April 2011 - Control Global

a s k t h e e x p e r t S
cations state if the sensors were individually tested (or only
sampled), and state that the drift was measured, say, “for a
period of X hours and was found to be Y% of output span.”
Also. as I have illustrated in Figure 1, the total error (inaccuracy)
of a measurement is the sum of its systematic and its
random errors.
The error caused by drift (some also call it shift), illustrated
in Figure 2, is the difference between the specified
and the actual performance of a sensor over a time period.
The total drift error is the sum of two error components: the
zero and the span shifts over some time period. When running
a test to determine these values, one has to be careful
to evaluate the whole system, not only the transmitter electronics.
Let me illustrate that point by a recent experience:
In connection with a lawsuit I, as an expert witness, was
asked to evaluate the performance of a flow loop that was
periodically calibrated only by checking the electronics of
the transmitter using a simulator (not the actual signal from
the sensor). Based on the simulated input signal to the analog
transmitter (the secondary), the calibration appeared to
be fine; the transmitter was generating a 4-mA output when
the simulated input corresponded to a zero flow signal, and
20 mA at a simulated 100% measurement signal. Yet, while
the transmitter correctly measured the simulated input signal,
the actual signal from the sensor (because of zero and
the span shift) has drifted so much over the decades of uncalibrated
operation of the primary that the total system error
amounted to 50% when the flow averaged about 20% of
span. (Naturally, as shown in Figure 2, this error percentage
becomes a smaller fraction of the total flow.)
Therefore, the evaluation of the drift should also be based
on testing the primary and the secondary together using the
actual measurement signal and not a simulated one.
Stability is a more general term, as it can include not only
drift (which is a function of the passage of time), but also
the environmental effects (pressure, temperature, humidity,
vibration), process effects (coating, corrosion, aging) and
other factors, such as cycling, hysteresis, linearity, noise, etc.
Offset (some also call it droop) is a term used in connection
with proportional-only controllers because such devices
(thermostats, pressure regulators, etc.) start making a correction
only after an error has already developed. The amount
of offset rises as the gain of the controller drops (proportional
band increases).
Bias occurs when a constant amount is added to or subtracted
from a signal. For example, zero shift can bias the
output of a sensor. A positive bias of a sensor output results
in over- reporting and a negative bias in underreporting the
variable measured.
Béla Lipták
liptakbela@aol.com
100
% Output y
0
0 % Input x
100
the drift test
Actual calibration curve
Figure 2. The results of a drift test should state the time period of
testing and should report the errors caused by both the zero and
the span shift that was measured at the end of that period over the
full range (span) of the sensor.
Q
I read your article on steam quality measurement
with great interest. I am working in a thermal project
pilot plant and would like to measure the steam
quality at the injection well where the line size is 3 ins.
Can you give me some information on “throttling calorimeters”?
Also please recommend a steam quality measurement
device that is not cumbersome to measure SQ in the
3-in. line.
Vijay Ramlal
VRamlal@kockw.com
A
Standard technique in large power plants in the United
States to assure steam quality on super heated steam is
to measure sodium ion concentration, usually in ppb
in two or more locations. Particular attention should be paid
to obtain representative samples before and after the mud
flow cycle. Several companies offer products, but sample
conditioning is difficult. After several manufacturers’ trial
application, I found one English company’s product to be
easy to maintain and reliable. You can write to me directly if
you need more information.
Ram.G.Ramachandran
ramacg@cox.net
Specified characteristic curve
Span
Span shift
Zero shift
Elevation
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