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Field-Checking and Maintenance of HVAC Humidity and CO2 SensorsWebinar – October 5th, 2016Lars Stormbom & Maria Uusimaa
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Meet the presenters
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Lars Stormbom. Product Manager at Vaisala, with over 30 years of experience of industrial and HVAC measurements.
Maria Uusimaa. Product Manager for Vaisala’s handhelds and industrial transmitters. She has over 10 years of experience in CO2 measurements in various measurement applications.
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Covered Topics
The importance of field-calibration in HVACWhat causes RH and CO2 sensors to drift?How to check RH and CO2 measurement reliably on the fieldField-checking options
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Why calibrate in HVAC?
Why is calibration important? Anything that is worth measuring is worth measuring right. All electronics and sensors are subject to drift over time.
What kind of energy savings can be achieved with calibration? According to the REHVA ETIAQ project DCV can save 20-50 %
energy in public buildingsWith poorly functioning sensors you may loose part or all of that
potential saving
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Field Calibration
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Calibration or adjustment?
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Calibration = comparing the output of a measurement instrument against a reference instrument and merely reporting the result
Adjustment = changing the output to correspond to the output of a reference instrument
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When to calibrate? Depends on the accuracy
need of the measurement and the environment. No single correct answer! Are there stability
specifications? Do you have a back record of
drift values? Start with a shorter inspection
interval – lengthen gradually
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Field check vs adjustmentA good strategy may be to first perform field checks against a
reference. This should be relatively fast to perform. If the observed difference is larger than a trigger value, for
instance: <2 %RH difference: Do nothing 2 ... 4 %RH difference: Make a one-point field adjustment >4 %RH difference: Send device for service/ Change the
measurement module
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One-point or multipoint?One point adjustment:
The measured condition varies only 20% of the measurement range Usually OK for HVAC
Multipoint adjustment: If the whole measurement range is used Usually requires laboratory adjustment If a large correction is indicated in the filed check
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You choose the maintenance strategy –we support it
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Do nothing because the specified stability is good enough
OR
Use convenient field checking and adjustment options
Buy field exchangeable measurement modules – with certificate!
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Relative Humidity
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How can we measure water vapor?
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Water is unique among common ambient gases
N2 CO2H2O
It has a positively and a negatively charged end
+--
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Solution: capacitive measurement
Water molecules align with alternating electrical field between two electrodes.
- -+
- -+
Energy stored and increased capacitance
Electrodes
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With a polymer between the electrodes we can multiply the number of water molecules and increase the sensitivity
air Polymer
Use a polymer film to increase sensitivity
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What can cause drift of RH measurement?
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• Cleaning• Detergents, floor
wax
• New materials• Chipboards• Paint
• Stored materials• Packaging materials• Truck exhausts
A humidity sensor must breath the ambient air – it can´t be hermetic like a temperature sensor
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What does humidity sensor drift look like?
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Vaisala HUMICAP®
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The HUMICAP® Advantage
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Vaisala HUMICAP® Technology
We use in-house synthesized polymers for best stabilityThe HUMICAP® structure with a porous upper electrode
makes the sensor insensitive to chemicals as well as dust and dirt
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Humidity calibration options
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Calibration Options for RH
Reference instrumentUsing salt solution calibrator - HMK15Exchange of module and probesSending the product to Vaisala service
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Using a handheld device Handheld devices are especially good for quick pass/fail checks, but can
with a bit of care be used also for one-point adjustments in the field. Allow enough time for the temperature to stabilize. Remember that an 1°C
temperature difference can mean a 6 %RH difference. Graphical displays are good tools to check stabilization. Avoid heating the handheld sensor with your hand. Avoid breathing on the sensor and the device you are checking. Some wall sensors give off a lot of heat – keep the handheld below a wall
sensor. Get close enough to the checked device – there are significant temperature
gradients in most rooms! For duct sensors it is good to plan for field checking in advance: Make a
sealed hole for the handheld probe when installing the duct sensor.
