Xylem SIT65 SATELLITE INTERNET TELEMETRY - Manual (Page 13)

Global Water Instrumentation
13
Sensor Data Format
Analog sensor data is transmitted as a 10-bit number in the range of 0-1023. A
calibration equation is applied to the raw data number at the database to scale the
sensor reading in the appropriate range and engineering units. Digital input
accumulators count input events from things like tipping bucket rain gauges and flow
monitors. The accumulator is transmitted as a 11-bit number in the range of 0-2047
and calibration factors are also used to scale the data. Once reaching 2047, the
accumulator counter rolls back to zero with the next count. Digital status inputs
transmit all channel data in a single status register with 1-bit for each channel, one or
zero, true/false, on/off, open/closed, etc. Digital control outputs also have their
status transmitted in a single register with 1-bit for each output channel.
Because digital status inputs and control outputs only need one data bit to transmit
their state, these channels can be combined into one data value. The state of up to
4 status inputs are combined into one status register in binary format, channel one
shown by bit one, channel two with bit two, etc. In the same manner, the current
state of the control outputs is transmitted in a single status register. Register
contents returned as 0011 (binary) shows channels 1 and 2 High (ON), and channels
3 and 4 Low (OFF). Features in the databases allow the mapping of these bits to
text labels.
Calibration Numbers
Data values are transmitted to the hosting databases as raw data numbers. This
number is scaled to the desired engineering units (EU) by first multiplying it with a
scaling factor (Multiplier), then adding an offset (Adder). These numbers are the
coefficients in a straight line equation: EU = Adder + (Multiplier * RawData). These
calibration numbers are entered into the databases and the scaling takes place as
the data comes in. Thus, all the calibration is done at the database, independent of
the field station. There is an Excel spreadsheet calculator for finding these
calibration factors that can be downloaded from our website at globalw.com/support.
Some examples are also given in Appendix B.
Another important feature is available for calculating data. When the raw data has
been adjusted to the proper engineering units using the straight line equation
discussed above, the database calibrations allows for additional mathematical
operations. For example; these equations can be flume or weir equations for