Xylem SDI-12 RADAR User Manual
Page 23
SDI-12 Command & Response
21
The radar supports two different “am1” measurement commands based on the Power mode.
Data values generated in response to this command are stored in the sensor’s buffer for subsequent
collection using “D” commands. The data will be retained in the sensor until another “m”, “ C”,
or “V” command is executed.
when Power mode = 0, the radar internally performs the following measurement sequence:
1. makes 181 measurements at a precise1.0 second interval.
2. Computes the standard deviation for the data set.
3. multiplies the standard deviation by 3 to obtain a High and Low outlier threshold.
4. Sifts through the data set and discards data points above and below the outlier thresholds.
5. Computes the mean and standard deviation again for the data set with the outliers removed.
when Power mode = 1, the radar internally performs the following measurement sequence:
1. Collect a set of 1-Hz samples centered in the previous six minute time block, the length of
which is selected by the user (ranging from 1-360 data points)
2. Computes the standard deviation for the data set.
3. multiplies the standard deviation by 3 to obtain a High and Low outlier threshold.
4. Sifts through the data set and discards data points above and below the outlier thresholds.
5. Computes the mean and standard deviation again for the data set with the outliers removed.
The standard deviation is computed as follows:
1. Compute the mean for the data set
2. Compute the deviation by subtracting the mean from each value
3. Square each individual deviation
4. Divide by one less than the sample size
5. Take the square root
Command
response
Description
“am1!”
“atttn
initiate measurement
where:
a
is the sensor address (“0-9”, “a-Z”, “a-z”, “*”, “?”).
m
is an upper-case aSCii character
ttt
is a three digit integer (000-999) specifying the maximum time, in seconds, the sensor
will take to complete the command and have measurement data available in its buffer.
n
is a single digit integer (0-9) specifying the number of values that will be placed in the
data buffer. if “n” is zero (0), no data will be available using subsequent “D” commands.
“am1” (Noaa) measure Command