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Vref warm-up time
The Vref warm-up time is the time that the SDI-12 AnalogPlus module allows voltages to stabilize before it
takes a reading. If a sensor is a ratiometric sensor that is powered off of Vref, then the warm-up time may
have to be increased based upon cable capacitance and sensor stabilization times. Many ratiometric sensors
are resistive dividers or resistive bridges that are relatively high impedance. They may require more time to
fully charge the cable capacitance, particularly for long cable runs. This parameter can be increased to tailor
the measurement to the sensor‟s requirements.
Switched voltage and Switched voltage warm-up time
The SDI-12 AnalogPlus module provides a switched voltage output. The switched voltage can be
programmed to come on for a particular measurement or measurements. The voltage level can also be
programmed to be either 5V or 12V. Note: The 12V is the +12 supplied to the module via the SDI-12 bus
and as such is unregulated and can vary from 8V to 16V. The switched voltage provides a means to power up
sensors that only need to be powered for the measurement. The switched voltage warm-up time allows the
user to specify how long the switched voltage must be on before the measurement is taken.
Note: There is only one switched voltage terminal connection. Do not connect a 5V only device to the
switched voltage and then perform a measurement that has been configured to use a 12V switched voltage
because 12V would then be applied to the 5V device.
Example Setup Tutorials
Note all command examples will show the SDI-12 address as a lower case “a”. The “a” will need to be
replaced with the actual address of the module. As shipped from the factory, this will be the number zero “0”.
AT/RH Probe
The 5600-0315 AT/RH probe needs a supply voltage of at least 3.5V and will accept supply voltages up to
16V without any problems. It provides two 0-1V outputs. Since this sensor can run off of 5 Volts or 12
Volts, we have the choice of switched 5, switched 12, or a continuous 5V or 12V. The choice would depend
upon what other sensors are connected and their requirements, since this sensor can accommodate any of the
options. It should be noted that the minimum excitation time for this sensor is 3 seconds, therefore if we are
going to use a switched voltage then the warm up time would need to be set to at least 3 seconds. To measure
both air temperature and relative humidity using the switched excitation would require separate measurement
commands being configured for each parameter. This also allows the user to specify a slope and offset for
each so the module would report degrees C and %RH. The other choice is to provide continuous excitation
to the probe, and then utilize the “M” command to read both channels at once. In this case, the module would
report voltages and the data recorder would have to handle the conversion to degrees C and %RH. An
example of both ways will be given.
Continuously Powered
If there was another sensor that needed the switched voltage out, then the AT/RH could be powered by the
12V from the data recorder, but the recommended connection would be to switched voltage connection since
the SDI-12 AnalogPlus module would protect the AT/RH Probe from any voltage transients on the SDI-12
bus. To continuously power the AT/RH probe at 5 volts off of the switched voltage connection, issue the
aXW+5!
Command to the unit. To collect the readings, issue the
aM!
command to the unit followed a couple of seconds later by the
0D0!
