Power supply
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Warm-up time
If the SDI-12 network is powered down between measurements
for greater battery economy, a warm-up time should be included
in the logger program at the start of each measurement block.
The warm-up time allows the micro-controller in the sensor to
boot up and prepare to receive SDI-12 commands. The length
of warm-up time depends on the type of sensor, the slew rate of
the system power supply and the size of the network.
For a WET150 using the internal GP2 logger 12V power rail, the
total power-up and boot time is less than 100ms, even with up to
26 WET150s connected together in a small network. The
shortest GP2 warm-up setting is 1 second, so use this value in
the GP2 program.
Multiple WET150s in a network
Sequential measurements
One WET150 is instructed to take a measurement and then
return its results before moving onto other WET150s in the
network.
Only one WET150 is fully active at a time. All the others are
either in SDI-12 network activity mode when the data line is
active, or idle mode when it is not.
Concurrent Measurements
All WET150s in the network are individually instructed to take a
measurement as soon as possible without waiting for any other
WET150. They then hold their measurement results until the
logger requests them from each sensor individually in sequence.
Due to the short active sensing time of the WET150 and the
slow communication speed of SDI-12, in practice, only one
WET150 in a network is actively sensing at any given time even
when using concurrent measurements. The worst-case
scenario is that one WET150 is actively sensing whilst another is
actively computing and all others are in SDI-12 network activity
mode.
Therefore, the maximum network current demand during a
concurrent measurement is little more than if performing a
sequential measurement.
Care should be taken when performing concurrent
measurements if sensors other than the WET150 are also used
in the network. Their sensing time and power supply current
demands may be greater than the WET150, which could limit
