or volumetric flask will be overfilled. The pipet or volumetric flask will be underfilled if your eye level is below the calibration mark. 

Parallax errors are occasioned by failure to read the thermometer from a position exactly normal to its tube axis. Parallax can be reduced by taking the mean of several observations by skilled observers but can be eliminated only by use of a filar telescope mounted so as to move along an axis parallel to the thermometer stem. 

A graticule is normally provided on the screen to assist in the measurement. Parallax errors can cause problems unless this graticule is actually engraved on the screen itself, and the final accuracy will depend on the degree of beam focusing which can be achieved. Accuracy of better than 1.5 per cent should be possible in most cases. 

The use of a light beam as an indicator avoids the errors caused by friction with a normal pointer, while a hairline incorporated in the projection fens eliminates errors due to parallax, and with the scale at a distance of 1 m from the mirror, a sensitivity of up to 1 500 mm/ tA can be achieved. 

As with other graduated glassware, burettes are produced to both Class A and Class B specifications in accordance with the appropriate standard [BS 846 (1985) ISO 385 (1984)], and Class A burettes may be purchased with BST Certificates. All Class A and some Class B burettes have graduation marks which completely encircle the burette this is a very important feature for the avoidance of parallax errors in reading the burette. Typical values for the tolerances permitted for Class A burettes are ... 

Palladium, D. of as dimethylglyoximate. (g) 463 as nioximate, (g) 474 by EDTA, (ti) 329 Paper chromatography 229 see Thin layer chromatography Parallax errors due to, 85 Parallel determinations 132 Partial ionic equations 850, (T) 851 Partition chromatography 13. 217 Partition coefficient 162 Patton and Reeder s indicator 317, 328 Peptisation 419. 421... 

Paraformaldehyde, 2 623 22 123 Paraformaldehyde solids, 22 119 para-hydrogen, 23 759, 760—761, 764 vapor pressure of, 23 764 Parainfluenza vaccine, 25 498 Paraldehyde, 2 103 Parallax errors, in liquid-in-glass thermometers, 24 465 Parallel-counterflow exchanger, heat-exchanger effectiveness for, 23 255 Parallel flow heat exchangers heat-exchanger effectiveness for,... 

Systematic errors usually arise from specific shortcomings in the measuring instrument, the observer, or the way in which the measurement is taken. Sources of systematic error include a badly calibrated measuring device, a faulty instrument movement, an incorrect action by the experimenter (e.g., misreading a volume measurement), or the parallax effect when incorrectly viewing a scale. Repeating the measurement does not necessarily help, because the error may be repeated, and the analyst may... 

Parallax error is one of the main sources of error in liquid measurements. Figure 4.2 illustrates that the liquid surface in a container is not even, and a meniscus is formed. For most liquids the surface is concave. The uneven surface is due to surface tension, and if not read properly, may result in an error commonly known as parallax error. When a container with graduations for volumes has a liquid in it, the reading must always be made at the bottom of the meniscus. The parallax error is shown in Figure 4.2. 

British Standards Institution (B.S. 846 1962) has laid down specifications for burettes and these are produced to either Class A or Class B specifications. All Class A and a few of Class B burettes have graduations that extend right round the barrel (or stem) of the burette to minimise errors due to parallax while taking the exact burette reading. It may be noted that Class B burettes are normally graduated on one side only. Permitted tolerances on capacity for burettes used in common practice are stated in Table 2.2. 

When reading the liquid level in a buret, your eye should be at the same height as the top of the liquid. If your eye is too high, the liquid seems to be higher than it really is. If your eye is too low, the liquid appears too low. The error that occurs when your eye is not at the same height as the liquid is called parallax. 

A frequently used simple mercury manometer which is employed for the measurement of pressure in the range 0.5-17 cm (6.6 to 226 mbar) is the U-tube design illustrated in Fig. 2.124 (Anschutz manometer, Gallenkamp). It consists of a U-tube charged with mercury and mounted in a wooden stand. The scale B, graduated in millimetres and sometimes made of mirror glass in order to eliminate errors due to parallax, is sited between the two arms and is movable. This... 

H.F. Carroll, Avoid Parallax Error When Reading a Mercury Manometer, Journal... 

A common source of random error is carelessness, e.g. reading a scale in the wrong direction or parallax errors. This can be reduced greatly by careful recording and may be detected by repeating the measurement. Other errors arise from faulty or inaccurate equipment, but even a perfectly functioning machine has distinct limits to the accuracy and precision of its measurements. These limits are often quoted in manufacturers specifications and are applicable when an instrument is new however, you should allow for some deterioration with age. 

Digital displays are increasingly used in preference to needle-type meters, as they are not prone to parallax errors and misreading of the absorbance scale. Some digital instruments can be calibrated to give a direct readout of the concentration of the test substance. 

In reading volumes, the eye must be at the level of the liquid surface to avoid an error due to parallax, a condition that causes the volume to appear smaller than its... 

Parallax Apparent change in position of an object as a result of the movement of the observer results in systematic errors in reading burets, pipets, and meters with pointers. 

Another limitation of MWPCs is that photons entering a flat chamber at an angle 26 to the plane of the detector suffer from a parallax error (figure 5.22(b)). If the sensitive depth of the chamber is g, then the parallax effect is of size g tan 26. For a 10 mm gap and 20=20°, then g tan 26 is 3.6 mm. There are three solutions to the parallax problem. Firstly, two or more flat chambers can be used whereby each can tilt towards the sample, on either side of the direct beam. If the tilt angle is a, then the parallax becomes gtan(20—a). This is the method employed by Xuong et al (1978) and is a commercially available system. There are plans to install a second chamber in such a fashion at SSRL, Stanford. 

Note. Should the volume of liquid go over the cahbration mark, it is still possible to save the solution as follows. Paste against the neck of the flask a thin strip of paper and mark on it with a sharp pencil the position of the meniscus, avoiding parallax error. After removing the thoroughly mixed solution from the flask, fill the flask with water to the calibration mark. Then by means of a buret or small volume graduated pipet, add water to the flask until the meniscus is raised to the mark on the strip of paper. Note and record the volume so added and use it to mathematically correct the concentration calculation. 

Care in reading the volume will avoid parallax error, that is, error due to incorrect alignment of the observer s eye, the meniscus, and the scale. This applies in the reading of any scale, such as the pointer scale of an analytical balance. Correct position is with your eye at the same level as the menicus. If the-eye level is above the meniscus, the volume read will be smaller than that taken the opposite will be true if the eye level is too low. 

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