Lower warning limit limits

The upper and lower warning limits, which are located at +2S, should only be exceeded by 5% of the data thus... 

The upper and lower warning limits (UWL and LWL) are drawn at 2s above and below, respectively, of the mean recovery. The upper and lower control limits (UCL and LCL) are defined at 3s value about the mean. If ary data point falls outside UCL or LCL, an error in analysis is inferred that must be determined and corrected. The recoveries should fall between both the warning limits (UWL... 

Precision control charts may, alternatively, be constructed by plotting the RPDs of duplicate analysis measured in each analytical batch against frequency of analysis (or number of days). The mean and the standard deviation of an appropriate number (e g., 20) of RPDs are determined. The upper and lower warning limits and the uppper and lower control limits are defined at 2 and 3.v, respectively. Such a control chart, however, would measure only the quality of precision in the analysis. This may be done as an additional precision check in conjunction with the spike recovery control chart. 

Lower warning limit Lower action limit -------- ... 

In addition to the control limits as given for both types of charts, very frequentlv lower and upper warning values are also given as the 2a limits. These are calculated in the same fashion as the control limits in regard to sample size. The lower warning limit... 

Interpreting Control Charts The purpose of a control chart is to determine if a system is in statistical control. This determination is made by examining the location of individual points in relation to the warning limits and the control limits, and the distribution of the points around the central line. If we assume that the data are normally distributed, then the probability of finding a point at any distance from the mean value can be determined from the normal distribution curve. The upper and lower control limits for a property control chart, for example, are set to +3S, which, if S is a good approximation for O, includes 99.74% of the data. The probability that a point will fall outside the UCL or LCL, therefore, is only 0.26%. The... 

The same rules apply to precision control charts with the exception that there are no lower warning and lower control limits. 

The primary objective of gas odorization is safety. Odorization serves as a warning in the detection of natural gas in air before it reaches combustible levels. Certain federal pipeline safety regulations require that combustible gases in pipelines be detectable at one-fifth of the lower explosive limit by a person with a normal sense of smell, either by the natural odor of the gas or by means of artificial odorization [574]. Therefore the proper odorization and odorants are integral parts of safety [813,1753]. 

Flammable gas detection systems are typically used to initiate an alarm at a concentration level below the lower flammable limit (LFL). Two gas alarm levels (low and high) are often utilized to allow early warning prior to taking automatic actions. Detection systems may also be used to stop electrical power and initiate process shutdown. The low alarm setpoint should be —20% LFL and the high alarm level set point should be between 40%-60% LFL. Where these devices are used to initiate process shutdown or activate fire protection systems, it is common practice to use some form of voting, typically 2 out of 2, such that the frequency of spurious shutdowns or system activation is minimized. 

Two aspects are important for IQC (1) the analysis of control materials such as reference materials or spiked samples to monitor trueness and (2) replication of analysis to monitor precision. Of high value in IQC are also blank samples and blind samples. Both IQC aspects form a part of statistical control, a tool for monitoring the accuracy of an analytical system. In a control chart, such as a Shewhart control chart, measured values of repeated analyses of a reference material are plotted against the run number. Based on the data in a control chart, a method is defined either as an analytical system under control or as an analytical system out of control. This interpretation is possible by drawing horizontal lines on the chart x(mean value), x + s (SD) and x - s, x + 2s (upper warning limit) and x-2s (lower warning limit), and x + 3s (upper action or control limit) and x- 3s (lower action or control limit). An analytical system is under control if no more than 5% of the measured values exceed the warning limits [2,6, 85]. 

The average range of the data is multiplied by D to give the lower control limit (Dq ooi). lower warning limit (/I(i.(i25). upper warning limit ( >0.975) and upper control limit ( >0.999). Adapted from Oakland (1992). 

An example of a recovery control chart is shown in Figure 4.7. The mean recovery of individual measurements is represented by the centreline. The upper warning limit (UWL) and the lower warning limit (LWL) are calculated as plus/minus two standard deviations (mean recovery + 2s) and correspond to a statistical confidence interval of 95 percent. The upper control limit (UCL) and the lower control limit (LCL) are calculated as plus/minus three standard deviations (mean recovery 3s), and represent a statistical confidence interval of 99 percent. Control limits vary from laboratory to laboratory as they depend on the analytical procedure and the skill of the analysts. 

It is good to integrate as many system failure alarm/warning systems as possible. For hydrogen leaks, for instance, a variety of different leak detectors are available. The concentration level to set a warning alarm is usually about 1% (10,000 ppm) of hydrogen by volume in air which is about 25% of the lower flammable limit, which is 4% (40,000 ppm) in air. 

FIGURE 12.22 Control charts and outliers. (A) pEC50 values (ordinates) run as a quality control for a drug screen over the days on which the screen is run (abscissae). Dotted lines are the 95% c.l. (upper and lower warning lines) and the solid lines the 99.7% c.l. (upper and lower action lines) Data points that drift beyond die action lines indicate significant concern over the quality of the data obtained from die screen on those days. (B) The effect of significant outliers on the criteria for rejection. For the data set shown, the inclusion of points A and B lead to a c.l. for 95% confidence that includes point B. Removal of point A causes the 95% limits to fall below point B, causing them to be suspect as well. Thus, die presence of the data to be possibly rejected affects die criteria for rejection of other data. 

Fig. 2.12. Shcwhart control chart using data obtained with the B. cereus RM (courtesy of P. in t Veld ref [52]). The use of back-transformed data to draw the chart leads to asymmetrical upper and lower warning and control limits.

When the measurement process is under control, that is when any variation in counts is basically random, the counts obtained with the RMs, in 95% of the cases, fall between the lower and upper warning limits and in 99.1% of cases between the lower and upper action limits. When the variation in the counts does not conform to the pattern that might reasonably be produced by chance variation, then it is concluded that the process is out of control. It means that one or more systematic errors have been introduced into the system. Several tests for detecting out of control situations have been developed. The purpose of each test is to detect a particular non-random pattern in the points plotted on the control chart. These tests have been evaluated by Nelson [42,43]. The following criteria are used for interpreting the microhiological control charts [38,42] ... 

The X-chart is based on the use of a standard reference material analyzed preferably with each batch of unknowns. After a reasonable number of analyses of reference material samples (typically n>20), the mean and standard deviation of the data are calculated and a control chart constructed. The center line represents the mean, the two outer lines represent the upper and lower control limits (UCL and LCL), or 99% confidence limits, and the two lines closest to the mean line are the 95% confidence limits, or upper and lower warning limits (UWL and LWL). One analysis outside the 95% confidence limits is not cause for alarm however, two consecutive analyses falling on one side of the mean line between the 95% and 99% limits would certainly be cause for an investigation. Control charts are very useful in visualizing trends (Fig. 10.6). 

UWL = upper warning limit LWL = lower warning limit UCL = upper control limit LCL = lower control limit... 

Coauthor Shannon warned that the safety hazard from fire or explosion with hydrogen requires that a hydrogen-inert gas mix be used only below the lower limit of flammability. The lower explosive limit is 4% hydrogen in a hydrogen-air mix. The upper limit is 74.2% hydrogen in an H2-air mix. 

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