Instrumentation evaluations are difficult to accomplish in the
field. Independent evaluations by users and associations prove
to be the best sources for determining whether a particular
instrument will work in a given application. Manufacturers’
specifications can provide many details about the instrument,
but comparing apples to apples is sometimes difficult.
Instrumentation undergoes rapid changes every year. The
pressure on manufacturers to implement techniques that are
novel in theory proves to be a challenge. The development
cycle is furious, resulting in instruments that do not meet
users’ expectations. This results in:
1. Delays in design/engineering, construction, and startup
2. Increased cost for commissioning
3. Decreased performance of the plant’s process or
adverse consequences on personnel, plant, or the environment
End users cannot assume these risks in today’s market.
Although the supplier usually corrects problems with the

instruments, the cost of any resulting damage is still paid by
the end user.
Independent evaluation has become difficult for users
whose time constrains do not allow them to perform tests
that verify certain design criteria of a particular instrument.
When it was found that cooperative evaluations could substantially
reduce the costs and workloads for the individual
users, internationally orientated instrument users’ associations
were founded.
EVALUATION RESULTS
Statistics show the potential problems one might expect from
an instrument (see Table 1.11a). Improvements have increased
over the years. This implies that evaluation results have provided
manufacturers with feedback that benefits the user. The
types of instruments evaluated are shown in Figure 1.11b. It
is desirable for a manufacturer to have a good quality management
system, based on internationally accepted standards,
such as those of International Standards Organization (ISO),

exchange of reports and other information. The three associations
together have approximately 100 members, covering
many multinationals in markets varying from oil, chemical,
steel, food, and beer to utilities such as the electricity, gas,
water, and nuclear industries.
EVALUATION METHODOLOGY
Evaluations of instruments and systems aim at confirming
the claims made by manufacturers’ specifications. Where
these are lacking, user requirements or relevant IEC/ISO
evaluation standards may be used. The methodology in use
in the evaluation laboratories of TNO, Netherlands, provides
guidance to the engineers involved in the evaluation. An
evaluation is an iterative process in which the evaluator needs
the involvement of user and manufacturer for setting up the
evaluation objectives and techniques. Evaluation is becoming
increasingly complex because of the introduction of sophisticated
electronics and software-based data processing, measurement,
and communication techniques.
Evaluation requires an analysis of four main aspects of
the device being tested, as follows:
1. System configuration in terms of functional and physical
modules
2. Functions relevant for the user
3. Properties relevant to the quality of the functions
4. External environmental and operating conditions that
affect performance
The first three items can be interrelated with the last one
in a multidimensional matrix that has the structure shown in
Table 1.11c. Each field represents a test. In actual practice,
not all possible tests will be performed. The choices by the
parties involved (user, manufacturer, evaluator) may depend
on priorities of the intended application, cost and time constraints,
and knowledge of system performance based on
other applications. The completed matrix then forms the
unambiguous agreement on the evaluation program, which
still has to be supplemented by a description of the tests.
SYSTEM CONFIGURATION
The description of a system to be evaluated is based on the
functional models of Figure 1.11d and the physical mode of
Figure 1.11e.
In the functional model the following modules are distinguished:
• The input module converting the physical quantities
(analog and/or digital) to be measured into signals
suitable for further processing.

http://www.abzardaghigh.ir/duh/doc_download/144-instrumentation-evaluations.html