SOP: Measurement of the Internal Pressure (Negative or Positive) of Foods in Hermetically Sealed Rigid Containers

GIFSQ — Global Institute for Food Safety & Quality

1. Principle

A direct-reading Bourdon-tube gauge fitted with a self-sealing port is used to puncture the container and take a direct reading of its internal vacuum or pressure. The gauge measures the vacuum or pressure inside the sealed container relative to the standard atmospheric pressure at sea level (14.7 psi or 29.9 inches of Hg or 101.3 kPa). The method applies to two- or three-piece hermetically sealed metal cans and to glass containers with metal lids.

2. Apparatus and materials required

• Bourdon-tube gauge for pressure and vacuum, direct-reading (see the source’s Appendix A for the diagram), with a scale that permits measurement of vacuum and/or pressure; the vacuum scale is usually graduated from 0 to 30 inches of mercury in 1-inch increments; fitted with a penetrating/sealing device (preferably a side-opening self-sealing port) that punctures the container while sealing it from the atmosphere; the internal gauge volume (Bourdon tube and attachments) must not exceed 6 mL; calibrated to ±2.0 psi/in. Hg
• Barometer, accuracy ±0.1 in. Hg
• Thermometer, accuracy ±1.0°C
• Bacti-Disc Cutters (Wilkens-Anderson Co., Dixie Canner Equipment Co.)
• Gross headspace measuring device, accuracy ±1/16 inch
• Pressure calibrator for calibrating pressure devices or systems, traceable to N.I.S.T. (calibration in accordance with the ISO 9000 quality standard)
• Silicon grease, for moistening the gauge’s rubber gasket

3. Procedure

Follow the safety procedures for foods in hermetically sealed containers given in methods MFHPB-01 and MFHPB-06

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Ensure the container has been identified as described in method MFHPB-06

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Remove the label, identify it, and perform the visual inspection as described in method MFHPB-06

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Clean dirt or soil from the container surface (scrubbing with water and soap may be necessary)

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Bring the container and its contents to 21–26°C; record the atmospheric pressure and the room temperature (if the temperature is below 21°C or above 26°C, apply the vacuum correction factor from the temperature correction table in section 4)

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For cans, select the manufacturer’s end to be used for the measurement (usually the uncoded end); measuring the vacuum at the end carrying the headspace is advisable — solid-pack products and meats can give problems, since it is difficult to determine which end has the headspace and such products do not move with the changing position of the can; tapping for a hollow sound may help in making the judgement

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To avoid deforming the end plate, select the first bead near the double seam, as far from the centre as possible, as the puncture site (for glass jars, a point as close to the edge of the cap as practical); the site selected must permit an effective seal by the gauge gasket

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Moisten the rubber gasket with silicon grease and seat it firmly on the can surface

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Puncture the can end with a single stroke, taking care to avoid excess pressure or force that may deform the end and induce an erroneous increase in headspace pressure

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Read and record the vacuum reading

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Using a Bacti-Disc cutter, cut a hole large enough to determine the gross headspace distance with the calibrated headspace gauge (the reading is used to calculate the net headspace volume for the additional vacuum correction in section 4)

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Record all measurements and observations for each container: read and record the scale readings to the nearest 1 inch of mercury or 1 pound per square inch (metric scales to the nearest unit); where readings require correction for differences in atmospheric pressure, such as elevation, apply the correction using the elevation table in section 4; where a true vacuum is required, convert the readings as described in section 4 — alternatively, a direct reading of the true vacuum can be taken with the Bourdon-tube gauge calibrated with a certified pressure calibrator; summary tables are recommended

4. Calculations and corrections

Atmospheric-pressure (elevation) correction

Variations in atmospheric pressure from elevation or climate change the apparent headspace vacuum or pressure, so all readings are corrected to the standard atmospheric pressure at sea level: 101.3 kPa (29.9 inches of mercury or 14 psi).

Corrected vacuum reading = (Actual vacuum reading × Standard atmospheric pressure) / Actual barometric pressure

Where,

• Actual vacuum reading = the vacuum reading taken on the gauge
• Standard atmospheric pressure = 101.3 kPa (29.9 inches of Hg; 14 psi)
• Actual barometric pressure = the local barometric pressure

Both atmospheric readings must be from the same scale (kilopascals, inches of mercury, or pounds per square inch absolute). The correction only has a significant effect when barometric pressure varies by more than 6.5 kPa (2 inches of mercury, 1 psi) from the standard atmospheric pressure.

Temperature correction

Take the measurements with the contents at 21–26°C. Where the temperature is outside that range, add or subtract the correction below to the vacuum gauge reading.

