700-SDI-PT-KEL-Man Rev 4 06 Jan 2020
9/19
3.4.1
Concurrent Measurement Commands
Concurrent Measurement commands (C-commands) are the same as M-commands, except that no
service request is generated by the C-command. Therefore, the SDI-12 master has to wait at least
1-3 seconds after the C-command to collect the data. The addition of the second C in the command
string will include the CRC (cyclic redundancy check).
CONCURRENT MEASUREMENT COMMANDS
SDI-12 Command
Response
Values Measured
(send
a
DO! command to retrieve)
aC!
aCC!
a00102
After 1 second, 2 measurements available:
•
Pressure
•
Temperature
aC1!
aCC1!
a00002
After 0 seconds, 2 measurements available:
•
Minimum Pressure in bar
1
•
Maximum Pressure in bar
1
aC2!
aCC2!
a00002
After 0 seconds, 32measurements available:
•
Minimum Temperature in
C
1
•
Maximum Temperature in
C
1
1 – the values for the C1 and C2 commands are always returned in the stated units of measurement regardless of
the selected temperature and pressure units.
3.4.2
Continuous Measurement (R-Commands)
Continuous measurement must be enabled using the extended command aXR! (refer to section
3.6.2.3 for details).
The pressure and the temperature are always displayed in the unit set with the ‘XP’ and ‘XT’
command. Internally all values are calculated in bar and °C. .
The values are displayed with a polarity sign followed by a maximum of seven digits which may or
may not have a decimal point (optional).
The maximum number of characters in a data value is 9 (polarity sign +7 decimal point).
Calculation of the average pressure is done using the following formula:
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃
𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴
_
𝑛𝑛𝐴𝐴𝑛𝑛
=
∑
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃
𝑖𝑖
𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐴𝐴𝐴𝐴𝐵𝐵𝑖𝑖𝐵𝐵𝐴𝐴
𝑖𝑖=1
𝐵𝐵𝑃𝑃𝐵𝐵𝐵𝐵𝑃𝑃𝑃𝑃𝑃𝑃𝐵𝐵𝐵𝐵𝑃𝑃
The response to an R-command is always
a
<CR><LF>. The values returned in response to an
a
DO!
command after the R-command are as follows:
CONTINUOUS MEASUREMENT COMMANDS
R-Command
aDO! Response
Values Returned
aR0!
aRC0!
a±uu.uu±uu.uu±uu.uu±uu.uu
Mean pressure
Minimum pressure
Maximum pressure
Temperature
