
SmartReader Data Loggers
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thus be expected to read a positive voltage when left unconnected. When connected,
however, they will read the correct input voltage.
3. The 25mA channels read positive current only. Make sure to observe polarity. If you are
using both channels simultaneously, ensure that both 4-20mA transmitters operate with
their negative (-) terminals tied together.
1.9.3 Choosing Input Channels
To choose which of the SmartReader 7's seven external input channels is best for your
particular applications, an understanding of resolution is required.
Maximizing Resolution
The resolution of your SmartReader 7 Logger is eight bits. This means that it can resolve
analog signals, with a defined range, to 256 discrete steps (2 to the power of 8). When
logging from the 2.5V channel, for example, the SmartReader 7 will record digital values in
increments of 2.5 volts divided by 256, or approximately 10 millivolts.
Resolution is usually not a significant factor until you have logging applications that will
produce input signals far less than that of the range of the logger's channel itself.
The 2.5V channel will have a resolution of 0.4% over a range of 0 to 2.5 volts (.01/2.5 x 100)
which should be more than adequate for most applications. If, however, you are intending to
measure signals ranging from 50 to 150 millivolts (with the same 2.5V channel), resolution will
be much coarser (10%). By switching the 50 to 150 millivolt input to the 200mV channel,
resolution can be improved to approximately 1%.
Choosing Input Channels
It is important to maximize resolution when measuring and recording from the SmartReader
7's current and voltage channels. Usually you can do this simply by choosing the channels
that match or approximate the input signals you will be using. The table below lists several
possible input signal ranges and the recommended channels to use. It also lists alternative
channels that can be used for additional simultaneous monitoring capability. These alternative
channels will, however, require the addition of external resistors (see Customizing Input
Ranges).
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