The factor is added to or subtracted from the vacuum gauge reading.

Gauge-volume correction

The volume of air trapped in the gauge tube is normally the greatest source of error in vacuum measurements. With a gauge having the side self-sealing port in the piercing assembly, this error can be largely overcome by disregarding the first reading (acceptable when the number of cans being measured is not critical). If the number of cans is critical or limited, pre-evacuate the gauge tube and piercing assembly using another can (not part of the sample) or a laboratory vacuum source.

Where the gauge supplier has not provided the gauge assembly volume, it can be determined from the gas laws with the apparatus shown in the source’s Figure 2, as follows:

Open valve A, and close valve B

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Hold the gauge firmly on its seat and open valve B to obtain a reading near midscale

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Close valve B, tap the gauge glass, and record the reading as R1

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Close valve A and remove the gauge

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Return the gauge to its seat and open valve A

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Tap the gauge and record the reading as R2

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Calculate the gauge volume from the formula below.

Vg = Vc (R1 − R2) / R2

Where,

• Vg = gauge volume
• Vc = known chamber volume, including the volume of valve A but not of valve B
• R1 = first reading
• R2 = second reading

Net headspace volume (for the additional correction of vacuum readings)

Measure the gross headspace distance with the headspace gauge (calibrated to at least 1/16 inch, i.e. 1.5875 mm, increments): the distance between the top of the can seam flange and the top liquid surface of the can contents

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Calculate the net headspace volume with the formula below, taking the volume factor, the countersink displacement, and the vacuum end displacement from the tables and figure in this section.

H.S.V. = G.H.S. × V.F. − (V.D. + E.D.)

Where,

• H.S.V. = net headspace volume, in millilitres
• G.H.S. = gross headspace, in sixteenths of an inch
• V.F. = factor for determining volume per sixteenth of an inch, from the can headspace table (column 2)
• V.D. = volume displacement due to countersink, from the can headspace table (column 3)
• E.D. = end displacement of the cover due to vacuum, from the source’s Figure 3

Worked example from the source: for a 202-diameter can at 15 inches of vacuum with a measured gross headspace of 4/16 inch: H.S.V. = 4 × 3.42 − (6 + 1.05) = 6.63 millilitres.

Can headspace volume and volume displacement from end unit profiles

True vacuum

The headspace-volume and gauge-volume correction to true vacuum can be carried out with the source’s nomogram: the net gauge reading corrected for altitude is taken on line Y and the net headspace in millilitres on line X; joining the two values with a straight line and reading the vacuum where this line crosses the line for the total gauge volume (3, 4, 5 or 6 millilitres, including the Bourdon-tube tip and connections) gives the true vacuum at that altitude. As an example, for the container in the worked example above with a 3-millilitre gauge: joining 6.63 mL on line X to 15 inches of vacuum on line Y crosses the 3-millilitre gauge-volume line at 22 inches — the true vacuum in that container.

Atmospheric and barometric pressures at different elevations above sea level

Vacuum and absolute pressure relationship

Note: 1 inch of Hg = 3.386 kPa = 0.491 psi.

5. Precautions

• Follow all safety procedures for foods in hermetically sealed containers given in methods MFHPB-01 and MFHPB-06.
• Check the gauge against a standard test gauge having an accuracy equal to or better than the gauge being tested before taking any vacuum or pressure readings; recheck it frequently and record the results, since calibration changes with continued use (wear in the gear mechanism, changes in Bourdon-tube stiffness, and blockage of the tube by small food particles — occasionally this causes a uniform error over the entire scale).
• Keep the piercing tip sharp and puncture near the side seam to minimize distortion of the end plate and false readings.
• Record the atmospheric pressure and the room temperature before measuring.
• Take the measurements with the contents at 21–26°C; apply the temperature correction factor where the temperature is outside that range.

References

• “Canned Foods Principles of Thermal Process Control Acidification and Container Closure Evaluation.” Revised Third Edition (1980), the Food Processors Institute, 1133 20th Street, N.W., Washington, D.C. 20036.
• “Evaluating a Doubleseam.” A technical publication (1971) by Dewey and Almy Chemical Division of W.R. Grace & Co., Cambridge, Massachusetts, U.S.A.
• “Doubleseam Manual.” A technical publication (1978) by Metal Box Limited, England.
• “Metal Can Defects, Identification and Classification Manual” (1999), Canadian Food Inspection Agency, Government of Canada.
• Health Canada, Compendium of Analytical Methods, Volume 2 — HPB Method MFHPB-04, February 2003: “Method to measure the internal pressure (negative or positive) of foods in hermetically sealed rigid containers.”