Quiz
Cross-word Puzzle

As we have mentioned before, the transmitter receives a non-standard signal and transmits a signal within a standard range. A number of signals can be used to transmit the value of a variable. The most common are pneumatic, electronic, and optical.

In most exsisting plants pneumatic and electronic signals are predominant. Pneumatic signals are normally 3-15 pounds per square inch (psi), and electronic signals are normally 4-20 milliamps (mA). Optical signals are also used with fiber optic systems or when a direct line of sight exists.

Radio and hydraulic signals are also used, though they are not as common because of inherent problems such as radio signal interference and leakage of hydraulic systems. However, radio signals commonly are used when sensors and transmitters are great distances (on pipelines, for example) from control centers.

Signal ranges vary, but are important to the calculation of process functions. The concept of signal ranges is synonymous with steady-state gain.

Electronic Transmitter Adjusted Range

This figure shows how the range of the measured variable, on the left, relates to the range of the transmitted signal, on the right. The measured variable is also the input and the transmitted signal is the output. The measurement range is 100 to 500 degrees C. Thus, the span of measurement is 400 degrees C. The transmitter range is 4 to 20 milliAmps (mA), and the transmitter span is 16 mA. At the lowest measurement (100 C), the transmitter output should be 4 mA. At fifty percent of the measurement (300 C), the transmitter output should be 12 mA.

Using standard signal ranges, current output in a process can be readily calculated using the formula for steady-state gain:

Steady-State Gain (K) = Change in Output / Change in Input that caused output change

The gain for this transmitter is .04 (for example, 16 divided by 400). Once the gain is known, the temperature can be calculated from the output of the transmitter. For example, let's say the the transmitter output is 8 mA. Remember, 4 mA is zero percent of span, so (subtracting 4 mA from 8 mA) the transmitter output change is 4 mA. Dividing 4 mA by the gain (.04) produces an answer of 100. The lowest temperature in the range is 100 degrees C. Adding 100 and 100 produces the actual temperature 200 degrees C.

Pneumatic Transmitter Adjusted Range

The figure above shows an example of a pneumatic transmitter. The previous formulas can be used to calculate the adjusted range of a pneumatic transmitter. Using the figure, the input range is 100-200 psi (pounds per square inch) for a 3-15 psi output or transmitted signal range. The span is 100 psi for the input and 12 psi for the output.

The gain for this transmitter is .12. For the same transmitter, a reading of 12.72 psi transmitter indicates a process pressure of 181 psi. Try the calculations, to see for yourself.