IEA Annex 40 Final Report Subtask B2 Development of Functional Performance Testing procedures
Pressure Sensors in HVAC systems Date: 02/06/2005
Authors: Cleide APARECIDA SILVA Jules HANNAY Jean LEBRUN University of Liège Laboratory of Thermodynamics LIEGE Christophe ADAM Philippe ANDRE Patrick LACÔTE University of Liège Department of Environmental Sciences and Management ARLON
1 Description of the considered object This specification is concerning to the pressure sensors installed in buildings and HVAC systems. A more general information about sensors validation is given in reference [15]. The information given in this first chapter is directly extracted from a reference document of IEA annex 40 [16], which should be recommanded for more details.
1.1 Operating principles [16] General consideration about sensors are valid for pressure sensors. Pressure measurement can be performed by various techniques: velocity probes manometers capacitance pressure sensors strain gauges piezoresistive sensors linear variable differential transformers (LVDT) Their characteristics are compared in the following tables. The capacitance, strain gauge, and piezoresistive technologies compete with each other since they offer accuracy at a reasonable cost in a variety of levels of quality. LVTDs compete with the lower end versions of these technologies. Table 1 : Pressure Measurement Technologies [16] Technology
Function
Advantages
Disadvantages
Velocity Probes
Measure pressure differential based on velocity through a tube across the pressure difference.
Can accurately and repeatable measure to thousandths and ten thousandths of an inch water column; good for low pressure applications like clean rooms, building pressure control and isolation room monitoring or control where pressure differentials in the range of 0.05 in.w.c.or less must be accurately measured and maintained.
More expensive that other technologies, sensing line length critical since there is active flow through the lines - long lines could impact the accuracy of the input because the pressure drop through them would affect the flow and flow is the indicator used to measure pressure differential.
Example application: isolation rooms
Manometer Example application: filter pressure drop
Measure the change in height of a column of liquid between a reference pressure and the pressure being measured. Typically a manual instrument used to calibrate and verify other instruments.
Capacitance
Pressure changes cause a change in capacitance between a metal diaphragm and an electrode. The capacitance is measured and used to generate an output signal.
Low hysteresis, high repeatability, high resolution, fast response, and the ability to measure low pressures.
Must be zeroed routinely since temperature or change in physical position can affect performance. May not be as rugged as some other technologies.
Strain Gauge
The deflection of a diaphragm due to pressure change is measured by strain gauges.
High accuracy, long-term stability; very tolerant of extreme overpressurization in some packages.
Strain gauge bond with diaphragm may degrade
Piezoresistive
A pressure change causes the resistance of a semiconductor (solid-state chip) to vary.
Detects larger pressure differences than capacitive transmitters (greater than 5”). Withstands vibrations.
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Linear Variable Differential Transformer (LVDT)
An electric output is produced in proportion to the displacement of a movable transformer core. Usually coupled to a bourdon tube to measure pressure.
High reliability since no mechanical wear/friction between the transformer core and coil. High resolution. Lower cost for a given accuracy spec as compared some other technologies.
Inherent nonlinearity of standard LVTDs is about 0.5% of full scale. Not as rugged or accurate as some other technologies.
Specific (pressure-based) devices are used for differential pressure measurements. They are dealt with in the “Flowrate sensors” specification.
1.2 Data provided by the manufacturers [16] Data are generally provided as "data sheets". A typical example is given by Figure 1.
Figure 1: Example of a pressure sensors data sheet.
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1.3 Problems to be considered [16] 1.3.1 General problems The problems generally concerned with the sensors (as listed in the reference document, [15]) are relevant for pressure sensor.
1.3.2 Specific problems for pressure sensors Furthermore, pressure sensors have to fulfill specific installation and calibration requirements, as described hereafter.
1.3.2.1 Pressure transients For all of the devices presented hereabove, pressure transients in the air or water may cause erroneous readings. For example, pressure pulses from the pumps can create noise in the pressure measurement. A pressure pulse can also be created from a door opening that is near a diffuser location and near the duct static pressure measurement point. In this case, the pressure pulse can cause the control loop to hunt.
1.3.2.2 Air pressure measurements Most often, static pressures have to be measured: dynamic pressure effect has to be avoided. More over, static pressure measurements are very sensible to turbulence effects and the localization of the measurement sensor has to fulfill a number of requirements: - no measurement in duct elbows - measurements are usually located at duct mid height - no measurement upstream of an other sensor (temperature, humidity) - use special fixing devices normally located on the duct wall
1.3.2.3 Water pressure measurement Use the same gage for both pressures when determining a pressure difference. When making water pressure difference measurements, never use two separate gages for the two measurements. Either use one gage in the system or use your own gage for both measurements. Do not trust gages in the mechanical room. Have a healthy skepticism about the accuracy of gages you find in the system. If a reading is reasonable and you can make a pressure change that you are reasonably certain of, and the gage readings are both appropriate, go ahead and use the existing gages. If you need a precise measurement, use a recently calibrated gage. Make height correction. measurements
Be sure to consider height differences when making ∆p
Fill the host with water. Be sure to completely bleed the host so it is filled with water when making pressure readings.
