USE OF REFERENCE MATERIAL AND CONTROL CHARTS
Control samples are typically analysed in duplicate and the average value plotted on a control chart. The average value is only calculated if the difference between the duplicate analyses is acceptable. As an example, the acceptable difference for Calorific Value is 0.12MJ/Kg. A control chart must have the following lines drawn on it as guides when plotting the daily values: the average value line, warning limit lines (above and below the average line), the upper control limit, lower control limit and the range. It is imperative that the control sample used is stable as degradation can result in a bias on the control chart. The bias can then be incorrectly assigned to faulty equipment, incompetent analyst, flawed technique or unknown discrepancy during analysis. If the average result of the control sample is found to be out of the upper or lower control limits, the analytical results of the batch of samples analysed should be treated as suspect. Further testing should stop and the root cause of the problem ascertained. Once the root cause has been identified and eliminated, work may commence and the control sample re-analysed.
FREQUENTLY ASKED QUESTIONS
Q: Should the values obtained for the CRM be exactly equal to the values on the specification sheet?
– Coal is hygroscopic and depends on the humidity result in your laboratory.
-The humidity affects the moisture value.
-Condition your sample at the same humidity at which the analysis takes place.
-Determine the moisture result for the CRM after conditioning
-Analyse for the various parameters in your laboratory e.g. proximate, CV , Sulphur etc. These will be your air dry results
-Use your moisture and convert your results to dry base
-Your dry base results should be within reproducibility of the assigned value
Q: How do you condition samples?
A: Moisture in analysis sample is also known in most laboratories in as “Inherent Moisture”. When samples are pulverized in the preparation area and bottled, they are taken to the conditioning area of the laboratory. The mass of each sample could be between 60 to 100g. The sample is placed into a conditioning tray. It should be spread with a distribution of about 0.1g/cm2 to 0.15g/cm2
Q: How do I know my conditioning tray is large enough?
A : 100g sample that has to be distributed at 0.1g/cm2, the size of the tray can be calculated as follows:
0.1 = 100/〖xcm〗^2
= 1000 〖cm〗^2
Hence a reasonable tray dimension for 100g sample spread at 0.1g/cm2 will be about 50cm x 20cm = 1000 cm2