Calibration and Auditing Procedure

A MiniVol sampler consists of three basic components: a specially designed inlet, size-fractionating impactors, and a flow rate controlling system. The particle size discrimination characteristics of both the inlet and the impactors depend critically on specified air velocities; a change in velocity will result in a change in the nominal particle size collected. For this reason, it is imperative that the flow rate through the sampler be maintained at a constant value that is as close as possible to the design flow rates. The design flow rate for the MiniVol is 5.0 L/min. The sampler’s flow rate should be within ±10% of 5 L/min or 4.5 to 5.5 L/min.

As indicated above, the true or actual flow through the sampler inlet must be known and controlled to ensure that only those particles nominally less than 10 µm/2.5 µm are being collected. A common source of error in a PM10/PM2.5 monitoring program is confusion between various air measurement units. Before calibration procedures are initiated, the operating agency personnel should review the following flow rate designations:

Qact: Actual volumetric air flow rates that are measured and expressed at existing conditions of temperature and pressure are denoted by Qact (Qactual). Typical units are L/min and cubic meters per minute (m3/min). Inlet design flow rates are always given in actual volumetric flow units.

Qstd: Air flow rates that have been corrected to EPA standard conditions of temperature and pressure (25 EC or 298 EK, and 760 mm Hg or 101 kPa) are denoted by Qstd (Qstandard). Typical units are std. L/min and std. m3/min. Standard volume flow rates are often used by engineers and scientists and are equivalent to mass flow units.

The MiniVol sampler is designed to operate at a total actual flow rate of 5.0 L/min. To ensure correct fractionation of particles at the inlet, this flow rate must be maintained within ±10% of 5.0 L/min.

Accurate calibration data for each MiniVol sampler is essential for the following:

1. To determine sampler flow rate set points.
2. To calculate sampler flow rate during routine QC field flow checks and QA performance audits.
3. To calculate total sample volume for the computation of PM10/PM2.5 mass concentrations.

Calibration of the sampler rotameter must be traceable to the U. S. National Institute of Standards and Technology (NIST) standards. A primary standard is used to calibrate a transfer standard, which in turn is used to calibrate the sampler rotameters.

Several commercially-available transfer standards can be used in calibrations. This procedure is applicable to orifice meter transfer standards, specifically, the Airmetrics MiniFlo Transfer Standard. The following are essential considerations in choosing another transfer standard for subsequent rotameter calibrations.

1. The transfer standard must be traceable to NIST through the calibration procedures referenced.
2. The transfer standard must be calibrated in the appropriate flow range.
3. The transfer standard should not have a significant pressure drop that affects normal sampler operation.

Regardless of the transfer standard employed, a leak-free adaptive device must be used to connect the transfer standard to the sampler inlet. The MiniFlo transfer standard is designed to connect to the inlet tube of the MiniVol filter holder. Adaptors for other devices may be purchased from Airmetrics or fabricated in-house. Obviously, the corresponding outlet on the transfer standard will determine how the transfer standard will be attached.

A MiniFlo flow rate transfer standard device is used as the flow rate reference to calibrate the sampler’s rotameter. To be valid, the MiniFlo transfer standard must be calibrated against a primary standard traceable to the NIST within the last year.

A calibrated digital manometer is required to measure the pressure drop across the MiniFlo orifice element.

The actual ambient temperature and barometric pressure must also be measured or obtained locally.

Procedures for the precalibration system check are as follows:

1. Place a filter into the MiniVol sampler filter holder and attach the filter holder assembly to the sampler. (Filters used for flow rate calibrations should not be used for subsequent sampling.)
2. Turn on the sampler and allow it to warm up to full operating temperature (at least 2 minutes).
3. While the sampler is running, close off the inlet using the palm of your hand. Observe the rotameter, if there are no leaks the rotameter should drop to zero and remain there.
4. Verify that the transfer standard calibration equation is current and traceable to an acceptable primary standard.
5. Adjust manometer to read zero and connect to the pressure tap on MiniFlo transfer standard.

Procedures for calibrating the sampler rotameter are as follows:

1. Install the MiniFlo transfer standard on the inlet tube of the MinVol sampler filter holder.
2. Turn on the sampler and allow it to warm up to normal operating temperature (at least 2 minutes). The transfer standard must also equilibrate before proceeding with the calibration.
3. Read and record the following parameters:
   1. Ambient temperature (Tact), °K
   2. Barometric pressure (Pact), mm Hg
4. Adjust the flow control until the rotameter displays 6.5 L/min.
5. Read and record the following parameters:
   1. Transfer standard pressure drop
   2. (H), inches of water
   3. Sampler rotameter indication (Qind), liters per minute

Repeat steps 4 and 5 for rotameter settings representing flow rates of 6.0, 5.5, 5.0, 4.5, and 4.0 L/min.

Gather together all the calibration data, including the transfer standard calibration information and the MiniVol sampler calibration data sheet. The following calibration calculation procedures are recommended.

Note: These calculations should be done at the time of the calibration, rather than later. This approach will allow additional calibration points to be taken if questions arise about the data that has already been obtained.

