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Field inspection manual — volumetric measuring devices

Category: Volume
Part: 4-STP
Section: 41
Publication date:
Revision number: 1

Part 4, section 41—Specialized test procedure—Procedure for density determination

Application

This procedure can be used to determine the density of normally liquid products and liquefied gas products when a liquid meter is tested gravimetrically.

Note: This procedure covers the use of pycnometers with normally liquid products and pressure hydrometers for liquefied gases. It does not currently cover other density determination methods such as using portable density meters. Specific instructions for use of alternate equipment will be provided as this equipment is approved for use.

Purpose

The procedure provides guidance to inspectors needing to determine the density of a product during gravimetric testing of liquid meters.

Note: The accuracy and suitability of any density measurement are dependent upon the density sample being representative of the product measured during the meter evaluation. The sample must be homogeneous and identical to the product that will be used during testing. The product sample must be taken carefully to reduce the amount of entrained air and eliminate the possibility of trapped air in the pycnometer. The sample must be allowed to reach a stable temperature.

Procedure (normally liquid products)

Equipment

  • Certified 500 mL pycnometer standard.
  • Appropriate reference scale (typically class II or better with a graduation ≤ 0.1 g).
  • Appropriate certified local standards of mass appropriate to determine the accuracy, repeatability and sensitivity errors as per STP-45.
  • Appropriate density conversion tables authorized for the product (e.g., API Table 53D or Measurement Canada-authorized equivalent).

Weigh the sample

Test the reference scales for accuracy and repeatability using local mass standards prior to and following the density determination process, as per STP-45.

  1. Warm-up and exercise the scale as per manufacturer's instructions.
  2. Place the empty, clean and dry pycnometer on the scale and tare off the weight value.
  3. Remove the pycnometer from the scale and then place it back on the scale at least three times to ensure there is no change in the tare value.
  4. Add an appropriate reference mass (typically 500 g for a 500 mL pycnometer) to the empty pycnometer and record the scale reading (Rref). This represents the approximate mass of the filled pycnometer. Remove the reference mass from the pycnometer.
  5. Fill the pycnometer with a sample of the test product (care must be taken to prevent air entrapment or contamination of the sample). Do not place the filled pycnometer in the sun or on a heat source, as this will affect the temperature of the product. Allow the product and pycnometer temperature to reach thermal equilibrium and stabilize to ± 0.1 °C (the required time will depend on the temperature difference between the liquid and pycnometer shell).
  6. Insert a certified thermometer into the product. Do not let the thermometer touch the sides or bottom of the pycnometer, as this may affect the temperature reading. Record the temperature of the sample in the pycnometer. Correct the recorded temperature using the error information stated on the thermometer's certificate of calibration.
  7. Top up the pycnometer if required. Install the lid on the pycnometer and ensure that it is seated completely. A small amount of sample product will squeeze out of the hole in the lid to confirm that it is filled completely. Wipe away all liquid which may have run out on to the top and outside of the shell. Ensure that the outside of the pycnometer and its lid are clean and dry.
  8. Place the filled pycnometer on the reference scale and record the weight indicated (Rpyc). Remove the pycnometer from the scale and then place it back on the scale another two times to ensure there is no change in the reading. Immediately remove the lid and record the temperature, which must be within 0.1 °C of the temperature recorded in (f); if it is not, repeat steps (f) to (h).
  9. Empty the pycnometer and, without resetting the scale to zero or taring off the weight on the scale, repeat steps (e) to (h) above with a second sample of the test product.
  10. Empty, clean and dry the pycnometer. Without resetting the scale to zero or taring off the weight on the scale, place the empty pycnometer on the scale and verify the tare reading. The zero reading must be within ± 1 graduation of the reference scale.
  11. Add the same reference mass that was added in step (d) above and record the reading. The scale reading must be within ± 1 graduation of the initial reading recorded in step (d) to confirm the scale's stability.

