Alarm rule 5 is triggered when multiple points (in a series) are in Zone A or beyond (upper or lower zone A).
The area between a control limit and the center line is divided into thirds, which are called Zones. Standard control limits are located 3 sigma away from the center line. For standard control limits, the distance from the center line to the control limit is divided into 3 equal parts of one sigma each.
Zone A, often referred to as 3-sigma zone, is closest to the control limit (typically between two and three standard deviations from the center line). Zone B is the middle zone (typically between one and two standard deviations from the center line). Zone C is closest to the center line (typically between the center line and one standard deviation from the center line).
For alarm rule 5, the limit and length values are used for both the upper and lower charts. The data points on the chart are checked for the specified number of points in zone A or beyond. For alarm rule 5, an alarm is triggered when there are <Limit> number of data points in the <Length> that are either in or beyond Zone A. You can also use other alarms, 32, 33, 34, or 35.
General guidelines to detect instability in Zone A for different chart types are as follows:
Chart Type | Maximum Points to apply Rule |
---|---|
X-Bar | 2 of 3 successive points in upper or lower Zone A or beyond |
Individual Chart | |
Median | |
R-chart when the minimum subgroup size is 4 | |
R-chart when the minimum subgroup size is less than 4 | 2 successive points in upper Zone A or beyond |
S | 1 point in upper or lower Zone A or beyond |
Moving Average | |
Moving Range | |
P | |
NP | |
C | |
U |
Following are the alarm properties defined for Zone A Limit Alarm:
Alarm Name | Property Name | Parameter | Data Type | Description |
---|---|---|---|---|
Allow Upper Chart Zone A Limit Alarm | UpperWEZoneAAlarmEnabled | Boolean | Enables Zone A limit alarms for upper chart | |
Allow Lower Chart Zone A Limit Alarm | LowerWEZoneAAlarmEnabled | Boolean | Enables Zone A alarm points for lower charts | |
Alarm 5 | Limit of Zone A Limit Alarm | WEZoneAAlarmLimit | Integer | Sets the limit and length values to check data points within Zone A |
Length of Zone A Limit Alarm | WEZoneAAlarmLength | Integer | ||
Alarm 32 | Limit of Upper Chart Upper Zone A Limit Alarm | WEUpperChartUpperZoneAAlarmLimit | Integer | Defines the limit and length values to check points in upper Zone A of the upper chart |
Length of Upper Chart Upper Zone A Limit Alarm | WEUpperChartUpperZoneAAlarmLength | Integer | ||
Alarm 33 | Limit of Upper Chart Lower Zone A Limit Alarm | WEUpperChartLowerZoneAAlarmLimit | Integer | Defines the limit and length values to check points in lower Zone A of the upper chart |
Length of Upper Chart Lower Zone A Limit Alarm | WEUpperChartLowerZoneAAlarmLength | Integer | ||
Alarm 34 | Limit of Lower Chart Upper Zone A Limit Alarm | WELowerChartUpperZoneAAlarmLimit | Integer | Defines the limit and length values to check points in upper Zone A of the lower chart |
Length of Lower Chart Upper Zone A Limit Alarm | WELowerChartUpperZoneAAlarmLength | Integer | ||
Alarm 35 | Limit of Lower Chart Lower Zone A Limit Alarm | WELowerChartLowerZoneAAlarmLimit | Integer | Defines the limit and length values to check points in lower Zone A of the lower chart |
Length of Lower Chart Lower Zone A Limit Alarm | WELowerChartLowerZoneAAlarmLength | Integer |
If you define alarm rule 32 with limit and length as 2 and 3 respectively, the alarm rule 32 is violated if only 2 points in a string of 3 adjacent points in the upper chart are lower than or beyond Zone A.
Graphical representation of Zone A Limit Alarm: