What you put into a cylinder may not be what comes out. where the components are added by weight of addition, and no analysis of the resulting mixture is carried out.īOC do not, as a rule, follow this methodology for calibration gases. This is true where the mixture is gravimetrically filled, i.e. If, during analysis by BOC, the analytical instrument gives a result of 20.4%, then BOC will certify that the mixture will have a concentration between 20.0% and 20.8%.Ĭertification accuracy is often quoted by the manufacturers to be the same as filling accuracy. If the requested level of certification is a Beta certificate then the result will have a certification accuracy of +/- 2% relative. the mixture will be filled within +/- 5% of the required value. 14ppm +/- 1.4ppm)īOC normally quotes the relative accuracy, but will clearly mark where it does not.Ĭonfusion may occur when comparing certificates which do not state the method of expression used, for example 1ppm +/- 0.2ppm looks more impressive than 1ppm +/-20% ExampleĪ filling accuracy of +/- 5% relative, means that the actual filled value of oxygen will be between 19% and 21% i.e. Absolute accuracy means that the quantity of the error is quoted (i.e. This quotes the proportion of the error to the whole value (i.e. Volumetrically prepared mixtures are certified by analysis. Partial pressure of oxygen filled = 180 bar Component level Partial pressure of nitrogen filled = 20 bar This is known as volumetric filling.įor example, 10% nitrogen in oxygen, final pressure 200 bar Gas mixtures may be filled by putting a measured pressure of each component into the cylinder. filling by putting a measured pressure of each component into the cylinder Gas manufacturers employ two main methods to cylinders in the right proportions for the gas mixture requested by the customer, these are: When considering the 'accuracy' of a gas mixture we need to ask if we are looking at the accuracy of the filling or the accuracy of the certification.įilling accuracy relates to how good the operator and equipment are at putting the components of the required mixture into the cylinder as close to the requested value as possible. Temperature, air pressure, humidity and many other conditions can affect the measuring instrument or the item being measured.
For example, do not place a thermometer on a wall near an air conditioning outlet to find the temperature at a workbench. The measurements taken must be properly representative of the process being assessed.
(Gross mistakes are a different matter and are not to be accounted for as uncertainties). The use of an instrument such as a stopwatch depends on the reaction time of the operator. One person may be better than another at the delicate work of setting up a measurement, or at reading fine detail by eye. Some measurements depend upon the skill and judgment of the operator.
For example, measuring the weight of a lively animal presents particular difficulties in getting the subject to cooperate.Ĭalibration of the instrument has an uncertainty which is then built into the uncertainty of the measurements made.
The measurement itself may be difficult to make. The item being measured may not be stable, for example measuring the size of an ice cube in a warm room. Instruments can suffer from errors including bias, changes due to ageing, wear, or other forms of drift, poor readability, and noise. Sulphur Recovery Burner Technology (SUREburner)Īs real measurements can never be made under perfect conditions, errors and uncertainties can come from: Source Refrigeration & Air Conditioning Processes Official BOC UK Online | Industrial Gases | Products & Solutions | BOConline UK