Correct pycnometer volume for temperature

If the corrected temperature in step (f) above is different from the temperature stated on the pycnometer's certificate of calibration, a correction of the pycnometer's certified volume is required to determine the true volume of the sample.

Where temperature-corrected volumes are indicated on the pycnometer's certificate of calibration, select the appropriate volume stated for the temperature of the sample (interpolating as required). Otherwise, calculate the true pycnometer volume at the temperature of the sample, using the following formula:

Volume=Vol_pyc×(1+T_s-T_ref)×γ

Where:

  • Volpyc = certified volume of the pycnometer at the reference temperature (from certificate)
  • γ = cubical coefficient of expansion /°C of the pycnometer (from certificate)
  • Tref = reference temperature for the pycnometer calibration (from certificate – usually 15 °C)
  • Ts = corrected temperature of the sample

Note: The values for the pycnometer's certified volume, the cubical coefficient of expansion and the reference temperature for the pycnometer are indicated on the certificate of calibration.

Calculate sample density

Calculate the sample density (ρt) in kg/m³ at the corrected temperature by dividing the recorded weight by the corrected pycnometer volume. The scale calibration factor (SCF) is used to correct the recorded weight.

Density(ρ_t )[(R_pyc⁄Volume)×1000]×SCF

Where:

  • Rpyc = scale reading of full pycnometer (g)
  • Volume = temperature-corrected pycnometer volume (mL)
  • SCF = scale calibration factor
  • 1000 = conversion factor to kg/m³

Convert the sample density to a corrected density at a temperature of 15 °C.

Using the appropriate values from API Table 53D or Measurement Canada-authorized equivalent, convert the calculated sample density at the corrected temperature to a density at 15 °C. Refer to the appropriate section of the tables to correctly interpolate values, if required.

Since two sample densities are required, repeat the calculations for the second density sample.

Ensure the readings of the corrected density (corrected to 15 °C) are within ± 0.4 kg/m³ of each other if a scale with graduations of 0.1 g is used, or within ± 0.2 kg/m³ if a scale with graduations of 0.01 g or less is used. If the density values do not agree within these values, repeat the density determination with two new samples.

Average the results from the two samples. The average density value is used in the remaining calculations.

Note: If density values continually do not agree, then the product is likely not sufficiently homogeneous and will not support gravimetric testing. Alternate means must be found to continue. Consult your Volumetric Specialist for more information.

Use of approved electronic worksheets

As an alternative to using the above calculations to determine the density of the product, electronic worksheets developed by Measurement Canada can be used. They were developed for laboratory tests, but can be used in the field. Available spreadsheets include:

  • Density_by_pycnometer.xls (using a pycnometer to determine the density of a sample)
  • GravLubeOilMeters-CompteursHuileLubrifianteGrav_en.xls (gravimetric testing of a lube oil meter)
  • GravDEF.xls (gravimetric testing of a diesel exhaust fluid meter)

Note: Industry-developed spreadsheets must be authorized for use by Measurement Canada Engineering prior to use in performing examinations.

Procedure (liquefied gas products)

Equipment

  • Certified hydrometer standard (minimum graduation 0.5 kg/m³ or less) appropriate for the density range of the product.
  • Suitable pressure cylinder.
  • Certified thermometer standard (minimum graduation ≤ 0.1 °C).
  • Appropriate density conversion tables authorized for the product (e.g., ASTM‑IP Table 53 or Measurement Canada-authorized equivalent).

Measure density of sample

Ensure the pressure cylinder is completely purged of air; otherwise, purge as necessary.

Note: If the pressure cylinder requires purging, see the procedure in Appendix 1.

  • Circulate the product until the temperature has stabilized within ± 0.2 °C.
  • Fill the pressure cylinder with just enough product to ensure one or several hydrometers are floating freely. Close all valves. Ensure no leaks are present. Record hydrometer reading only after the temperature of the product has stabilized to within one graduation of temperature standard.

Note: For more information on reading a hydrometer, refer to the procedure in Appendix 1 below.

