40 C.F.R. § 82 app E to Subpart F of Part 82
This appendix is based on the California Air Resources Board (CARB) standard TP-503: Test Procedure for Leaks from Small Cans of Automotive Refrigerant, as amended on January 5, 2010; and CARB standard BP-A1: Balance Protocol for Gravimetric Determination of Sample Weights using a Precision Balance, as amended January 5, 2010.
Section 1. Applicability
This test procedure is used by manufacturers of containers holding two pounds or less of refrigerant for use in a motor vehicle air conditioner (MVAC) to determine the leakage rate of small containers of automotive refrigerant that are subject to the requirements of 40 CFR part 82, subpart F. Specifically, this test procedure will specify the equipment, procedures, and calculations to determine if a container holding two pounds or less of refrigerant for use in an MVAC complies with the leakage rate specified in § 82.154(c)(2)(ii) . All terms in this appendix will follow the definitions in § 82.152 unless otherwise defined in this appendix.
All containers holding two pounds or less of refrigerant for use in an MVAC must comply with other applicable codes and regulations such as local, state, or Federal safety codes and regulations.
This test procedure involves the use of materials under pressure and operations and should only be used by or under the supervision of those familiar and experienced in the use of such materials and operations. Appropriate safety precautions should be observed at all times while performing this test procedure.
Section 2. Principle and Summary of Test Procedure
This procedure is used to determine the leakage rate of containers holding two pounds or less of refrigerant for use in an MVAC (small cans). Testing will involve subjecting both full and partially empty cans in both upright and inverted positions at two temperatures: 73 °F and 130 °F.
Thirty small cans are tested under each condition for a total of 240 small cans tested. Small cans are brought to temperature stability, weighed, then stored for 30 days under specified conditions of temperature, orientation, and state of fill, then re-weighed. Leakage rate (grams/year) is estimated by (weight loss in grams) x 365/(days duration). The leakage rate is then compared to a standard of 3.00 grams/year to determine if a given small can complies with the leakage rate specified in § 82.154(c)(2)(ii) .
Section 3. Biases and Interferences
3.1 Contaminants on the operator's hands can affect the weight of the small can and the ability of the small can to absorb moisture. To avoid contamination of the small can, the balance operator should wear gloves while handling the small cans.
3.2 Weight determinations can be interfered with by moisture condensing on the small can and by thermal currents generated by temperature differences between the small can and the room temperature. The small cans cool during discharge and could cause condensation. For these reasons, small cans must be equilibrated to balance room temperature for at least four hours before weighing.
3.3 Variations in the temperature, pressure, and humidity of the ambient air will cause variations in the buoyancy of the small can. These variations should typically be less than 25 mg for a small can. If the small can is not leaking at all, then the uncorrected weight changes will be within the range of 0 ± 25 mg, which is about ten percent of the 247 mg loss expected after thirty days for a can leaking at 3 g/yr. In that case buoyancy corrections can be omitted. If the absolute value of the uncorrected weight change exceeds 25 mg, then all calculations must be made using weights corrected for buoyancy based on the temperature, pressure, and humidity of the weighing room.
3.4 Some electronic balances are sensitive to the effects of small static charges. The small can should be placed directly on the balance pan, ensuring metal to metal contact. If the balance pan is not grounded, the small can and balance pan should be statically discharged before weighing.
Section 4. Sensitivity and Range
The mass of a full small can could range from roughly 50 g to 1000 g depending on the container capacity. A top loading balance, capable of a maximum weight measurement of not less than 1,000 g and having a minimum readability of 0.001 g, reproducibility and linearity of ± 0.002 g, must be used to perform mass measurements.
Section 5. Equipment
5.1 A top loading balance that meets the requirements of Section 4 above.
5.2 A NIST traceable working standard mass for balance calibration. A NIST traceable working standard mass for a balance linearity check. A reference mass to serve as a "blank" small can.
5.3 An enclosure capable of controlling the internal air temperature from 73 °F ± 5 °F, and an enclosure capable of controlling the internal air temperature to 130 °F ± 5 °F.
5.4 A temperature instrument capable of measuring the internal temperature of the temperature conditioning enclosures and the balance room with a sensitivity of ± 2 °F.
5.5 A barometric pressure instrument capable of measuring atmospheric pressure at the location of the balance to within ± 0.02 inches of mercury.
5.6 A relative humidity measuring instrument capable of measuring the relative humidity (RH) at the location of the balance with a sensitivity of ± 2 percent RH.
