Calibrating Measurement Tools
To quote the wise Yogi Berra, “You’ve got to be very careful if you don’t know where you are going, because you might not get there.” In terms of calibration you could say, “If you don’t know where you started, you won’t be able to tell how far you’ve gone.” With a calibrated test instrument, we can start at the same place every time.
There has been a remarkable transformation in the tools that we use to analyze the performance of homes. Blower doors have been around for only about 35 years and digital manometers, which relied on the development of more sophisticated electronic technologies, are even more recent. It isn’t that the older, analog gauges didn’t need calibration, but the refinements to the data have been such that calibration has increased in importance. Since 1 inch of water column (WC) is equal to 248.843 Pa, we need instruments that can read in tenths of a pascal, and that is virtually impossible with an analog gauge. A Magnehelic analog gauge has minor division of 0.005-inch WC, or 1.244 Pa. When we get down to values that small, the gauge needs to be accurately calibrated so that we have reliable, repeatable values and not just noise.
Table 1. The Pressure Accuracy Requirements of Various Standards
Table 2. Test Equipment Representative Calibration
Not all instruments need to be calibrated (although their operational integrity should be regularly validated). Some diagnostic devices, such as some of the gas leak detectors, provide a simple go/no go type of information. When the device doesn’t provide values and has operational, adjustable sensitivity, there is either a gas leak or there isn’t a gas leak. The size or seriousness of the leak is not displayed.
The need for accuracy and calibration of the device depends on how the information will be used. For example, a window box fan and an incense stick can be used during the construction of a house to depressurize the house and find gross leaks. They won’t provide comparative performance data, but they will indicate where the holes are that need to be filled. The Canadian garbage bag test can provide quite reasonable operational information on the exhaust flow from a bath fan, the balance of a heat or energy recovery ventilator, or the supply flow from a heating or cooling register. But if the information is to stand up in court to meet legal requirements, the measurement device should be carefully calibrated and the information should be documented.
For a program that requires quality control validation, the building analyst and the quality control technician should have similar measurements that are calibrated to the same standard. There will undoubtedly be variations in how the devices are treated, so programs need to have acceptable tolerances. Houses do not exist under laboratory conditions. A myriad of events affect the functioning of the house, including the impacts of weather, occupants, and mechanical equipment.
If their equipment is calibrated to the same standards, the technicians are starting off at the same point with a similar scale. Colin Genge of Retrotec says, “I checked three duct testers recently at training centers. They had errors of 10–25% when one range was compared to another. When did they go out of calibration? Were they always out? Who knows? Which is why they should be field-tested regularly.” Operators should have a system of daily, weekly, or monthly operational checks, and annual calibrations, and that information should be kept in a log as required by oversight organizations.
It is interesting to note that different standards organizations have different tolerances for manometer accuracy. Manufacturers design and build their pressure measurement tools to meet or exceed these standards (see Table 1).
Calibration is generally performed to meet the National Institute of Standards and Technology (NIST) standards. NIST is the national reference repository. It was founded in 1901 as the National Bureau of Standards and Testing and is now part of the U.S. Department of Commerce. NIST states that its mission is “to promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life.” The NIST calibration standards provide a traceable path to comparable measurement values, and since NIST is an industry-neutral organization, its information is unbiased.
For manometers and their associated fans, the U.S. Army Corps of Engineers provides some specific calibration information in its Air Leakage Testing Protocol:
Pressure gauges must be digital with a resolution of 0.1 Pa and accurate to within ±1% of reading or ±0.25 Pa, whichever is greater, and must have a means of adjustable time averaging to compensate for wind. Pressure gauges shall have their calibration checked and accuracy verified minimum every two (2) years (or sooner, based on the gauge manufacturer’s recommendations) against a National Institute of Standards and Technology (NIST) traceable standard over at least 16 Pressures from at least +250 to -250 Pa or to the greatest pressure used during a test.
Test fan measurement equipment shall have their calibration verified at least every four (4) years in compliance with ASTM E1258-88 (2008). Calibration Certificates must show the deviations from the calibration equations that must not exceed ± 5% of the flow reading for a range of air flows and backpressures (the pressure across the fan). For each test fan flow range configuration used in a test, the calibrations shall include the minimum and maximum air flows allowed by the manufacturer for that range plus at least one intermediate flow. For each flow rate, calibrations shall include data at backpressures within ± 10% of 25, 50, and 75 Pa. Digital pressure gauges and test fans may be calibrated separately and used interchangeably as long as they meet the requirements of this section.
RESNET National Home Energy Rating Standards, Chapter 8 states that “blower door and associated pressure testing instruments shall be tested annually for calibration.” It goes on to say, “The provider shall use a standard for field testing of calibration provided by the equipment manufacturer. Magnehelic Gauges cannot be field tested and shall be recalibrated by the Blower Door manufacturer annually.” It also says that the Provider and Rater will maintain a written log of the annual calibration for a period of three years. In a section on Equipment Accuracy, the standard says that “Blower door fans used for building air leakage testing shall measure airflow (after making any necessary air density corrections) with an accuracy of +/- 5%. Pressure gauges shall measure pressure differences with a resolution of 0.1 Pa and have an accuracy of +/- 1% of reading or 0.5 Pa, whichever is greater.”
For combustion safety testing, RESNET requires that the equipment be capable of measuring CO levels from 0 to 2,000 ppm, have a resolution of 1 ppm, have an accuracy rate of +/- 5 ppm, and be calibrated annually by the manufacturer (or using manufacturer’s instructions), and that “evidence of the calibration shall be submitted to the Rating Provider Quality Assurance Designee.”
