Chemical Engineering December 2016 - 70

down and measuring accuracy in
the mechanical differential pressure
measurement are independent
of the pressure range - at
least when the temperature difference
is low.
Selection criteria
In addition to the desired accuracy,
a number of other factors are important
in the selection of a DP technology.
Table 2 provides a concise
comparison of the performance of
mechanical and electronic DP measuring
systems for a number of different
criteria.
FIGURE 2. This photo shows flow measurement via an orifice plate and mechanical DP measurement.
The static pressure is measured by an additional sensor
sures of up to 9.140 psi (630 bars);
minimum pressure differences of
a few millibars; high process temperatures
of up to 750°F (400°C);
strong temperature fluctuations; extreme
continuous vacuum; suction
and pressure surges; abrasion by
the measuring medium; and more.
As will be seen below, these operating
conditions can help in selecting
the DP technology that is most suitable
for a given application.
Calculating total deviation
The total deviation of a pressure
transmitter indicates the maximum
measuring error to be expected in
practice. According to the German
Pressure Standard DIN 16086, the
total deviation Ftotal is the sum of the
basic accuracy Fperf and the longterm
stability Fstab:
Ftotal
= Fperf
+Fstab
(1)
The basic accuracy Fperf is made
up of the thermal change of the zero
signal and output range FT as well as
the measuring deviation FKl:
Fperf
2
= FT
+FKI
2
(2)
Initially, this only applies to the
digital signal output via HART, Profibus
PA or Foundation Fieldbus. For
a 4-20-mA output, there is also the
thermal change of the current out64
put
Fa:
Fperf
Application examples
The following three examples illustrate
specific
applications
where
2
=
FT
+FKI
2
+Fa
2
(3)
Where:
Ftotal = Total deviation
Fperf = Basic accuracy
Fstab = Long-term drift
FT = Thermal change of zero signal
and output range (temperature error)
FKl = Measuring deviation
Fa = Thermal change of
the current output
Calculation example
As an example, consider
the measurement of level
in a pressure vessel. Using
the German Pressure
Standard DIN 16086 as
outlined above, we can
calculate and compare the
total deviations for both
the mechanical and the
electronic DP measurements.
This is done for
four different examples, as
shown in Table 1.
Table 1 shows that the
electronic DP measurement
delivers a higher
measuring accuracy, especially
at large temperature
differences. This
is also the case with a
high turndown ratio. But
it also shows that turnelectronic
DP measurement can be
advantageous over traditional mechanical
DP technology.
The pharmaceutical industry. In
this example, solvents for an active
ingredient extraction are recovered
in a distillation tower that
is approximately 8.2 ft (2.5 m) tall.
Evaporation talkes place at about
120-140°F (50-60°C) under a
vacuum of approximately 0.75 psi
(50 mbar abs).
Mechanical DP systems with
FIGURE 3. Shown here is a master sensor for an electronic DP
measurement in a distillation tower
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
DECEMBER 2016

http://WWW.CHEMENGONLINE.COM

Chemical Engineering December 2016

Table of Contents for the Digital Edition of Chemical Engineering December 2016