Campbell Scientific CS215 Temperature and Relative Humidity Probe - Manual (Page 21)

CS215 Temperature and Relative Humidity Probe
8.1.1.2 Edlog Dataloggers
The aM! command takes about 700 milliseconds in total to make a
measurement from the CS215. If it is included in the main program table
(table 1) the program will be delayed for this interval which will limit the
maximum scan rate for fast running programs.
For Edlog dataloggers, it is possible to put the SDI-12 instruction in table 2,
which allows table 1 to interrupt and pause the SDI-12 instruction letting it run
other instructions. However, table 1 cannot interrupt the instruction while
SDI-12 communications are taking place, only when datalogger is waiting for
the sensor to take the measurement. As the initiation of the sensor and also the
transfer of data from the sensor each take approximately 200 milliseconds this
limits the scan rate of table 1 to about 250 milliseconds, and only then if table 1
itself takes little time to execute.
The aC! concurrent measurement command can also be used where the sensor
measurement is initiated with one command and data is collected after a
minimum delay of one second or any time thereafter. With Campbell
Scientific dataloggers, this is done by using the SDI-12 recorder instruction
with the aC! command. The datalogger will return –99999 for the temperature
reading for the call of the instruction that initiates the measurement. At the
next call of the instruction, the datalogger will request the data and record the
correct temperature.
Using the aC! command requires more detailed programming to ensure the
out-of-range values are not recorded as real temperatures. It also has similar
limitations to running the instruction in table 2 when trying to make other
measurements at a fast scan rate. This is because the program will still be
delayed by approximately 200 ms for both the initiation of the measurement
and the subsequent reading of data from the sensor. Using the aC! command
can be useful where predictable timing of the program is required (without the
complications of working out how different program tables will interrupt each
other).
8.1.2 Long Cables
Digital data transfer eliminates offset errors due to cable lengths. However,
digital communications can break down when cables are too long, resulting in
either no response from the sensor or corrupted readings. The original SDI-12
standard specifies the maximum total cable length to be 61 m (200 ft). To
ensure proper operation with long cables, follow these guidelines:
• Use low capacitance, low resistance, screened cable (as fitted by Campbell
Scientific) to reach distances of several hundred meters.
• Ensure that the power ground cable has low resistance and is connected to
the same ground reference as the datalogger control ports.
• Be aware that “daisy-chaining” sensors reduces the maximum cable length
roughly in proportion to the number of sensors connected in parallel.
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