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Chapter 3 Starting and Stopping this Device
3-11-2 Standard Deviation
This function calculates the standard deviation of the input pressure. It can be used for diagnostic
processes that detect a change in process conditions.
(1) Equation
The following equation is used to calculate the standard deviation (s).
s: Standard deviation
x: Input pressure
n: Number of samples
(2) Parameters Related to the Standard Deviation
Standard deviation-related parameters are shown below. For details on the attributes, see (4) below,
“Checking the Settings.”
Standard Deviation Standard deviation
Standard Deviation Maximum amount of standard deviation
Standard Deviation Minimum amount of standard deviation
Average Pressure Average pressure
Standard Deviation Unit The unit of measurement for standard deviation
Reset Standard Deviation Reset of the standard deviation and average
Standard Deviation Sample Count Number of samples used for calculation
Standard Deviation Alarm Use Operating mode of the standard deviation alarm
Standard Deviation High Limit Upper limit of standard deviation (alarm threshold—high limit)
Standard Deviation Low Limit Lower limit of standard deviation (alarm threshold—low limit)
(3) Preparation
First, it is necessary to monitor the process under normal conditions. The process should be
monitored for a time period of one to several hours (the amount of time required for each
condition to occur, if there is a significant change in the operating conditions).
The way to monitor the process under normal conditions is to monitor trends in the standard
deviation. If that is not possible, an easier way is to obtain the maximum and minimum standard
deviation.
(i) Monitoring trends in standard deviation
To monitor trends in the standard deviation, see the CommStaff Operation Manual (HART 7).
(ii) Obtaining the maximum and minimum standard deviation
Follow the procedure below to obtain the maximum and minimum standard deviation.
Reset the standard deviation and its maximum and minimum values when the process is stable
under normal conditions. Recording of the maximum and minimum values then begins anew .
To reset, execute Reset Standard Deviation. This resets the standard deviation, its maximum and
minimum values, and the average pressure.
Once the set amount of time has elapsed, observe the value of the following two variables.
Standard Deviation Max Maximum amount of standard deviation
Standard Deviation Min Minimum amount of standard deviation
2
=
n
− (x)
2
= x
2
− (x)
2
∑
x
2
i
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