Change Value Alarm Calculations

There are four variations of the Change Value alarm calculation that can be implemented with the CygNet Enhanced Point Scheme. See Understanding the CVS Metadata File for more information about the elements and attributes that define the CygNet Enhanced Point Scheme.

For more information, see Alarm Calculation Types.

See the following subsections for details:

• Change Value
• Change Value Percent
• Change Value Positive Percent
• Change Value Negative Percent

Change Value

The Change Alarm uses the Change Value (CHANGE_VAL) calculation. On each update, the system will set the alarm bit if the point’s value is changed. It is cleared every time a point is updated with the same value. There are no calculation parameters for this calculation type.

• If the previous value ≠current value, the alarm is set.
• If the previous value = current value, the alarm is not set.

Example

• Previous value = 10
• Updated value = 11 (alarm is set)

Change Value Percent

The Rate of Change Alarm uses the Change Value Percent (CHANGE_VAL_PCT) calculation. On each update, the system calculates the percentage change between the new value and the newest value older than the time interval, divided by the time interval in seconds, and sets the alarm bit if the absolute value of the percentage change exceeds the threshold.

Calculation Parameters

• Maximum allowed percentage change (calcparm1) — maximum allowed percentage change over the specified time interval before the alarm bit is set

• Time Interval (sec) (calcparm2) — the time span during which the maximum percentage change is allowed

Preliminary Calculations:

Equation

• If the (ABS) Rate Change > maximum allowed percentage change, the alarm is set.
• If the (ABS) Rate Change ≤ maximum allowed percentage change, the alarm is not set.

Example

• calcparm1 = 10%
• calcparm2 = 60 seconds

In this example, the newest value older than the time interval is 100 and the updated value is 89. The Rate of Change bit is set because the absolute value of the percentage change is 18.33%, which exceeds the calcparm1 value (10%).

Change Value Positive Percent

The Positive Rate of Change alarm uses the Change Value Positive Percent (CHANGE_VAL_POS_PCT) calculation. On each update, the system calculates the percentage change between the new value and the newest value older than the time interval, divided by the time interval in seconds, and sets the alarm bit if the value is positive and exceeds the threshold.

The following image shows the area of an Analog Alarm Settings page of a PNT Editor with an enabled Positive Rate of Change.

Calculation Parameters

• Maximum % difference allowed (calcparm1) — maximum allowed positive percentage change over the specified time interval before the alarm bit is set

• Time Interval (sec) (calcparm2) — the time span during which the maximum percentage change is allowed

Preliminary Calculations:

Equation

• If the Rate Change is positive and > maximum allowed percentage change, the alarm is set.
• If the Rate Change is positive and ≤ maximum allowed percentage change, the alarm is not set.

Example

• calcparm1 = 10%
• calcparm2 = 60 seconds
  

In this example, the Positive Rate of Change bit is set because the rate of change is positive (23.17%) and exceeds the calcparm1 value (10%).

• Updated Value = 393.21
• Previous Value (newest older than time interval) = 345.214

Change Value Negative Percent

The Negative Rate of Change alarm uses the Change Value Negative Percent (CHANGE_VAL_NEG_PCT) calculation. On each update, the system calculates the percentage change between the new value and the newest value older than the time interval, divided by the time interval in seconds, and sets the alarm bit if the value is negative and exceeds the threshold.

The following image shows the area of an Analog Alarm Settings page of a PNT Editor with an enabled Negative Rate of Change.

Calculation Parameters

• Maximum % difference allowed (calcparm1) — maximum allowed negative percentage change over the specified time interval before the alarm bit is set

• Time Interval (sec) (calcparm2) — the time span during which the maximum percentage change is allowed

Preliminary Calculations:

Equation

• If the Rate Change is negative and > maximum allowed percentage change, the alarm is set.
• If the Rate Change is negative and ≤ maximum allowed percentage change, the alarm is not set.

Example

• calcparm1 = 10%
• calcparm2 = 60 seconds
  

In this example, the Negative Rate of Change bit is set because the rate of change is negative (-18.33%) and exceeds the calcparm1 value (10%).

• Updated Value = 89
• Previous Value (newest older than time interval) = 100