Method of Finding Calibration Intervals
For initial choice of Interval factor to be taken into account are:
- The equipment manufacturer’s recommendations.
- The extent and severity of use
- The influence of environment
- The accuracy of measurement sought
A range of methods is available for reviewing the confirmation intervals. These differ according to whether:
- Items of equipment are treated individually or as a groups( e.g. by make or by type)
- Item fail to comply with their specifications due to drift with the lapse of time, or by use
- Data are available and importance is attached to the history of calibration of the equipment.
No single method is ideally suited for the full range of equipment encountered.
There are five different methods for that
a.Automatic or “Staircase “ adjustment method
b.Control Chart Method
c.Calendar time method
d.“In-use” time method
e.In-service or “black-box testing” method
a. Automatic or “Staircase “ adjustment method :
Each time an item or equipment is confirmed on a routine basis, the subsequent interval is extended if it is found to be with in tolerance.
b.Control Chart Method:
The same calibration points are chosen from every confirmation and the results are plotted against time. From these plots, scattered and drift are calculated, the drift being either the mean drift over one confirmation interval or, in case of very stable equipment, the drift over several intervals. From these figures the effective drift may be calculated.
c. Calendar time method:
Item of measuring equipment are initially arranged into groups on the bases of their similarity of construction and of their expected similar reliability and stability. A confirmation interval is assign to the group, initially on the bases of engineering intuition. In each group, the quantity of items which return at their assigned confirmation interval and are found to have excessive error or to be otherwise non-confirming is determined and expressed as a proportion of the total quantity of item in that group which are confirmed during a given period. If the proportion of nonconforming items of equipment is excessively high, the confirmation interval should be reduced. If it appears that a particular sub-group of items( such as a particular make or type) does not behave like the other member of the group, this sub-group should be removed to a different group with a different confirming level. If the proportion of nonconforming items of equipment in a given group proves to be very low, it may be economically justifiable to increase the confirmation level.
d.“In-use” time method
This is a variation on the foregoing methods. The basic method remain unchanged but the confirmation level is expressed in hour of use rather than in calendar months of elapsed time.
e. In-service or “black-box testing” method
This method is complementary to a full confirmation. It can provide useful interim information on characteristics of measuring equipment between full confirmation and can give guidance on the appropriateness of the confirmation programme. This method is suitable for complex instruments. Critical parameters are checked frequently ( once per day or even more often) by portable calibration gear or preferably, by a “black-box” made up specifically to check the selected parameters. If the equipment is found to be nonconforming by using “black-box”, it is returned for a full confirmation.