Page 104 - Mechanical Engineers' Handbook (Volume 2)

6 Resistance Bridge Transducer Measurement System Calibration 93

As alluded to earlier, the addition of a balance network to a bridge transducer may react
unfavorably with temperature compensation resistors placed in the transducer’s circuitry by
its manufacturer. Temperature compensation can be severely modified by the presence of
this balancing network. The prerequisite to insertion of a balance network should be an exact
knowledge of the circuit of the transducer. For this reason, and for reasons associated with
desensitization of the transducer, balance networks should be avoided unless required.

6 RESISTANCE BRIDGE TRANSDUCER MEASUREMENT SYSTEM CALIBRATION
A basic component in any measurement system is the transducer. The measurement system
can be as simple as a transducer, a cable from the transducer, and a recorder. Alternately,
the measuring system can contain many more of the elements of the transmitting and re-
ceiving system defined in Section 1. Cables, amplifiers, filters, digitizers, tape recorders, and
so on all have the capability, when inserted into a measurement system, to modify both the
amplitude and spectral content of the signal from the transducer defining the measurand. The
response of these components may also drift with time.
To obtain measurements of the highest possible quality, one must accurately and care-
fully calibrate the entire measurement system as near to the time of actual measurement as
possible. The calibrations may be conducted prior to, immediately after, or even during the
time of actual measurement. Such calibration of an entire measurement system is referred
to as ‘‘end-to-end’’ calibration. This calibration ordinarily does not replace the evaluation of
individual components of the measurement system.
One group concerned with ‘‘end-to-end’’ calibration of measurement systems is the
Telemetry Group/Range Commanders Council whose Secretariat is headquartered at White
Sands Missile Range, New Mexico. The Transducer Committee of the Telemetry Group
coordinated the writing of Chapter 2 of Ref. 9, entitled ‘‘Test Methods for Transducer-Based
System Calibrations.’’ The following information is largely extracted from that chapter, which
also deals with piezoelectric transducers, servo transducers, capacitive and inductive trans-
ducers, and thermoelectric transducers.
A preferred calibration procedure is one in which a known value of the measurand is
applied directly to the transducer in the measurement system (transmitting, receiving, and
display) in which it will be used. This procedure permits the output display to be read directly
in terms of units of the measurand.

6.1 Static Calibration
The basic equipment for the static calibration of transducer systems consists of a measurand
source supplying accurately known and precisely repeatable values of the measurand and an
output-indicating or recording system. The combined errors or uncertainties of the calibration
system should be sufficiently smaller than the permissible tolerance of the system perform-
ance characteristic under evaluation so as to result in meaningful calibration values. All
calibration system components should be periodically checked against standards. Environ-
mental conditions during calibrations should be constant and specified to permit corrections
to the data, as required.
The procedures which will be specified are based on the assumption that the measuring
system is linear. For systems that will ultimately measure dynamic data, linearity is a pre-
requisite.
The static calibration sequence consists of the following steps: