Principles of Float & Tape Tank Gauge Transmitters
Tank gauge transmitters vary in capabilities and application, from float & tape driven switches for the indication of alarms or relays, to level and temperature transmitters that can be mounted to all standard float gauges. When a tank gauge transmitter is used, communications and power are required at the gauge head.
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, optical, magnetic or the latest capacitive sensors.
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 backup 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.
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. The latest capacitive sensors provide the most accurate output to date.
Do I need a transmitter on my float & tape gauge?
If you have a stand alone float and tape gauge, such as a 2500 ATG, currently installed and can answer yes to any of the following requirements - you need a Varec tank gauge transmitter.
- Visualize gauge data via your control room computer system?
- Integrate temperature measurement?
- Use standard net volume or mass calculations for accounting?
- Integrate gauge data into PLC or DCS systems?
Temperature sensors connected to the transmitter also allow the inventory system, such as FuelsManager®, to calculate volumes that are temperature corrected. Spot temperature sensors are appropriate for float & tape tank gauging, while average temperature devices are generally associated with radar or servo tank gauges due to the increased accuracy obtained from the level device.