What is a Bilinear Output?

The typical 4 to 20mA analog signal is a linear output that represents the measurement range of a sensor. For example, the 4 to 20 mA signal of a VisiFerm Optical DO Sensor may be scaled to match a dissolved oxygen reading of 0 to 62.85% O2 by volume. This can be seen in the graph below.

A bilinear output splits the 4 to 20mA signal into two parts. The first half uses 4 to 12mA with the second half uses 12 to 20mA. If we consider the example above, 4 to 12mA portion of the signal may be scaled for 0 to 10% O2. The 12 to 20mA half of the signal can be scaled from 10 to 62.85% O2. Once again this is shown in the graph below.

What Measurement Applications use Bilinear Output?

Bilinear outputs work well for process analytics applications where high resolution is required for a portion of the measurement range but a much larger measurement range must also be captured.

Low Dissolved Oxygen Measurement

In applications such as boiler water monitoring low dissolved oxygen must be maintained to prevent corrosion. Oxygen is monitored throughout the system to ensure proper chemical feed control for deaeration.

The bilinear 4 to 20mA output is used in same way as illustrated by the graph above. Low range oxygen measurement is scaled for the 4 to 12mA signal to provide high resolution for small changes in the DO value. During process start-up or if a leak occurs, the oxygen value may be much higher but still needs to be monitored. The 12 to 20mA signal is used to capture the higher oxygen values.

Purified Water Conductivity Measurement

Conductivity measurement is used to determine the quality of purified water. In these applications, dissolved solids are removed from the water through filtration, ion exchange resins or other methods. Low level conductivity measurements are represented by the 4 to 12mA half of the analog signal. If a failure occurs such as breakthrough of the filter, then the water is contaminated and the conductivity will increase quickly. The 12 to 20mA portion of the signal is used to capture the high range conductivity until the process can be corrected.

This same idea of using bilinear conductivity may also find applications in monitoring steam condensate return lines, heat exchanger leak detection, and other applications where maintaining water purity is critical.

Related Content

  • Analog output signals are often connected to PLCs. Learn about different wiring schemes to connect to these devices.
  • Local displays from other vendors can also be used with Arc sensors. See an example in this article.
  • Visit our Main Arc Knowledge Base Article Page to see all articles on Arc Intelligent Sensor Technology.