BPI Standard for Residential Building Quality Assurance Related to Post-Installation Field Inspections (BPI 6300-S-201x) states that diagnostic testing equipment “shall be calibrated in compliance with the manufacturer’s specifications.” And in the section on combustion testing, the standard states, “Inspectors shall use gas leakage detection equipment that is capable of detecting gas leaks at 20 ppm and shall follow equipment manufacturer’s calibration requirements.”
ACCA Standard 12 Manual, Existing Home Evaluation and Performance Improvement, states that equipment used to measure CO shall be “calibrated annually by the manufacturer and have evidence of the calibration.” The standard also says that thermostats shall be “calibrated as required by the manufacturer and have evidence of the calibration.”
Field Checks and “Bump” Tests
As described below, various individual manufacturers have varying calibration requirements. Technicians can perform a variety of field checks or bump tests to verify that their devices are at least functional. These do not qualify as calibration of the instrument. Personal CO detectors, for example, can be placed in a Ziploc® bag with ground coffee to set them off. (ToxiRae has a bump test and calibration station that can calibrate and automatically keep a log for its QRae II personal CO monitor.) Combustible-gas leak detectors can be exposed to the gas from a cigarette lighter.
Retrotec recommends that the gauge and the hoses of a manometer be checked weekly by individually connecting each hose in the umbilical between the two reference taps. This simple test will verify the integrity of the hose as well as the function of the manometer. The manometer should be turned on, be set so both channels are reading pressure, and be operating on the four-second time average. The two readings should be within 2% of each other.
Colin Genge of Retrotec also recommends checking duct testers regularly in the field using a calibration plate or a thin piece of poster board with a 4-inch x 4.65-inch rectangular hole. The duct tester flex duct should be connected to the exhaust side of the fan, to pressurize the duct. With the duct stretched to its full length, the flow should be at 100 CFM on all ranges (50 CFM through a 4-inch x 2.35-inch rectangular hole). If the flow reads 80 CFM, for example, it will be 20% low.
Retrotec provides through its web site a lot of extremely explanatory information on field-validating the performance of its equipment.
The Energy Conservatory (TEC) recommends that all of its digital pressure gauges be recalibrated once per year in order “to maintain the accuracy specifications of 1% of reading (or 0.15 Pa, whichever is greater).” Calibration is performed at TEC in Minneapolis, Minnesota. TEC will provide a NIST traceable calibration certificate that includes “as found” as well as “current condition” pressure data.
Alternatively, TEC digital pressure gauges can be field-checked by following the procedure outlined in the Digital Gauge Field Calibration Check document on the TEC web site. This procedure compares the functions of the gauge to those of a recently calibrated DG-700 gauge that is being used as an in-house calibration standard. The calibration checks should be logged for any in-house or third-party quality control process.
TEC also has a field calibration check procedure for its Duct Blaster System that is very similar to the Retrotec procedure, using the TEC calibration plate in a depressurization mode. Details of this procedure can be found on the TEC web site.
Bacharach has a B-Smart calibration program. Through this program, Bacharach will send out a pre-calibrated sensor after a time-period you set. You install the sensor in your device and send the old sensor back to Bacharach. According to Rick Wanek of Bacharach, “this can be done in less than five minutes to provide a fresh certified calibration for CO, NO, NO2, and SO2 sensors.” The calibration time period is entered into the device and a notice is displayed on the screen when calibration is required.
Bacharach devices used to measure indoor air quality are fitted with IR TVOC and electrochemical sensors. Some of these sensors must be calibrated as frequently as every three months.
The Bacharach temperature and pressure sensors have little or no drift, according to the company, and need to be calibrated, as the company puts it, “less frequently,” which it didn’t define.
Extech suggests that its equipment be sent back to its NIST calibration laboratory. (The company does indicate that there are other companies out there with traveling calibration techs who can provide on-site calibration of Extech equipment.) Extech’s lab provides Certificates of Traceability to NIST standards. Turnaround, Extech says, “is quick in order to minimize inconvenience and downtime for the customer.”
Find out more about NIST.
Information about the RESNET calibration protocols.
Get information on field-validating the performance of Retrotec equipment.
Download ACCA’s Standard 12 Manual, Existing Home Evaluation and Performance Improvement, or view all of ACCA's Standard Manuals.
Learn about or purchase the ToxiRae bump test and calibration station that can calibrate and automatically keep a log for the QRae II personal CO monitor.
Read Bacharach’s B-Smart Calibration Program document.
Testo combustion analyzers have a warning on the screen that indicates when the oxygen sensor should be replaced. There is also an over-range warning on the screen when the CO sensor has been exposed to excessive amounts of CO (in excess of 4,000 ppm). When this happens, the probe should be removed from the source and exposed to CO-free air.
Table 2 summarizes manufacturer’s calibration requirements.
How Much Accuracy Is Needed?
If we are all going to start from the same place and know where we are when we get there, each piece of test equipment should be calibrated in the same way. If we are going to require quality control checks of energy performance, it’s important to calibrate the test gear. Energy efficiency organizations should weigh carefully whether that degree of accuracy is necessary for all equipment in all situations. Maybe we just need regular validation of the equipment to know whether the house or the ducts are leaky or very leaky. The accuracy of combustion safety measurements is a different story. Those readings affect the life and health of the building occupants. Regular validation checks and an up-to-date log of performance and calibration should be a part of every technician’s and organization’s tool kit.
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