A Low Differential Pressure Transmitter for HVAC Applications
DIN-rail mounting, a small footprint, and a novel method of calibration make the Ashcroft DXLdp well suited to high-end HVAC applications.
Benjamin J. D’Acunto and William S. Kosh Dresser Instrument, Dresser, Inc.
The current methods used for in-place diagnostics and calibration verification of low differential pressure transmitters in high-precision or critical airflow systems are very labor intensive and can lead to problems. Agency regulations typically require monitoring and certified calibration on a regular basis. With conventional units, you must periodically remove the pressure tubing and transmitter wires to check calibration against traceable standards.
TABLE 1
Performance Data
Accuracy Class (F.S.)
0.25%
0.5%
1.0%
Nonlinearity
Terminal point*
B.F.S.L.
0.2
0.15
0.04
0.3
0.8
0.6
Hysteresis
0.02
0.02
0.05
Nonrepeatability
0.03
0.05
0.10
Interchangeability
0.25
0.5
1.0
Based on the Si-Glas, a silicon-on-glass capacitive sensor, the Ashcroft DXLdp is the first of a new generation of low differential
Photo 1. The Ashcroft DXLdp uses a rail-mount design to keep its footprint small. When making a calibration (far left unit), a shorting tube is an excellent way to achieve true zero pressure across the high and low differential pressure ports. The Si-Glas sensor (enlarged) is the DXLdp's fundamental building block.
pressure transmitters (see the sidebar, “The Capacitive Differential Pressure Sensor”). The Si-Glas ensures long-term stability, high performance, and reliability (see Table 1), while the DXLdp’s specially designed valve actuator allows you to quickly redirect the process without having to remove the process lines.
This novel approach does not translate into a larger unit—quite the opposite, in fact. The DXLdp maintains a small footprint by mounting on DIN rails (see Photo 1). Besides making transmitter installation and system expansion easier, rail mounting reduces the DXLdp’s required footprint area by a factor of four compared to other transmitters. An additional advantage is that mounting screws are
Photo 2. With the 4:1 reduction in transmitter footprint, cabinet enclosures can be significantly smaller. Onsite calibration (as with the Ashcroft ATE-100 portable calibrator, shown), can be achieved without disconnecting pressure ports or electrical connections.
required only for the DIN rail and not for each transmitter. This means smaller and less costly cabinets for the same number of transmitters (see Photo 2), reducing installation costs. The DXLdp is designed to snap onto the DIN rail and is easily removed without additional tools.
Spinning the SpoolCal
The DXLdp can be configured with options to allow greater user flexibility. Most important is the SpoolCal actuator valve option (see Photo 3). This clever valve has two positions: CALIBRATE and MONITOR. In CALIBRATE, the transmitter sensor is isolated from the process and allows externally generated test pressure input for calibration. In MONITOR mode, the valve provides online, uninterrupted output of the process pressure without physically
Photo 3. The SpoolCal actuator tool is keyed to index the SpoolCal valve assembly located on the DXLdp transmitter. The tool is shown here with a shorting tube used primarily in the zero calibration mode.
disconnecting the process tubing. In either mode, the pressure can be monitored or recorded with pressure measurement equipment such as a handheld calibrator, as shown in Photo 2. In both modes, online access to the actual electrical output is available through test jacks on the front panel of the DXLdp.
Straightforward Operation
To verify a DXLdp calibration, simply insert the SpoolCal valve actuator probe, referencing high and low pressure. A 90° clockwise rotation isolates the internal sensor from the process pressure. This allows external pressure input through the probe. Shorting the probe’s high and low ports provides a quick and accurate zero differential pressure. Additional calibration points can be generated with an external NIST-traceable pressure source. The ZERO and SPAN adjustments, located on the front panel, are noninteractive.
To activate the monitor mode and make online pressure and electrical signal measurements, simply rotate the probe 90° counterclockwise. This is the process pressure both to the DXLdp sensor and out through the probe tubing to a pressure-measuring device. You can therefore monitor or record an online pressure and electrical signal at the transmitter location without taking the system off line.
