Q: What is uncertainty of measurement and does it affect my measurements?
Ans: Uncertainty of Measurement as the word “Uncertainty” means is the doubt in the measurement. How confident are we when we say what we say. (Confidence level considered is normally 95%) In other words it also is the Accuracy of Determination of the measured size. This Uncertainty exists and cannot be totally eliminated. The element of doubt is due to reasons, some, that can be controlled and some that cannot be. In both cases the severity of the affect can be controlled or minimized. For more information, look up the net.
However the measurement is not affected but it is important that we know and understand the uncertainty of measurement while making measurements.
Please also see the other answer below in this page.

Q: How can I confirm the size of the setting master?
Ans: To be able to calibrate one should have a clean environment, equipment that has the ability & capability of measurement, a suitable master when comparing and a skilled person. The calibrated size is always given between two diametrical opposite points. Measurements maybe made at various angles and at various levels to ascertain the size and the form of the master. Two-point calibration following the ABBE’s principle is standard practice.

Q: How do I co-relate my dial bore gauge measurement with that of Air gauge?
Ans: Firstly it should be understood that Air gauge is a higher capability gauge and should be treated as such. Being non-contact, the gauging is by sensing the backpressure by a column of compressed air over an area. Where as Dial bore gauge is contact gauging where the gauging is by a point contact plunger, which is apart of mechanical chain to indicate the reading.  Air gauging will always measure the average of the peaks and valleys of the surface where as any contact gauge will always measure on the peaks thereby results in showing the bore as less than that checked by air gauge. The minimum difference would be equal to the surface finish value in microns Ra. 

Q: How often should my gauges/instruments be calibrated?
Ans: The frequency with which calibrations should be carried out is an important, sometimes difficult question to answer. There is no fixed period for the validity of a calibration certificate. However, measurement results stated on certificates are usually 'on the day' values and no allowance is made for subsequent drift. There are two main considerations when determining re-calibration intervals.. Firstly, all measuring devices - whether they are simple, 'fundamental', or sophisticated - change characteristics with time; the issue is how much do they change? New devices should be calibrated relatively often in order to establish their reproducibility - essentially their metrological stability or the change in their measuring ability between calibrations. Initial estimates of reproducibility are sometimes made using type-test data from earlier calibration results of similar instruments but the resultant uncertainty of measurement has to be cautiously higher, until real data is available. Secondly, the required uncertainty of measurement should be assessed. If the instrument's reproducibility is shown, by successive calibrations, to be substantially better than the uncertainty required then the interval between calibrations can be extended - perhaps even up to 5 years or so, but at the other extreme - where the instrument's reproducibility approaches the uncertainty needed - the calibration intervals should be much shorter, perhaps even daily in some cases.

Q: Is there a difference between 'accuracy' and 'uncertainty'?
Ans: Yes there is a difference.
The words accuracy and uncertainty are sometimes interchanged but the difference between them is significant and, in many applications it is vital.
Accuracy of measurement is the older phrase and its internationally agreed definition is '… the closeness of the agreement between the result of a measurement and a true value of the measurand' (e.g. length). The definition also notes that '... accuracy is a qualitative concept' - it can be high or low for example but strictly is should not be used quantitatively.
In practice, though, it is often used quantitatively by bending the definition to something like '... the difference between a measured value and the true value' - and this leads to phrases like '... accurate to ± X'. Unfortunately this unofficial definition breaks down because it inherently assumes that a true value can be defined, known and realised perfectly. Even in the finest national measurement laboratories, however, perfect values cannot be realised; it is simply impossible to define or make perfect measurements - nature, and hence the laws of physics, don't allow it.
Uncertainty of measurement acknowledges that no measurements can be perfect and is defined as a '… parameter, associated with the result of a measurement, that characterises the dispersion of values that could reasonably be attributed to the measurand'. It is typically expressed as a range of values in which the value is estimated to lie, within a given statistical confidence, but it does not attempt to define or rely on a unique true value.
In summary, common usage of the word accuracy for quantitatively describing the characteristics of measuring instruments, is incompatible with its official meaning but, even ignoring this point, its common usage definition is significantly cruder than the proper metrological term uncertainty.

Q: Does the difference really matter?
Ans: In many situations the difference really doesn't matter at all and it remains much easier to say 'This instrument is accurate to…." rather than "This instrument is uncertain by…" (Swopping convention might have been easier if the term was certainty and not uncertainty; but it isn't.) And an accurate device sounds more impressive than an uncertain one too, which is probably why much equipment sales literature tends to stick with the word accuracy.
In recent years, however, great strides have been made in developing methods for better quantifying the performance of measuring instrumentation - a task that can be relatively complex for even a simple instrument. If you are attempting to make a serious estimate of such performance - and persuade others that the result is valid - you will have to use the modern tools available and they are fundamentally structured around the philosophy of uncertainty, and its adoption from the outset is thoroughly recommended.