Mass Standards Calibrations
Mass Standards Calibrations are generally inter-comparisons of one "generation" of mass standard to another of a higher "generation". It is this hierarchy that ensures all errors introduced into the calibration cycle are within allowable limits.
Most calibrations are done with standards which have an "apparent density" of 8 000 kg/m3 and are performed in air with an "apparent density" of 1.20 kg/m3. These factors are important only in high precision calibrations and will be discussed in further detail later on.
The prototype kilogram is a cylinder measuring approximately 39mm * 39mm and is comprised of Platinum-Iridium alloy. The composition is 90% Platinum - 10% Iridium and the approximate density is 21 500 kg/m3
Mass standard calibration is generally performed on an equal arm balance. There are several ways to use an equal arm balance for calibration of weights each having it's own advantages and disadvantages.
The standard is placed upon one pan, the weight to be calibrated is placed upon the other pan and weight is added or removed to bring the balance to it's center or zero position. To be of any use the length of the arms of the balance must be very near equal.
Because the length of the arms of a balance are never exactly equal, we must use a different method of ensuring that the weight to be calibrated is not influenced by these differences if we wish to calibrate a weight somewhat more accurately. The simplest way to accomplish this is to use Substitution Weighing. For this procedure, the standard is placed upon one of the pans of the balance and material is added to the other pan to bring the balance to a position of rest or zero position. The standard is then removed from the balance and the weight to be calibrated is placed upon the same pan which held the standard. If the balance is then brought back to the same position of rest by adding or removing weight from the weight to be calibrated, the final weight will be the same as the standard. This is the most common method employed for calibration of working class standards.
The final method involves placing the weight to be calibrated upon one pan and the standard on the other. The balance is then brought to a central zero position by adding unknown weights to one of the pans. This extra material is to remain in place for the rest of the process and must not be disturbed. This initial rest point is noted and the standard and weight are then transposed or interchanged so that they are on opposite pans of the balance. Known weights are then added to the light (or high) pan to bring the balance back to the same position of rest noted previously. The difference in weight between the standard and the weight to be calibrated is then equal to one half the value of the weights just added. Alternately, the new rest point could have been noted without adding any new weights. The difference in weight between the standard and the weight to be calibrated would then be one half of the absolute difference which is found by the multiplying the rest point shift in scale graduations by the sensitivity of the balance in weight per graduation (see Calculated Rest Point (CRP), Sensitivity and Repeatability).
For a further discussion of High Accuracy Mass Calibrations, please see the section on Buoyancy in Mass Calibrations.
OIML Weight Classifications.
|OIML R111-1:2004 (max. deviation in mg unless otherwise noted)|
|5 000 kg||||||25 g||80 g||250 g||800 g||2 500 g|
|2 000 kg||||||10 g||30 g||100 g||300 g||1 000 g|
|1 000 kg||||1,6 g||5 g||16 g||50 g||160 g||500 g|
|500 kg||||800||2,5 g||8 g||25 g||80 g||250 g|
|200 kg||||300||1 g||3 g||10 g||30 g||100 g|
|100 kg||||160||500||1,6 g||5 g||16 g||50 g|
|50 kg||25||80||250||800||2,5 g||8 g||25 g|
|20 kg||10||30||100||300||1 g||3 g||10 g|
|5 kg||5||16||50||160||500||1,6 g||5 g|
|5 kg||2,5||8||25||80||250||800||2,5 g|
|2 kg||1,0||3,0||10||30||100||300||1 g|
Note: Based upon OIML R111-1:2004