Making a Measurement
There are four classes of measurement commands which will be referred to as M commands
(Measurement Commands), C commands (Concurrent Measurement Commands), MC commands
(Measurement commands with CRC-16), and CC commands (Concurrent Measurement Commands
with CRC-16). Concurrent measurement commands are new to version 1.2 of the SDI-12
specification. The commands with CRC-16 are new to version 1.3 of the SDI-12 specification. In
the original class of “M” measurement commands the data recorder issued the measurement
command and then waited for the sensor to complete the measurement before continuing the data
collection cycle. Only one sensor could be accessed at a time and a maximum of nine parameters
could be returned. With version 1.2 of the specification, concurrent measurements were defined.
With a concurrent measurement, the data recorder can request the sensor to take a measurement,
determine how long it will be until the sensor has a reading, and then continue on making requests
to other sensors on the SDI-12 bus. This way multiple sensors are taking measurements concurrent
with each other. Once the measurement time for a sensor has expired the data recorder polls the
sensor for the data. The CRC-16 commands that were added in version 1.3 of the specification add
a 16 bit cyclic redundancy check (CRC-16) to the returned data values. This provides an additional
means for the data recorder to ensure that the collected data has not been corrupted. Software
support for SDI-12 version 1.3 was added in software revision V2.0.
Selecting a measurement command class
Always supported
The first requirement is that the data recorder support the command. All SDI-12 data recorders
support the non-concurrent measurement M command. With the M command the data recorder
collects data from the sensors one at a time.
Multiple long measurement time sensors
When collecting data from several SDI-12 sensors that have long measurement times, the complete
data collection cycle can be shortened by utilizing concurrent commands. The data recorder can
initiate the measurement on all the sensors and when each finishes, then collect the data from all of
them. Since the measurement times overlap, the complete data collection cycle is shorter. There is
no advantage to the concurrent measurement C command when there is only one sensor.
Improved data integrity checking
The measurement command classes with CRC-16 (MC and CC) offer additional data integrity
checking over the non CRC-16 commands (M and C). The non CRC-16 commands offer data
integrity checking in the form of parity and the SDI-12 command structure. The CRC-16
commands offer some additional data integrity through the addition of a CRC-16. Since the CRC-
16 commands are brand new in SDI-12 version 1.3, not as many data recorders support them. In
most applications, lack of this support on the part of the data recorder will not be missed since non
CRC-16 SDI-12 commands still offer significant data integrity checking. If the data recorder
supports CRC-16 commands, then it is recommended to use them when collecting data from this
sensor in order to benefit from the increased noise immunity.
Making a non-concurrent Measurement (M command)
The command to tell the ACCUBUBBLE to make a measurement with the original measurement
command is:
aM!
where a is the address character, and M is the command to
make a measurement
Most data recorders will issue this command and automatically handle the reply to collect data.
You can also issue the command yourself. In reply, the ACCUBUBBLE will respond with
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