1. Calculate Qact for each calibration point as determined by the transfer standard calibration equation (Eq.A from MiniFlo calibration certificate).
2. Calculate and record the standard corrections (Q@std) for each calibration point as:
   
   where:
   Q@std = standard correction flow rate, L/min
   Qact = flow rate at ambient conditions (transfer standard), L/min
   Tact = ambient temperature, °K
   Pact = ambient barometric pressure, mm Hg
   Tstd = standard temperature, 298 °K
   Pstd = standard pressure, 760 mm Hg.
3. On a sheet of graph paper or computer spreadsheet plot the calculated standard correction flow rates, Q@std (y-axis) versus the corresponding rotameter indicated flow rate, Qind (x-axis). Using a programmable calculator or a spreadsheet and the following model, calculate the linear regression slope (mvol), intercept (bvol), and correlation coefficient (r) to obtain the MiniVol™ sampler flow rate calibration relationship.For the regression model y=mx+b
   
   So that the model is given by:
   
   A six-point calibration should yield a regression equation with a correlation coefficient of r > 0.990, with no point deviating more than 2% from the value predicted by the regression equation.Note: For actual sample periods, the sampler’s average actual operational flow rate Qact is calculated from the calibration slope and intercept using
   
   where
   Qact = sampler flow rate at ambient conditions, actual L/min
   Qind = rotameter response, L/min
   Tact = ambient temperature, °K
   Pact = ambient barometric pressure, mm Hg
   Tstd = standard temperature, 298 °K
   Pstd = standard pressure, 760 mm Hg
   mvol = slope of the MiniVol™ flow rate calibration relationship
   bvol = intercept of the MiniVol™ flow rate calibration relationship.Tact and Pact readings may be measured on-site or obtained from a nearby U.S. National Weather Service or airport weather station. Barometric pressure readings obtained from remote stations must be at station pressure (not corrected to sea level), and they may have to be corrected for differences between the elevation of the monitoring site and that of the remote station. If ambient temperature and pressure readings are not available, a seasonal average temperature and barometric pressure can also be used. Care must be taken, however, that the actual conditions at the site can be reasonably represented by such averages. It is therefore recommended that seasonal values represent actual values within 20 EC and 40 mm Hg.

1. Calculate and record the rotameter set point using temperatures and pressures expected on the day(s) sampling is conducted. These values can be seasonal set points or daily settings.
   

   where:
   Isp = sampler rotameter set point flow rate adjustment
   5.0 = MiniVol design flow rate, L/min
   Pact = ambient barometric pressure, mm Hg
   Tact= ambient temperature, EK.
   Tstd = standard temperature, 298 EK
   Pstd = standard pressure, 760 mm Hg
   mvol = slope of the MiniVol flow rate calibration relationship
   bvol = intercept of the MiniVol flow rate calibration relationship.

   Adjusting the sampler rotameter to seasonal average conditions will help minimize data loss caused by exceeding the MiniVol’s design condition specifications.

To ensure accurate measurement of the PM10/PM2.5 concentrations, calibrate the sampler upon installation and then recalibrate it as follows:

1. At least annually.
2. After any repairs that might affect sampler calibration.
3. If the field calibration flow check results exceed QC limits (±10% from the sampler’s indicated flow rate).

The following presents audit procedures specific to the MiniVol sampler which is designed to operate at an actual total flow of 5.0 L/min, and the MiniFlo transfer standard calibrated over the appropriate flow range.

Since the accurate measurement of PM10/PM2.5 mass concentration is dependent upon flow rates under actual conditions, the audit must also be conducted in terms of actual conditions.

A MiniFlo flow rate transfer device is used as the audit flow rate reference standard; however, the audit standard must be a different device from the one used to calibrate the sampler. Also, to be valid, the MiniFlo audit standard must have be calibrated against a primary standard traceable to the NIST within the last year.

A calibrated digital manometer is required to measure the pressure drop across the MiniFlo orifice element.

The actual ambient temperature and barometric pressure must also be measured or obtained locally.

1. Install a new filter in the filter holder and energize the sampler. Filters used for flow rate audits should not be used for sampling.
2. Adjust motor flow control to its operational set point for routine sampling. This set point should correspond to the calculated set point (Isp) determined by the sampler’s calibration relationship for the actual ambient conditions.
3. Allow the sampler to warm up for a minimum of 2 minutes while maintaining the proper rotameter set point.
4. Adjust manometer to read zero and connect to pressure tap on MiniFlo transfer standard.
5. Install the MiniFlo transfer standard on the inlet tube and allow to equilibrate for a minimum of 2 minutes.
6. Record the transfer standard pressure drop,(H) in inches of water, the ambient temperature (Tact) in degrees centigrade, ambient barometric pressure (Pact) in millimeters of mercury, and the rotameter indicated flow rate, (Qind) in liters per minute.

   Note: a Magnehelic® gage is sensitive to the position it is in when zeroed (vertical or horizontal). Maintain this position when taking flow readings.

7. Install the MiniFlo transfer standard on the inlet tube and allow to equilibrate for a minimum of 2 minutes.
8. Record the transfer standard pressure drop,(H) in inches of water, the ambient temperature (Tact) in degrees centigrade, ambient barometric pressure (Pact) in millimeters of mercury, and the rotameter indicated flow rate, (Qind) in liters per minute.

1. Calculate and record the audit flow rate (Qact-audit) by using the calibration equation accompanying the MiniFlo transfer standard. Each orifice has a unique pair of calibration constants (“mflo” and “bflo”) that are used in this equation to calculate the actual flow rate at specific ambient conditions. Record these values to the nearest 0.01 L/min (e.g., 5.03 L/min).
2. Calculate (using the MiniVol’s calibration relationship) the sampler flow rate (Qact-sampler) corresponding to the rotameter indicated flow rate (Qind) and record.
3. Determine the percentage difference between the sampler-indicated flow rate and the audit-measured flow rate as:
        
4. Record percent difference. Any deviation greater than 10% may require recalibration and the invalidation of the data from the last calibration forward.