| Pressure Measurements
Pressure is measured in DDC controls systems for HVAC in
order to control the operation and monitor the status of fans and pumps.
Space pressure is sometimes measured and used for control. Pressure is
also the basis of many flow and level measurements.
Types of Pressure Sensors
Diverse electrical principles are applied to pressure measurement.
Those commonly used with DDC control systems include capacitance and variable
resistance (piezoelectric and strain gage).
Capacitance
Capacitance pressure sensors typically use a capacitance cell (Figure
2.11) consisting of a diaphragm exposed to the pressure medium separated
from another plate by a fill fluid. When the applied pressure deflects
the diaphragm, the capacitance characteristic of the sensing element changes.
The capacitance cell is excited by a high frequency source. The frequency
changes as the capacitance of the cell changes. This frequency shift is
converted to the output signal by the transmitter electronics. Capacitance
transmitters are available configured for either differential or gauge
pressure measurement. Usual outputs are voltage or current.
Capacitance transmitters are available with ranges from
a few inches water column (in. w.c.) to thousands of pounds per square
inch (psi). Transmitter accuracy of 1% of full scale is common for inexpensive
versions. Accuracy to 0.1% of full scale is available with 'smart' transmitters
using microprocessor signal conditioning and compensation. Smart transmitters
can be calibrated using hand-held operator interface devices, or by digital
communication over analog signal wiring using any of several protocols.
Varying grades of transmitter packaging (molded plastic to forged stainless
steel) are available depending on the application and price.
Variable Resistance
Variable resistance technology includes both strain gage and piezo-resistive
or piezoelectric technologies.
Traditional strain gages are constructed of wire filament
bonded to a substrate. The resistance of the wire changes in proportion
to the strain in the substrate, which is transmitted to the wire through
the bond. Strain gauges are applied to diaphragms or other mechanical
pressure elements and change resistance in response to strains induced
in the element by the applied pressure. When arranged to form a Wheatstone
bridge circuit, an analog voltage signal is produced that is proportional
to applied pressure.
Piezo-resistive sensors operate on the principle that certain
semiconductor materials, such as silicon, change resistance with stress
or strain. These piezo-resistive elements are implanted on a solid-state
chip that is attached to a mechanical sensing element or used as the sensing
element. When the piezo-resistive elements are arranged to form a bridge
circuit (as with the wire filament strain gage sensor), an analog voltage
signal is produced that is proportional to the applied pressure.
Piezo-resistive type sensors have a sensitivity of approximately
100 times greater than a wire strain gage. Also, other strain gages must
usually be bonded to a dissimilar force sensing material with different
composition and thermal characteristics. The wire strain gage sensor is
subject to degradation from failure of the bond to the force sensing element,
thermal effects and plastic deformation of the force-sensing element.
In contrast, the silicon based piezo resistors may be integral with a
silicon wafer that serves as the force-sensing element. This eliminates
many of the inherent problems with thermal effects and bonding. Silicon
has very good elasticity throughout the typical operational range and
normally fails only by rupturing.
Strain gage and piezo-resistive transmitters are available
with ranges of a few inches water column (in. w.c.) to thousands of pounds
per square inch (psi). Transmitter accuracy of 1% of full scale is common
for inexpensive versions. Accuracy better than 0.1% of full scale is available
with 'smart' transmitters using microprocessor signal conditioning and
compensation. Smart transmitters can be calibrated using hand-held operator
interface devices, or by digital communication over analog signal wiring
using any of several protocols. Available transmitter packaging ranges
from molded plastic to forged stainless steel depending on the application
and price.
Installation
Process connections for pressure instruments are typically
made using piping or tubing. The majority of applications in the HVAC
DDC field fall into two categories, the first being ductwork and plenums,
and the second being piping.
Ductwork and Plenums
Special sensing tips are often used when connecting pressure instruments
to ductwork for measurement of static, velocity, or total pressures. This
is necessary because improper orientation of an open-ended tube type probe
can result in unreliable readings due to the directional nature of the
pressures being measured (with the exception of very low velocity flow).
Numerous types of pressure probes have been developed for these applications.
Many of these probes are adaptations of the Pitot tube used in pressure
and flow measurement and discussed in detail in the Differential Pressure
Measurement Systems section of this document
Piping
The major considerations for the installation of a pressure element in
a fluid system should include provisions for the following:
- sensor location (pipe mounted, tank
mounted, remote);
- isolation of the sensing element from
undesirable and potentially
- damaging transient pressures, such as
those resulting from water hammer and turbulence;
- temporary isolation from the pressure
source for maintenance and release of trapped pressure when removing the
sensor for maintenance or for setting zero during calibration;
- over-range protection for
differential pressure instruments;
- protection from process temperature
outside of the range of the sensor application;
- venting trapped, non-condensable
gases in liquid sensing piping;
- draining trapped liquids from gas.
Pressure snubbers or dampeners are used to reduce the magnitude
of pressure transients. These can be a sintered metal element with small
openings, a small orifice fitting, a high-pressure drop valve (such as
a needle valve), or a pressurized gas filled container mounted on the
sensing piping.
A variety of valving schemes to provide isolation, venting,
drain, and pressure relief for pressure instruments are shown in the Figures
2.12-2.14. One valve (not shown) or two-valve manifolds are commonly applied
to gauge and absolute pressure instruments. Three- and five- valve manifolds
are used with differential pressure instruments. The equalizing valve
in the three- and five- valve manifold insures a proper zero for the transmitter.
It also allows the pressure to be equalized to prevent exposing low differential
transmitters to potentially damaging gauge pressures during installation
and removal.
Continue with Analog Devices:
Flow Measurements
|