INTRODUCTIOX 5 



of the transfer function and of the errors may result. Although such 

 variations ma}^ lead to deviations in the instrument output, for prac- 

 tical reasons they are separated from the instrument errors described 

 under part 2 above. 



For other definitions of error terms, see, for instance, J. D. Trimmer, "Re- 

 sponse of Physical Systems," John Wiley & Sons, Inc., New York, 1950, and 

 C. S. Draper, W. McKay, and S. Lees, "Instrumentation Engineering," McGraw- 

 Hill Book Company, Inc., New York, vol. 1, 1952; vol. 2, 1953; vol. 3, 1955. 

 J. Mandel and R. D. Stieler, J. Research Natl. Bur. Standards, 53, 155 (1954), 

 and Tech. News Bull. Natl. Bur. Standards, 40, 139 (1956). 



C. Output Characteristics 



1 . Type of Output. Input transducers and modifiers have an elec- 

 tric output which may be a voltage, a current, an impedance, or a 

 variation or a time function of these magnitudes. The output from 

 an output transducer is a nonelectrical quantity and is, in general, 

 of a mechanical, thermal, or optical nature. 



2. Useful Output Level or Range. Like the input range, the useful 

 output range is limited at the lower end by noise considerations; an 

 upper limit is set by the maximum useful input level. Increase of 

 the output level is frequently technically possible (e.g., a greater gain 

 of an amplifier or a finer suspension of a galvanometer). However, 

 since such increases are frequently connected with an increase in 

 noise level, they may not offer an advantage. 



3. Output Impedance. The output impedance determines the 

 amount of power that can be transferred from the element to the 

 succeeding element or stage at a given output-signal level. If the 

 output impedance of an element is low compared to the input im- 

 pedance of the succeeding stage, the output has the character of a 

 constant-voltage source. If the output impedance is high, the output 

 has the character of a constant-current source. 



The description of instrumentation elements given in this book 

 follows the general outline of characteristics enumerated above, 

 whenever the necessary information has been available. 



The material is organized according to input quantities; e.g., an 

 element that converts pressure into resistance variations will be 

 found under Pressure Transducers ( 1 - 5) . However, only such systems 

 which convert pressure variations directly into electric signals will be 

 found in this section. If the pressure is first converted into a dis- 

 placement (e.g., in a pressure capsule or Bourdon tube) and the dis- 

 placement is converted into a signal by an electrical system, such 

 systems will, naturally, be found under Displacement Transducers 

 (1-2). 