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2 Description of the testing procedures For commissioning pressure sensors, all methods proposed for verification of sensors are applicable
2.1 Visual observation of measuring results 2.1.1 Summary of the test specifications This "basic" method consists is observing the results of the measurement of one or several variables and to check there is no major apparent problems in the measurement. Typical "obvious" problems are : - interruption of measurement - values out of range for a long time - apparent random evolution of measurement Objectives and sequence of the test: Detection of anormalities in the sensors readings List of operational conditions to test: Current operation of the HVAC system Requisite: None Required material: None Time required for the test execution: Short : time required for observation of measurement (display on screen, printing of output file, ...)
2.1.2 Preparation phase: evaluation of available data and of expected performances -
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Measurement points available for the test : all pressure measurement points are potentially concerned by this test procedure. No specific measurement technique required. Data helpful for this testing procedure: - Information about problems having occured in the past; - Technical data about sensors : expected "noise", theoretical time constant, expected range of variation... - Scheme of the HVAC system showing sensors location. No additional instrumentation required.
2.1.3 Execution phase
2.1.3.1
Summary of the test method
This "basic" test method consists in visually observing the evolution of typical sensors readings on the BEMS.
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2.1.3.2 -
Select typical variables to display on the BEMS; Observe evolution of selected variables; Optionally print selected graphs or save for future processing.
2.1.3.3. -
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Experimental method
Contents of the test report
Date and time Current operation of the system List of selected variables For each selected variables : - typical evolution - comments Summary of test results.
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2.2 Visual observation of the sensors (in the plant) 2.2.1 Summary of specifications This simple method consists in observing each selected sensor on the plant, to check for problem in relation with: -
The type of sensor installed; Its location; Its configuration and of the part of the measurement chain which is "accessible"; The current state of the sensor : hardware failure, disconnection...
Objectives: Detection of anomalities in the sensor type, location, placement, maintenance, current state. Operating conditions: Current operation of the HVAC system. Pre-requisite : None. Required material: None. Required time: Short.
2.2.2 Preparation phase: evaluation of available data and of expected performance Cfr § 2.1.2.
2.2.3 Execution phase
2.2.3.1 Summary of the test method This simple test method consists in inspecting a selection of sensors in the system and in comparing the observed characteristics (type, location, etc.) with expected ones.
2.2.3.2 Experimental method -
Select sensors to be checked Go to the plant Inspect.
2.2.3.3 Contents of the test report -
Date and time Current operation of the system List of selected sensors For each selected sensor : - "A priori" information - Type, location 7
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Status of the sensor Photo, showing the sensor placement
2.2.4 Illustration example Figure 2 shows an example of wrong location: the pressure sensors are located too close to pipe bends.
Figure 2 : Wrong location of pressure sensors: too close to piping bends
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2.3 Manual checking 2.3.1 Summary of specifications This simple method consists in comparing “manually” (ie without the use of the BEMS) the response of a sensor with that of a reference sensor (in that case it is very similar to the “physical redundancy” method presented in the next paragraph) or in imposing defined and known conditions to the sensor and checking the response of the sensor. In that case, the testing corresponds to a “calibration” operation.
Objectives: (Re-) calibration of a sensor, detection of drift, offset in the sensors response. Operating conditions: Current operation of the HVAC system or calibration conditions Pre-requisite: None Required material: - For sensors comparison: a reference sensor to be installed close to the sensor to be checked - For sensors calibration: a calibration equipment corresponding to the type of sensor Required time: Short for sensors comparison. Longer for sensors (re-) calibration; the time required depends upon the selected calibration technique.
2.3.2 Preparation phase: evaluation of available data and of expected performance -
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Measurement points available for the tests : all pressure measurement points of the HVAC system are potentially concerned by this test procedure The following data are very helpful for this testing procedure o technical data about sensors installed : type, placement guidelines, ... o initial calibration results o scheme of the HVAC system showing sensors location Additional instrumentation to provide: None
2.3.3 Execution phase
2.3.3.1 Summary of the test method Calibrate the sensors using the data acquisition and processing system and a reference sensor of high quality. Manual sensor checking is used to compare the measurement sensor reading to the reading of a calibrated reference sensor. In practice it is convenient to use the building energy management system (BEMS) including the installed electrical wiring to collect data during the checking procedure. The obvious advantage of this approach is that possible errors in the
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sensor as well as possible errors in the wiring are contained in the reading, making them easier to detect.