5.7 A hose with appropriate fitting for dispensing refrigerant from the small can to a recovery machine.
5.8 A refrigerant recovery machine to collect the discharged refrigerant from small cans being tested.
Section 6. Calibration Procedures
6.1 Calibrations are applied to the balance and to the support equipment such as temperature, humidity, and pressure monitoring equipment. Procedures for calibration are not spelled out here. General calibration principals for the support equipment and the balance are described in Section 11, Quality Assurance/Quality Control. Detailed calibration procedures for measurements made using the balance are contained in Attachment A: "Balance Protocol for Gravimetric Determination of Sample Weights using a Precision Balance."
Section 7. Small Can Preparation
7.1 Receive a batch of 240 small cans of one design to be tested. These may include several SKUs from different manufacturers if the container and valve combination are the same.
7.2 Clean small cans with Alkanox solution or equivalent and dry with a lint free towel.
7.3 Confirm that the sample ID sticker on the small can matches the sample ID on the chain of custody forms.
7.4 Select a reference mass similar to the weight of a full small can. If multiple sets of similar sized small cans are being tested, only one reference mass is needed; it can be used with all sets. Store the reference mass in the balance area.
7.5 Evacuate the contents of one half of the small cans (120 cans) into the refrigerant recovery machine using normal DIY dispensing procedures until each small can is approximately half full.
7.6 Select a reference mass similar to the weight of the half-full small can. If multiple sets of similar size small cans are being tested, only one reference mass is needed; it can be used with all sets. Store the reference mass in the balance area.
Section 8. Small Can Weighing
Weighing cans on the balance is done in accordance with Attachment A to this appendix. Attachment A describes how to conduct weight determinations including appropriate calibration and QC data. This section, "Small Can Weighing," describes the overall process, not the details of how to use the balance.
Initial Weights
8.1 Put on gloves. Check the small cans for contamination.
8.2 Place the 240 small cans into a location where they can equilibrate to balance room temperature. Record the small can test IDs and the equilibration start time on the Small Can Test Data Forms available on EPA's Web site in sets of thirty, one form for each of the eight test conditions.
8.3 Let cans equilibrate for at least four hours.
8.4 Weigh the set of 240 small cans and the reference weights using Attachment A and log the results to the Balance Weighing Log Form available on EPA's Web site.
8.5 Transfer data from the Balance Weighing Log Form to the Small Can Test Data Form in sets of 30, one set for each of the eight conditions to be tested.
Thirty-Day Soak
8.6 Place each set of 30 small cans into the appropriate orientation and temperature for soaking:
30 full small cans-73 °F, upright
30 full small cans-73 °F, inverted
30 full small cans-130 °F, upright
30 full small cans-130 °F, inverted
30 half-full small cans-73 °F, upright
30 half-full small cans-73 °F, inverted
30 half-full small cans-130 °F, upright
30 half-full small cans-130 °F, inverted
8.7 Soak the small cans for 30 days undisturbed.
Final Weighing
8.8 Place the 240 small cans into a location where they can equilibrate to balance room temperature.
8.9 Let the small cans equilibrate for at least four hours.
8.10 Weigh the set of 240 small cans, the reference weights, and any additional sets of small cans using Attachment A.
8.11 Transfer data from the Balance Weighing Log Form to the corresponding Small Can Test Data Forms.
Section 9. Calculations
Corrections for Buoyancy
The calculations in this section are described in terms of "weight." Mass is a property of the small can, whereas weight is a force due to the effects of buoyancy and gravity. Procedures for correcting the effect of buoyancy are given in Attachment B of this appendix. Ignoring buoyancy, i.e., using weight data uncorrected for buoyancy effects, is acceptable for a thirty day test if the absolute magnitude of the weight change is less than 25 mg. If the uncorrected weight change exceeds 25 mg for any small can, then correct all small can weights for buoyancy using the procedures in Attachment B before performing the calculations described below.
Calculation of Leak Rate
The emission rate in grams/day for each small can is calculated by subtracting the final weight from the initial weight and then dividing the weight difference by the time difference measured in days to the nearest hour (nearest 1/24 of a day). The emission rate in g/day is multiplied by 365 to determine emission rate in grams/yr. If the annual emission rate for any small can exceeds the entire small can contents, then the annual emission rate for that small can is adjusted to equal the entire small can contents/year (e.g., about 350 g/yr for a 12 ounce small can). The annual emission rate for the purpose of the test is calculated by averaging the 240 individual adjusted annual emission rates and rounding to two decimal places. The cans fail the test if the adjusted annual emission rate averaged over 240 cans is greater than 3.00 g/yr. The calculations are described below.
