Transmitter Function

The signal transmitter converts a low level signal from any sensing device into a standardised process signal. These transmitters sometimes incorporate electromechanical devices such as strain gauges for measurement of physical values - pressure, flow, level, etc.

Signal transmitters can;

• Convert one specific type of signal into another type of signal
  Input: Thermocouple type J 0-800°C
  Output: 4-20mA.
  May provide galvanic isolation from one signal (input) to another (output).
  Note that APCS defines a Signal Isolator as a transmitter with isolation.

When choosing a a transmitter please consider the following;

• Input signal or measurement.
  
• Output and what powers the signal conditioning module are a major determining factor when choosing a transmitter.
• Is isolation required?

Most transmitters have two adjustments in common which are termed OFFSET (zero) and SPAN (gain). These adjustments allow the output signal to be varied considerably, generally 20% of the transmitter range.

The principle used in the calibration of transmitters is that if two points on a straight line are established then the line itself is established.

By examining Figure 2 it can be seen that at low magnitudes of input signal the effect of span adjustment is very small. Consequently the first calibration point should be selected close to the bottom of the input range. The offset adjustment is then used to give the required output signal for the input signal being fed to the transmitter.

Because alteration of the span results in a higher output deviation at the top end of the input range it follows that the second calibration point should be at this top end, say in the top 25%. Thus the calibration at this point is achieved by imposing an input signal of suitable magnitude and adjusting the span potentiometer to give the correct output.

In practice it is sometimes difficult to eliminate the interaction effects between offset and span so it is recommended that the procedure for setting these two points be repeated until both points are obtained without the need for further adjustments.

The principle used in the calibration of transmitters is that if two points on a straight line are established then the line itself is established. By examining Figure 2 it can be seen that at low magnitudes of input signal the effect of span adjustment is very small. Consequently the first calibration point should be selected close to the bottom of the input range. The offset adjustment is then used to give the required output signal for the input signal being fed to the transmitter.

Because alteration of the span results in a higher output deviation at the top end of the input range it follows that the second calibration point should be at this top end, say in the top 25%. Thus the calibration at this point is achieved by imposing an input signal of suitable magnitude and adjusting the span potentiometer to give the correct output.

In practice it is sometimes difficult to eliminate the interaction effects between offset and span so it is recommended that the procedure for setting these two points be repeated until both points are obtained without the need for further adjustments.