2.3.3.2 Experimental method Sensors comparison: -
install reference sensors connect to BEMS or to local recording device launch data collection compare results
Sensors (re-)calibration: -
remove sensor from plant place it in calibration equipment launch calibration procedure (physical variable dependent)
2.3.3.3 Content of the test report -
Date and time Current operation of the system List of selected sensors For each selected sensor : -
"A priori" information Type, location Status of the sensor Photo showing the sensor placement
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2.4 Diagnosis tests 2.4.1 Summary of specifications This method consists in performing test cycles: the system is brought into specific operational conditions where an expected and easily predictible relation between sensors can be observed. The method assumes that the HVAC system in which the sensors are installed can be considered as fault-free. Objectives: detection of the qualitative response of a sensor to a given sollicitation for which the expected response is known. Operating conditions: typical conditions which involve a predictible behaviour of the sensor response (eg: shut down of the ventilation system: all pressure sensors should indicate a “zero” relative pressure). Pre-requisite: the “artificial” configuration of the system should concern fault-free components. Required material: None (if using the BEMS). Required time: Short (a few minutes for each test).
2.4.2 Preparation phase: evaluation of available data and of expected performance -
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Measurement points available for the tests : all pressure measurement points of the HVAC system are potentially concerned by this test procedure. No specific measurement techniques required. Data are helpful for this testing procedure: o technical data about sensors installed : type, placement guidelines, ... o schemes of the HVAC system showing sensors location Additional instrumentation to provide: diagnosis tests are normally using the BEMS. Consequently, no additional instrumentation is normally required to perform the test.
2.4.3 Execution phase
2.4.3.1 Summary of the test method The test method consists in generating (or taking profit of) some specific operating conditions and use the BEMS to observe the behaviour of pre-defined sensors when specific conditions are applied.
2.4.3.2 Experimental method Testing conditions are either generated or are naturally occuring and presenting “interesting” characteristics. In both cases, the method consists in observing (or recording) the behaviour of some selected variables on the system.
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2.4.3.3 Contents of the test report Date and time Selected (or generated) operational conditions For each condition: - Description of the operation imposed - Method to generate (or obtain) it - Variables to observe - Typical evolution - Comments Summary of tests results
2.4.4 Illustration example Figure 3 shows an example of diagnosis of pressure sensors.
Figure 3: Comparison of pressure sensors when ventilation system is forced to shut down
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3 References [1] CYSSAU, R. “Manuel de la régulation et de la gestion climatique”. PYC Editions, 1995 [2] SELLERS et al “HPCBS Control System Design Guide”. California Energy Commission. Public Interest Energy Research Program, 2003 [3] DEXTER, A.L. ; PAKANEN, J. editors. IEA Annex 34 final report, IEA ECBCS, 2002 [4] VISIER, J. Ch. Editor “IEA Annex 40 final report”, IEA ECBCS, 2005 [5] XIAO, F.; WANG, S. “Sensors fault diagnosis foe re-commissioning of AHU monitoring instruments”. IEA Annex 40 working document n° , 2002 [6] WANG, J.-B.; WANG, S.; BURNETT, J. „FDD and soft fault estimation in commissioning BMS monitoring instruments of central chilling plant”. Proceedings System Simulation in Buildings 98, Liège, 1998 [7] NAJAFI, M.; CULP, Ch.; LANGARI, R. “Performance study of enhanced autoassociative neural networks for sensors fault detection”. Proceedings ICEBO’2004, Paris, 2004. [8] YOSHIDA, H.; KUMAR, S.; MORITA, Y. „Online fault detection and diagnosis in VAV air handling unit by RARX model”. Energy and Buildings, vol. 33, n°4, pp 391-410, 2001. [9] BERTON, A.; HODOUIN, « Linear and bilinear fault detection and diagnosis baed on mass and energy balance equations”. Control Engineering Practice, vol 11, pp 103-113, 2003 [10] FRANK, P.M. “Fault diagnosis in dynamic systems using analytical and knowledgebased redundancy – a survey and some new results”. Automatica, vol 26, n° 3, pp 459-474, 1990. [11] ISERMANN, R. “Supervision, fault-detection and fault-diagnosis methods: an introduction”. Control Engineering practice, vol. 5, n°5, pp 639-652, 1997. [12] ASHRAE Fundamentals Chapter 14: Measurement and Instruments [13] CAPUANO et al, "A metrological analysis of a DDC-based air conditioning system", Energy and Buildings, vol 29, pp 155-166, 1999 [14] http://www.nicrom-electronic.com/Pressure_Sensors.pdf [15] APARECIDA SILVA C., HANNAY J., LEBRUN J., ADAM C., ANDRE P., LACÔTE FPT S09: “Sensors of HVAC systems” , Laboratory of Thermodynamics and Department of Environmental Sciences and Management, University of Liège, Belgium, May 2005. [16] LIU M., CLARIDGE D.E., “ Tools and Equipment for Continuous Commissioning”, Annex 40 – subtask B2, March 2001
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