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Guide to DDC

Chapter 2

Input Devices and Sensors

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.



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