Loss rate for each small can
Eidaily = (Wifinal - Wiinitial)/(Difinal - Diinitial) g/day
Eiannual = 365 * Eidaily g/year
Eiadjusted = Minimum of (Eiadjusted, Ci/year) g/yr
Where,
Ei = emission rate
Wifinal = weight of can i after soaking (grams)
Wiinitial = weight of can I before soaking (grams)
Difinal = date/time of final weight measurements (days)
Diinitial = date/time of initial weight measurements (days)
Ci = original factory mass of refrigerant in can i
Note: Date/Times are measured in days. Microsoft Excel stores dates and times in days, and the calculations can be made directly in Excel. If calculations are made manually, calculate serial days to the nearest hour for each date and time as follows:
D = Julday + Hour/24
Where,
Julday = serial day of the year: Jan 1 = 1, Jan 31 = 31, Feb 1 = 32, etc.
Hour = hour of day using 24-hour clock, 0 to 23
Calculate the average loss rate for the 240 small cans as follows:
Emean = [Sum (Eadjustedi), i = 1 to 240]/240
Section 10. Recordkeeping
During small can weighing, record the small can weights and date/times on the Balance Weighing Log Form. After each weighing session, transfer the measured weights and date/times from the Balance Weighing Log Form to the Small Can Test Data Form.
At the end of the test, complete the calculations described in Section 9, Calculations, and record the results on the Small Can Test Data Form.
Section 11. Quality Assurance/Quality Control
11.1 All temperature, pressure, and humidity instruments should be calibrated annually against NIST traceable laboratory standards. The main purpose of the NIST traceable calibration is to establish the absolute accuracy of the device. The instruments should also be checked periodically such as weekly, monthly, or quarterly against intermediate standards or against independent instruments. For example, a thermocouple can be checked weekly against a wall thermometer. A barometer or pressure gauge can be checked weekly by adjusting to sea level and comparing with local airport data. The main purpose of the frequent checks is to verify that the device has not failed in some way. This is especially important for electronic devices such as a digital thermometer, but even a liquid filled thermometer can develop a problem such as a bubble.
11.2 The balance should be serviced and calibrated annually by an independent balance service company or agency using NIST traceable reference masses. Servicing verifies accuracy and linearity, and the maintenance performed helps ensure that a malfunction does not develop.
11.3 The balance must also be calibrated and its linearity checked with working standards before and after each weighing session, or before and after each group of 24 small cans if more than 24 small cans are weighed in a session. Procedures for calibrating and using the balance, as well as recording balance data, are described in the accompanying balance weighing protocol. These procedures include zero checks, calibration checks, and reference mass checks. Procedures for calculating quality control data from those checks are described in Attachment A.
11.4 The small cans are cleaned then handled using gloves to prevent contamination. All equilibration and soaking must be done in a dust free area.
Section 12. Balance Protocol for Gravimetric Determination of Sample Weights Using a Precision Balance
12.1 Scope and application
This Protocol summarizes a set of procedures and tolerances for weighing objects in the range of 0 to 1,000 g with a resolution of 0.001 g. This protocol only addresses balance operations, it does not address project requirements for equilibration, sample hold time limits, sample collection etc.
12.2 Summary of method
The balance is zeroed and calibrated using procedures defined herein. Object weight determinations are conducted along with control object weight determinations, zero checks, calibration checks, sensitivity checks, and replicate weightings in a defined sequence designed to control and quantitatively characterize precision and accuracy.
12.3 Definitions
N/A.
12.4 Interferences
Object weights can be affected by temperature and relative humidity of their environment, air currents, static electricity, gain and loss of water vapor, gain or loss of and loss of volatile compounds directly from the sample or from contaminants such as finger prints, marker ink, and adhesive tape.
Contamination, transfer of material to or from the samples, is controlled by conducting operations inside a clean area dedicated to the purpose and having a filtered laminar air flow where possible; by wearing gloves while handling all samples and related balance equipment; by using forceps to handle small objects, and by keeping the balance and all related equipment inside the clean area.
