Transmitters convert the measurement parameter (rotation, resistance, etc.) to an electrical signal for transmission over an instrumentation field bus or communications loop. Most transmitters use an incremental or absolute encoder with either brush or optical sensors.

Encoders - Incremental encoders count steps up or down to find level and then store the information in memory (think of it as a digital watch). A power loss can causes the transmitter to forget where it is so recalibration or a battery back-up may be required. Absolute encoders count the absolute position on the encoder disc to find level (think of it as an analog watch). After a power loss the encoder knows where it is, so no battery backup and no recalibration is required.

Sensors - Brush sensors make contact with the encoder disc causing the encoder to wear down over time (every 5-8 years) and brushes may “feather”, which will affect the accuracy or reading. Optical sensors, either linear or reflective, do not make contact with the encoder disc. Sunlight may affect calibration (except with infrared sensors) and photo-etched (painted) discs may fade, affecting the reliability and accuracy of the reading.