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Using salt calibrators Can be used for 2 ... 3 point calibrations at room temperature for applications
where accuracy is needed over a larger humidity range. A fixed set of humidity values are available (e.g. 11%, 33%, 54%, 75%, 97%) Can be used with an accredited probe as reference if needed Suitable only for probe type sensors Relatively straightforward to use When properly used gives highly repetable and reliable results Sensors are usually removed from the site a calibrated in a lab or other suitable
space with relatively stable temperature conditions – not really field calibration? Requires ~30 min per humidity point depending on the calibrated sensor Bringing the sensor into the same space as the calibrator well in advance for
temperature stabilization speeds up the process. Again: it is good practice to monitor stabilization of the readings!
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Exchaging probes and modules
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Exchangeable measurement modules or probes can be an easy way to maintain measurement accuracy and traceability, especially if you have only a few units Make sure you get a calibration certificate with the new probe/module One drawback is that there is no information on the performance of the
old measurement. Has there been drift? How much? Is the exchange interval suitable? May be combined with spot-checking with a handheld to determine if
some units need changing
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Carbon Dioxide
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CO2 Technology in HVACOptical (infra-red) measurement technologyLight source, filter and detector CO2 measurement at 4.26 µmAttenuation in intensity gives the CO2 concentration (e.g. 1000
ppmCO2)Changes in sensor structure cause drift: Light source intensity change Dirt in optical cuvette Any mechanical changes!
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One IR source, one filter CO2 absorption wavelength
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Ways to Overcome Drift: Adding Components
Two light sources
OR
Two filters and detectors
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Two IR sources, one filter reference source
One IR source, two filters CO2 and reference wavelengths
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Ways to Overcome Drift: Using Software Algorithms
ABC (Automatic background calibration) Zeroing against an assumed
background CO2 concentration (~400 ppmCO2)
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One IR source, one filter CO2 absorption wavelength
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Vaisala CARBOCAP® - It´s unique
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Vaisala CARBOCAP® sensorOne IR source, one tunable filter CO2 and reference wavelengths = true referencing!
Field calibration options for CO2
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Field calibration options for CO2
Reference gasReference instrument – a handheld
Exchange of modules or probesSending the product to Vaisala service center
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Making a good CO2 measurement in the fieldChoose a reference – either reference
gas or a reference instrumentAllow time for calibrationCalibration is best done in a stable
environmentNote temperature and pressure settings,
compensate if possible!
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CO2 field adjustment using calibration gas
CO2 adjustment against high accuracy bottled gasAllows multipoint calibration!
With gas bottle: 0.3 ... 0.5 l/min gas flow to gas port approximately 2 minutes of stabilization time
Vaisala does not sell gas bottles
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CO2 field adjustment
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One good solution is using Linde gas Ecocyl
Integrated flow adjustment with preset flow rates: 0.25, 0.3, 0.5, 1 l/min On/OFF valve Pressure meter Light-weight aluminiun cylinder contains 150 l gas
with ±0.5% accuracy specs. Good for about 75 adjustments with 2 min stabilization time at 0.5 l/min
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Reference instrument – a handheldCalibration against a freshly calibrated
hand-held deviceEasy to use!Vaisala CARBOCAP® Carbon Dioxide
Meter GM70 is a good choice for CO2 field checks Add HM70 probe for simultaneous humidity
and temperature check
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Exchange of modules
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Calibration orders via:http://store.vaisala.com (US & Europe)or www.vaisala.com/calibration
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We are happy to calibrate Vaisala instruments for you. Information at:www.vaisala.com/calibration
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Vaisala Service Agreements
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Summary
Define the approach for field-calibration based on your measurement needs!There are plenty of easy-to-use options for Vaisala HVAC
products
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Thank You
Download the HVAC eBook, see videos or get other useful information at:www.vaisala.com/hvac
