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ctllp05044Mistakes.qxd 4/18/2005 11:29 AM Page 2

THE FOUR BIGGEST MISTAKES IN INSTRUMENTATION ... A
1

Despite ongoing advancements in measure- The problem lies in the straightforward nature
ment and communications technology, instru- of instrumentation projects: each variable to be
menting a process for feedback control measured must be matched with the most
remains a technical challenge. Today’s sensors appropriate sensor; the sensor must be
are certainly more sophisticated than ever installed, calibrated, and interfaced to the con-
before, and fieldbus technology has simplified troller; and the information generated by the
many installation issues considerably. sensor must be filtered, factored, and filed in
Nonetheless, much can still go wrong with an order to give the controller an accurate picture
instrumentation project. of what’s going on in the process. This appar-
ent simplicity is what often leads to a false
sense of security and missteps in a minefield
of potential problems.

With that in mind, here are four circumstances
you definitely want to avoid.

>> Mistake #1: the rate at which a gas or liquid occurrrence would cause the con-
Selecting the wrong has been passing through a partic- troller to throttle back the flow rate
sensor ular section of pipe, but not all flow more than necessary, preventing
meters can measure all flows. A the required volume of fluid from
T Teecchhnnoollooggyy mmiissmmaattcchh magnetic flow meter or magmeter, reaching the downstream process.
Although it’s generally obvious for example, can only detect the The problem gets even worse if
what quantity needs to be meas- flow of electrically conductive the pipe is so full of air that it is
ured in a flow, temperature, or materials by means of magnetic only partially filled with liquid, a
pressure control application, it’s not induction. Non-conductive fluids condition known as open channel.
always obvious what kind of flow like pure water will pass through a Although recent technological inno-
meter, temperature sensor, or pres- magmeter undetected. vations allow certain magmeters to
sure gauge is best suited to the Magmeters also have trouble work in such a challenging environ-
job. A mismatch between the sens- distinguishing air bubbles from the ment, mechanical sensors such as
ing technology and the material to fluid in the pipe. As a result, a turbines yield artificially high read-
be sensed can lead to skewed magmeter will always yield an artifi- ings, since a trickle of fluid will
measurements and severely cially high reading when bubbles move the meter’s mechanism just
degraded control. pass through because it cannot as much as a full-pipe flow travel-
This is especially true when sense the decrease in fluid volume ing at the same speed. On the
measuring flow rates. All flow caused by the presence of the other hand, mechanical sensors
meters are designed to measure bubbles. In a feedback loop, this are not affected by the conductivity