What is the SDI-21 Support Group?

The SDI-12 Support Group is a non-profit corporation. We provide all interested parties with the SDI-12 Specification and other information, as appropriate, about SDI-12. We also publish the SDI-12 Specification and upgrade it when changes are needed.

The SDI-12 Support Group is exempt from federal income taxes under section 501(a) of the United States Internal Revenue Code. The corporation is registered in Utah (USA).

Please join the SDI-12 Support Group.

What is SDI-12?

SDI-12 stands for serial data interface at 1200 baud. It is a standard to interface battery powered data recorders with micro-processor based sensors designed for environmental data acquisition (EDA).

EDA is accomplished by means of a sensor, or sensors, and a data recorder which collects and saves the data. SDI-12 is a standard communications protocol which provides a means to transfer measurements taken by an intelligent sensor to a data recorder. An intelligent sensor typically takes a measurement, makes computations based on the raw sensor reading, and outputs the measured data in engineering units. For example, an SDI-12 pressure sensor may take a series of pressure measurements, average them, and then output pressure in psi, inches of mercury, bars, millibars, or torrs. The sensor's micro-processor makes the computations, converts sensor readings into the appropriate units, and uses the SDI-12 protocol to transfer data to the recorder.

What requirements make SDI-12 particularly useful?

SDI-12 is used in applications with the following requirements:

These requirements are necessary to acquire hydrologic data at remote sites. Most sites use battery powered data recorders to operate for long and unattended periods of time.

Why use micro-processor based sensors anyway?

A micro-processor in the sensor may calibrate the sensor, control sensor measurements, and convert raw sensor readings into engineering units. The micro-processor also controls the SDI-12 interface. It accepts and decodes instructions received from the data recorder, starts the measurements, controls all timing, and uses the SDI-12 protocol to communicate with the data recorder.

Can more than one than one sensor be connected to a single data recorder?

Yes. SDI-12 is a multi-drop interface that can communicate with multi-parameter sensors. Multi-drop means that more than one SDI-12 sensor can be connected to a data recorder. The SDI-12 bus is capable of having ten sensors connected to it. Having more than ten sensors, however, is possible. Some SDI-12 users connect more than ten sensors to a single data recorder.

Can a sensor take more than one measurement?

Multi-parameter means that a single sensor may return more than one measurement. For example, some water quality sensors return temperature, conductivity, dissolved oxygen, pH, turbidity, and depth.

What advantages does SDI-12 possess for environmental data collection?

A serial-digital interface is a logical choice for interfacing microprocessor-based sensors with a data recorder. This has advantages for sensors and data recorders.

In what applications are SDI-12 devices used?

SDI-12 is used in water resource research and management, industry, government, and agriculture. For example, the U.S. Geological Survey uses over 4000 SDI-12 sensors in its data collection networks. SDI-12 sensors are available that measure the following:

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