Why all the fuss about SDI-12?

Serial Digital Interface Protocol
SdiWhat is it?
In the mid 1980's a group of water monitoring instrumentation users met to discuss a specific problem - the unreliability of low power analog water sensors and the complexity of interfacing analog sensors and dataloggers.
SdiTheir eventual solution to the problem? SDI-12
The Serial Digital Interface Protocol is a set of rules that define exactly how a sensor must communicate with a datalogger.  Compatibility with SDI-12 requires that each sensor support a standard set of commands and meet specific electrical and power standards.  Circuitry and intelligence are added to create a "smart" sensing device allowing the user and manufacturer to program and calibrate the sensor independently of the datalogger.
SdiWhat does it do?
 An analog signal traveling along a wire is highly subject to corruption from a wide range of sources - electric motors, electromagnetic fields, radio signals, lightning pulses, microwaves, varying cable lengths and other factors.  The SDI-12 digital signal is low power and yet far more resistant to data errors than a low power analog signal, greatly reducing the likelihood of corrupt data due to interference.  SDI-12 requires that the sensor's analog signal be converted to a digital one right at the sensor, rather than at the datalogger.  The use of a digital communications protocol eliminates or reduces the problems from these interference sources.

What can I do with SDI-12 sensors (that I can't with analog sensors)?

  1. Use much longer cables and cables of varying length, from your sensors to your datalogger.
  2. Have several sensors transmitting along one cable, instead of one cable per sensor.
  3. Instantly unplug or plug-in sensors into the system, without affecting any other components in the system.  Sensor replacement or servicing is extremely simple.
  4. Remove sensors from the datalogger and replace with freshly calibrated sensors with no programming changes to the datalogger. Calibration information is stored within the sensor eliminating the need to transport the datalogger along with the sensor for calibration.
  5. Use any SDI compatible sensor to interface with any SDI compatible logger (with compatible connectors).
  6. Access an increasing number of compatible sensors and dataloggers and not get locked into proprietary sensor protocol or face custom wiring and programming jobs.
  7. Eliminate a separate power supply for each sensor as power is supplied through the cable.