Xylem SDI-12 RADAR - Manual (Page 7)

Normally, the radar transmits the service request within 530mS of receipt of the command. as
with many data loggers, this leaves insufficient time to sustain continuous measurements once
per second. The Power_mode is an internal setting which places the H-3611 in a fast measure
mode. in this mode, the radar is always awake and automatically collecting data from the
radar unit at a 1-second interval and stores the previous six minutes of data in the stack. in this
mode the sensor uses additional power. when an “am!” measurement is received, the sensor
response is “0014” (1-seconds, 4-parameters). The service request is sent within 55mS of receipt
of the command (170mS of receipt of the break). This leaves plenty of time for a data logger to
collect the data while sustaining a 1-second logging interval. The data placed in the sensor buffer
is the results of the previous internal measurement (1-second old). The data logger must
implement its own internal Noaa or other filtering and averaging computations.
when in fast measure mode the radar makes continuous measurements which allows a different
type of Noaa mode measurement to take place. when in fast measure mode the “am1!”
command is replaced with a command that takes the data from the previous six minutes and
performs the same procedure as the Noaa mode measurements, namely:
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)
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.
in addition to the “am1!” command, the “am2!” and “am3!” commands have been added which
take the simple average and standard deviation of the previous 1 minute and 15 second data sets
respectively. all of these commands have concurrent counterparts as well.
Introduction
5
4. Divide by one less than the sample size
5. Take the square root
The “am1” command response is “01844” (184 seconds, 4- parameters). The sensor buffer will
contain 4 parameters; mean, standard deviation, number of outliers and battery voltage.
if the built in “Noaa” mode is not workable, the H-3611 supports an alternate “fast measure”
mode.
Fast measure mode
FCC approval
This device complies with part 15 of the FCC rules. operation is subject to the following two
conditions: (1) This device may not cause harmful interference, and (2) this device must accept
any interference received, including interference that may cause undesired operation.