Additional Options
Developed for high-reliability and high-precision pressure and flow control in biotech and pharmaceutical applications, the DXLdp is also well suited for demanding HVAC building applications and fab plant air monitoring systems with the requisite pollution control. Common to all these applications is the need for easy access to status information and convenient adjustment of the transmitter. Three additional options address these needs.
LED Display. Provides quick diagnostic information. The display’s range status indicators are especially useful in cases where diverters or fans have failed, or blocked intakes drive the pressure beyond the design range of the system. An amber zero LED indicates little or no differential pressure. Two red LEDs highlight either an overpressure or an underpressure condition. Green LEDs show that the pressure is in the operational range of the transmitters, either bidirectional or unidirectional.
Front Access Test Jacks. Provide convenient, snap-in connections for online output signals without having to remove wiring on current loop controlling units. You can then measure the electrical output with a standard multimeter or data-collecting instrument such as the Ashcroft ATE handheld calibrator.
2:1 Turndown. Lets you rescale the units—in the field—to one-half the upper range pressure limit via a jumper accessed through a window on the side of the unit. For example, a 1.0 in.w.c. unit with 4–20 mA output could be adjusted to 0.5 in.w.c. while maintaining the same output.
The DXLdp, available in 0.25%, 0.5%, and 1.0% terminal point accuracies, provides 10 psi proof pressures for ranges of ±0.05 to 50 in.w.c. Both unidirectional and bidirectional pressure ranges are available with a wide selection of outputs, including 4–20 mA and 1–5, 1–6, 0–5, and 0–10 VDC. The front instrument panel zero and span adjustments are noninteractive, making field calibration and adjustments easy. The DXLdp has been tested and CE certified to exceed radiated and conducted immunity in heavy industrial applications. Additional performance options are available, including adjustable signal response time and custom ranges.
Summary
DIN-rail mounting makes for compact, less expensive, and more flexible installations. Local LED range-status indication lets you quickly diagnose control problems at system startup or when troubleshooting. Turndown allows more output signal when unexpected, lower pressures occur. Best of all, the SpoolCal feature addresses one of the most problematic issues with low differential pressure transmitters: convenient access to pressure and electrical signals. An automated airflow system can now continue to process the transmitter’s signal and at the same time let you monitor or record at the transmitter location while the system operates. All this helps improve system reliability and reduces downtime—a welcome combination for designers, fabricators, installers, and maintenance personnel working on critical and demanding HVAC air control systems.
Si-Glas and SpoolCal are trademarks, and Ashcroft is a registered trademark of Dresser Instrument, Dresser, Inc.
The Capacitive Differential Pressure Sensor
Figure 1. Differential pressure applied to the parts of the Si-Glas sensor deflects the diaphragm in proportion to the applied pressure. The full scale deflection of only a few microns provides a 6 pF change in bulk capacitance that is detected by the circuit.
The Ashcraft Si-Glas capacitive low differential pressure sensor is constructed by sandwiching an etched silicon diaphragm between two pieces of glass (see Figure 1). Before anodic bonding to the diaphragm, the glass is drilled and aluminum is sputtered onto the inner surfaces to create fixed capacitor plates. This metalization is then communicated through the pressure ports to the top and bottom surfaces of the glass plates for electrical connection. The silicon forms the moving center capacitive plate of the sensor in a configuration similar to that of a capacitive potentiometer.
Pressure applied to the sensor’s positive pressure port causes the diaphragm to deflect toward the bottom plate, increasing the capacitance on one side while decreasing it on the other. The imbalance, directly proportional to pressure, is detected by an electronic circuit. The glass forms an overpressure stop that gives the sensor extremely high proof and burst pressures. The use of silicon as a diaphragm material eliminates concerns about elasticity and yield. The sensor’s small size and low mass make it virtually free of any position sensitivity.
Benjamin J. D’Acunto is Sales and Marketing Manager, Transducers, and William S. Kosh is Product Development Manager, Dresser Instrument, Dresser Inc., 2 Research Dr., Shelton, CT 06484; 203-925-4000, fax 203-925-4010, bdacunto@dresser.com, bkosh@dresser.com.
For further reading on this and related topics, see these Sensors articles.