Air currents are controlled by conducting weighing operations inside a closed chamber or glove box and by allowing the substrates to reach temperature and relative humidity equilibrium. The chamber is maintained at 40 percent relative humidity and 25 °C by a continuous humidity and temperature control system. The temperature and RH conditions are recorded at least once per weighing sessions. Equilibration times for samples that are particularly sensitive to humidity or to loss of semi-volatiles species are specified by project requirements.
Static electric charges on the walls of the balance and the weighed objects, including samples, controls, and calibration weights, can significantly affect balance readings. Static is avoided by the operator ground himself and test objects as described in the balance manual.
12.5 Personnel health and safety
N/A
12.6 Equipment and supplies
Filtered, temperature and humidity controlled weighing chamber. Precision Balance Plastic forceps Nylon fabric gloves. Working calibration weights: ANSI Class 2, 1000g and 500 g Working sensitivity weight: 50 mg Reference objects: references are one or more objects that are typical of the objects to be weighed during a project, but that are stored permanently inside the balance glove box. Reference objects are labeled Test1, Test2, Test3, etc.12.7 Reagents and standard
N/A
12.8 Sample collection, preservation, and storage
N/A. See relevant project requirements and SOPs.
12.9 Quality control
Data quality is controlled by specifying frequencies and tolerances for Zero, Calibration, Linearity, and Sensitivity checks. If checks do not meet tolerance criteria, then samples must be re-weighed. In addition, the procedures specify frequencies for Control Object Checks.
Data quality is quantitatively characterized using Zero Check, Calibration Check, and Control Check data. These data are summarized monthly in statistics and QC charts.
12.10 Calibration and standardization
The absolute accuracy of the balance is established by calibration against an ANSI Class 2, stainless steel working weight: 1000.000 g ± 0.0025 g. Linearity is established checking the midpoint against an ANSI Class 2 stainless steel working weight: 500.000 ± 0.0012 g. Sensitivity is established using and ANSI Class 2 stainless steel or aluminum working weight: 50 mg. Precision is checked by periodically checking zero, calibration, and reference object weights.
12.11 Procedure
12.11.1 Overview of Weighing Sequence
Weighing a series of substrates consists of performing the following procedures in sequence, while observing the procedures for handling and the procedures for reading the balance:
1. Initial Adjustment
2. Weigh eight samples
3. Zero Check
4. Weigh eight samples
5. Zero Check
6. Weigh eight samples
7. Calibration Check
8. Return to step 2.
9. If less than 24 cans are weighed, perform a final Calibration Check at the end of weighing.
This sequence is interrupted and samples are reweighed if QC check tolerances are not met. Each of these procedures along with procedures for handling and reading the balance are described below. The QC tolerances referred to in these procedures are listed in Table 1.
12.11.2 Handling
1. Never touch samples, weights, balance pans, etc. with bare hands. Wear powder free gloves to handle the weights, controls, and samples.
12.11.3 Reading the Balance
1. Close the door. Wait for the balance stabilization light to come on, and note the reading.
2. Watch the balance reading for 30 sec (use a clock). If the reading has not changed by more than 0.001 g from the reading noted in step 1, then record the reading observed at the end of the 30 sec period.
3. If the reading has drifted more than 0.001 g note the new balance reading and go to step 2.
4. If the balance reading is flickering back and forth between two consecutive values choose the value that is displayed more often than the other.
5. If the balance reading is flickering equally back and forth between two consecutive values choose the higher value.
12.11.4 Initial Adjustment
1. Empty the sample pan Close the door. Select Range 1000 g
2. Wait for a stable reading
3. Record the reading with QC code IZC (initial zero check)
4. Press the Tare button
5. Record the reading in the logbook with QC code IZA (initial zero adjust)
6. Place the 1,000 g working calibration weight on the balance pan
7. Wait for a stable reading.
8. Record the reading with QC code ICC (initial cal check)
9. Press the Calibrate button
10. Record the reading with QC code ICA (initial cal adjust)
11. Remove the calibration weight.
12. Wait for a stable reading.
13. Record the reading with QC code IZC.
14. If the zero reading exceeds ± 0.002 g, go to step 4.
15. Place the 500 g calibration weight on the balance pan
16. After a stable reading, record the reading with QC code C500. Do not adjust the balance.
17. Add the 0.050 g weight to 500 g weight on the balance pan.
18. After a stable reading, record the reading with QC code C0.05. Do not adjust the balance.
19. Weigh reference object TEST1, record reading with QC code T1.
20. Weigh the reference object TEST2, TEST3, etc. that is similar in weight to the samples that you will be weighing. Record with QC code T2, T3, etc.
