Chemical Engineering October 2014 - 37

Pressure measurement PrinciPles
A
pressure transmitter measures the pressure of a gas or liquid and relays that information
back to a control system. This differentiates it from a pressure gage, which
gives only a local indication and does not communicate.
Gage, absolute or differential?
Any pressure measurement is by definition made against a reference pressure. In a
gage measurement (expressed, for example, as pounds per square inch gage, or psig)
the reference is ambient atmospheric pressure. An absolute pressure measurement
(psia) measures against vacuum, while differential pressure (DP) measures the difference
between two pressures.
While the uses of gage and absolute pressure measurements are generally fairly
straightforward (what is the pressure in the vessel?), differential pressure measurement
has much wider application. It is used in flow measurement, to measure the pressure
drop across an orifice plate or other device.
DP is also used in level measurement, to measure the depth of liquid in a tank: if the
density of the liquid is known, its depth can be calculated. But since many tanks are not
at atmospheric pressure (for many reasons, including the vapor pressure of the liquid
in the tank), a DP gage is used, with one side connected near the bottom of the tank
and the other near the top, above the surface of the liquid. (For more on DP transmitter
applications, see Part 2 on pp. 41-44).
Process connection
Figure 2. An in-line mounted transmitter
has a single connection to the
process (for gage or absolute pressure)
at the bottom of the unit
connection. The coplanar connection
enables measurement of differential,
absolute and gage pressure
applications.
A biplanar connection is a more
traditional way of connecting to the
process (Figure 4) and has two ports
on the side of the lower part of the
unit. This is the original process
connection used for DP measurement;
it supports gage, differential
and absolute pressure measurements.
It is heavier and more challenging
to connect than in-line or
coplanar designs.
A transmitter used in a DP flow
application can also be mounted
directly to a flange containing an
orifice plate, as shown in Figure 1
(second from the left).
When considering a connection
type, one should ask: is there a process
connection point available, or
will it be necessary to add a connection
point or tap into the process? Either
of these may require a process
Parts of a pressure transmitter
The major parts of a pressure transmitter are the pressure-sensing element, which transforms
pressure input to an electrical signal, an isolating diaphragm and a housing that
includes the necessary electrical interfaces.
Sensing element. There are three main types of pressure sensors in common use in these
applications. In a capacitive sensor, a diaphragm is one plate of an electrical capacitor.
An electronic circuit detects changes in the capacitance as the diaphragm flexes under
pressure. In a vibrating wire sensor, a wire that vibrates at its natural frequency is connected
to a diaphragm. Changes in force on the diaphragm change the tension in the
wire and hence its frequency. The electronic circuit that drives the vibration detects the
change in frequency and outputs it as a pressure reading. In a piezoresistive sensor,
a strain gage placed on a diaphragm changes its resistance as the diaphragm flexes
with pressure changes. In many modern sensors the sensing element is contained on a
silicon microchip.
Isolating diaphragm. With the exception of simple applications (measuring the pressure
of air or a noncorrosive gas), it is imperative to keep the process medium from reaching
the pressure-sensing element. This is most commonly done with an isolating diaphragm.
Isolating diaphragms are available in a variety of materials to meet the needs of different
process fluids. As pressure is applied to the process, the isolating diaphragm flexes
and transfers the pressure to the internal pressure-sensing element via a small volume of
oil. The pressure sensor then transforms the pressure input to an electrical signal.
Module housing. The pressure sensor is protected by the module housing, which supports
the isolating diaphragms, protects the pressure sensor and provides an electrical
connection for the transmitter housing. Transmitter housings are available in three basic
configurations for different applications. The dual-compartment housing is the most
common type. It separates the terminal block from the output electronics. It allows for
advanced functionality and allows for use of either an LCD display or a local operator
interface. A single-compartment transmitter housing is less common. It contains just a
wiring termination and junction box housing. A quick-connect transmitter housing is
compact and lightweight, and simplifies field wiring.
❏
shutdown, which can be costly and
potentially dangerous. It is also possible
to hot tap a process, a procedure
that requires highly trained personal,
but can keep the process up and running.
A new connection is usually put
in place during scheduled downtime
or if the location can be bypassed.
Other things to consider regarding
the connection are as follows:
* Can a flange be added to make the
connection? If so, what type of flange
is appropriate for the application?
* What threading is present?
* Is a shutoff valve available?
It is often impossible simply to
put a pressure measurement point
wherever you want it; you can put
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Chemical Engineering October 2014

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