12.11.5 Zero Check
1. Empty the sample pan. Close the door.
2. Wait for a stable reading
3. Record the reading with QC code ZC
4. If the ZC reading is less than or equal to the zero adjustment tolerance shown in Table 1, return to weighing and do not adjust the zero. If the ZC reading exceeded the zero adjustment tolerance, proceed with steps 5 through 7.
5. Press the Tare button
6. Record the reading in the logbook with QC code ZA.
7. If the ZC reading exceeded the zero re-weigh tolerance, change the QC code recorded in step 3 from ZC to FZC. Then enter a QC code of FZ into the QC code column of all samples weights obtained after the last valid zero check. Re- weigh all of those samples, recording new data in new rows of the logbook.
12.11.6 Calibration Check
1. First, follow procedures for Zero Check. If the ZC was within tolerance, tare the balance anyway (i.e., follow steps 5 and 6 of the Zero Check method)
2. Place the 1,000 g working calibration weight on the sample pan, wait for a stable reading.
3. Record the reading with QC code C1000
4. If the C1000 reading is less than or equal to the calibration adjustment tolerances, skip steps 5 through 8 and proceed to step 9. Do not adjust the calibration.
5. If the C100 reading exceeded the calibration adjust tolerance, press the Calibrate button.
6. Record the reading in the logbook with QC code CA
7. Perform a Zero Check (follow the Zero Check method)
8. If the C1000 reading exceeded the calibration re-weigh tolerance, change the code recorded in step 3 from C1000 to FC1000. Enter FC into the QC column for all sample weights obtained after the last valid calibration check. Re-weigh all of those samples, recording new data in new rows of the logbook.
12.11.7 Replicate Weighing Check
1. This protocol does not include reweigh samples to obtain replicates. The projects for which this protocol is intended already include procedures multiple weightings of each sample.
Table 1-QC Tolerances and Frequencies for Balance Protocol
| Reading Tolerance: | |
| 0.001 g, stable for 30 sec. | |
| Adjustment Tolerances: | |
| Zero: | -0.003 to +0.003 g. |
| Calibration: | 999.997 to 1000.003 g. |
| Controls: | none. |
| Replicates: | none. |
| Re-weigh Tolerances: | |
| Zero: | -0.005 to +0.005 g. |
| Calibration: | 999.995 to 1000.005 g. |
| Controls: | none. |
| Replicates: | none. |
| Reference Objects: | |
| Test 1-A reference object weighing about 400 g. | |
| Test 2-A reference object weighing about 200 g. | |
| Test 3-A reference object weighing about 700 g. | |
| QC Frequencies: | |
| Zero Checks: | once per 8 samples. |
| Calibration Checks: | once per 24 samples. |
| Repeat weighings: | none (test method includes replicate determinations). |
| Control objects: | once per weighing session. |
12.12 Data analysis and calculations
For Zero Checks, let Z equal the recorded Zero Check value. For control checks let T1, T2, etc. equal the recorded value for control object Test 1, Test 2, etc. For Calibration Checks, let C1000 equal C1000 reading minus 1000, M = C500-500, S = .C.050-C500-.050. For Replicate Checks, let D equal the loss that occurred between the first and second measurements. In summary:
T1 = T1
T2 = T2
T3 = T3
Z = ZC-0
C = C1000-1000
M = C500-500
G = C050-C500-.050
Tabulate the mean and standard deviation for each of the following: Z, C, M, G. T1, T2, T3. Depending on the number of operators using the balance and the number of protocols in use, analyze the data by subcategories to determine the effects of balance operator and protocol. Each of these standard deviations, SZ, SC, etc. is an estimate of the precision of single weight measurement.
For Z, C, M, and G, check the mean value for statistical difference from 0. If the means are statistically different than zero, troubleshooting to eliminate bias may be called for. For Z, C, M, G, T1, T2, T3, check that the standard deviations are all comparable. If there are systematic differences, then troubleshooting to eliminate the problem may be called for.
Note that the precision of a weight gain, involves two weight determinations, and therefore is larger than S by a factor of sqrt(2). On the other hand replicate weighings improves the precision of the determinations by a factor of sqrt(N). If N = 2, i.e., duplicates, then the factors cancel each other.
To estimate the overall uncertainty in a weight determination, a conservative estimate might be to combine the imprecision contributed by the zero with the imprecision contributed by the calibration.
U = Sqrt(SZ2 + SC2)
The uncertainty in a weight gain from N replicates is then given by:
Ugain = Sqrt(2) * Sqrt(SZ2 + SC2)/Sqrt(N)
But due to the balance adjustment and reweigh tolerances, we expect SZ to approximately equal SC, to approximately equal SM, etc. tolerances, so that the equation above becomes:
Ugain = 2 * S/Sqrt(N)
Where S is any individual standard deviation; or better, a pooled standard deviation.
12.13 Method performance
The data necessary to characterize the accuracy and precision of this method are still being collected. The method is used primarily to weigh objects before and after a period of soaking to determine weight loss by subtraction. Given the reweigh tolerances, we expect that the precision of weight gain determinations will be on the order of 0.006 g at the 1-sigma level. Bias in the weight gain determination, due to inaccuracy of the calibration weight and to fixed non-linearity of the balance response is on the order 0.005 percent of the gain.
12.14 Pollution prevention
When discharging half the can contents during can preparation, do not vent the contents of the small can to the atmosphere. Use an automotive recovery machine to transfer small can contest to a recovery cylinder.
12.15 Waste management
Dispose of the contents of the recycle cylinder through a service that consolidates waste for shipment to EPA certified facilities for reclaiming or destruction.
Section 13. Compensation of Weight Data for Buoyancy and Gravity Effects
13.1 Gravity
Variations in gravity are important only when weighing objects under different gravitational fields, i.e., at different locations or at different heights. Since the balance procedures calibrate the balance against a known mass (the calibration "weight") at the same location where sample objects are weighed, there is no need to correct for location. Although both the sample and the calibration weight are used at the same location, there will be a difference in the height of the center of gravity of the sample object (small can) and the center of gravity of the reference mass (calibration weight). However, this difference in height is maintained during both the initial weights and final weights, affecting the initial and final weights by the same amount, and affecting the scale of the weight difference by only a few ppm. In any event, the magnitude of this correction is on the order of 0.3 ug per kg per mm of height difference. A difference on the order of 100 mm would thus yield a weight difference of about 0.03 mg, which is insignificant compared to our balance resolution which is 0.001 g or 1 mg.
Based on the discussion above, no corrections for gravity are necessary when determining weight changes in small cans.
13.2 Buoyancy
Within a weighing session, the difference in density between the sample object and the calibration weight will cause the sample object weight value to differ from its mass value due to buoyancy. For a 1-liter object in air at 20 °C and at 1 atm, the buoyant force is about 1.2 g. The volume of a 1 kg object with a density of 8 g/cm3 (e.g., a calibration weight), is about 0.125 liters, and the buoyancy force is about 0.15 g. Variations in air density will affect both of these values in proportion. The net value being affected by variations in air density is thus on the order of 1.2 - 0.15 = 1.05 g. Air density can vary up or down by 2 percent or more due to variations in barometric pressure, temperature, and humidity. The buoyancy force will then vary up or down by 0.02 g, or 20 mg. This is significant compared to the weight change expected after one week for a can leaking at 3 grams per year, which is 57 mg.
Based on the discussion above, buoyancy corrections must be made.
Variables measured or calculated:
Vcan = volume of can (cm3). Estimate to within 10 percent by measuring the can dimensions or by water displacement. Error in the can volume will cause an error in the absolute amount of the buoyancy force, but will have only a small effect on the change in buoyancy force from day to day.
Wcan = nominal weight of a can (g), used to calculate the nominal density of the can.
[RHO]can = nominal density of a small can (g/cm3). The nominal values can be applied to corrections for all cans. It is not necessary to calculate a more exact density for each can. Calculate once for a full can and once for a half full can as follows:
[RHO]can = Wcan /Vcan
T = Temperature in balance chamber (degrees Celsius).
RH = Relative humidity in balance chamber (expressed a number between 0 and 100).
Pbaro = Barometric pressure in balance chamber (millibar). Use actual pressure, NOT pressure adjusted to sea level.
[RHO]air = density of air in the balance chamber (g/cm3). Calculate using the following approximation:
[RHO]air = 0.001*[0.348444*Pbaro-(RH/100) * (0.252 * T-2.0582)]/(T + 273.15)
[RHO]ref = the reference density of the calibration weight (g/cm3). Should be 8.0 g/cm3.
Equation to correct for buoyancy: Wcorrected = Wreading * (1-[RHO]air/[RHO]ref)/(1-[RHO]air/[RHO] can)
40 C.F.R. 82 app E to Subpart F of Part 82