  • Correct the readings of the hydrometer and temperature standards in accordance with correction values stated in the certificate of calibration for each standard.
  • Calculate the corrected density reading.

Convert sample density to standard reference density at 15 °C

Convert the observed density and temperature to a density at 15 °C using ASTM-IP Table 53 (liquefied petroleum gas) or Measurement Canada-authorized equivalent for other liquefied gas products.

Revisions

The purpose of revision 1 was to:

  • streamline formulas and procedures in order to make the procedures easier to follow.
  • eliminate the calculations for correction of air buoyancy when both the sample and test load are measured in the same environment.
  • update reference to Measurement Canada-authorized spreadsheets.
  • revise Appendix 1—Method of Using a Hydrometer and add pressure vessel purging to this section.
  • add Appendix 2—Method of Using a Pycnometer.

Appendix 1—Method of using a hydrometer

Density is defined as the mass of a unit of volume. There are various methods of determining the density of a product, but for most high vapour pressure products, Measurement Canada recognizes and uses hydrometers floating in a pressure jar filled with a sample of the product to be measured. This procedure addresses the use of this type of density determination equipment.

The following procedure should be read in conjunction with the density determination procedure in the above section.

Preparation

After the hydrometer pressure jar has been filled in accordance with previously established procedures, lightly agitate it in an attempt to equalize and stabilize the liquid temperature.

Note the temperature and compare it to a second temperature taken approximately 10 minutes after the first.

If the temperature has not changed by more than ± 0.2 °C, take density readings.

Reading the hydrometer

The correct method of reading an open type hydrometer is illustrated in Figure 1 (reading a closed system type, where the hydrometer is in a pressure jar, is identical).

Place the eye slightly below the plane of the surface of the liquid and then raise it slowly until the surface, seen as an ellipse, becomes a straight, or slightly concave, line.

The bottom of the meniscus or the lowest point at which this line cuts the graduated stem scale of the hydrometer is the reading of the instrument. Record the value to the nearest ½ graduation.

Figure 1—Reading a hydrometer

the long description is located below the image
Description of Figure 1

Figure 1 shows the correct method of reading an open type hydrometer. The user's eye must be level with the top of the liquid in the hydrometer jar in order to reduce or eliminate parallax errors

Correct the reading for errors in the hydrometer

When a hydrometer is certified, it is usually accompanied with a table of corrections. This table stipulates the error at various points throughout the hydrometer's certified range.

Correct each reading to indicate the true value using the information supplied in the certificate of calibration.

If necessary, calculate correction values by linear interpolation between adjacent correction values.

Temperature reading

Record the temperature inside the pressure jar immediately after the hydrometer reading is determined.

Correct the reading for errors in the thermometer

When a thermometer is certified, it is usually accompanied with a table of corrections. This table stipulates the error at various points throughout the thermometer's certified range.

Correct each reading to indicate the true value using the information supplied in the certificate of calibration.

If necessary, calculate the correction values by linear interpolation between adjacent correction values.

Correct density to reference temperature (15 °C)

Correct the density as read from the hydrometer, and corrected for any inherent error in the hydrometer, to an equivalent density at the appropriate reference temperature (15 °C) using the appropriate tables.

Repeatable results

In order to ensure that the final results are acceptable for use, take a second set of readings. The corrected results must agree to within two graduations of each other.

Repeat this procedure until two consecutive readings agree.

Purging a pressure jar

When a hydrometer is used to measure high vapour pressure products in a pressure jar, it is imperative that the jar only contain saturated vapour from the product. If the jar has been opened for any reason, the saturated vapour space will have been replaced with air. This air must be purged before accurate measurements can be taken. In order to purge the pressure jar, follow the procedure below:

  1. Connect a product supply line to the inlet liquid valve and slowly deliver a small quantity of product representative of the product under test into the cylinder.
  2. Once a small amount of the liquefied gas product has entered the cylinder, stop dispensing the product and close the inlet valve.
  3. Open the smaller purge valve slowly (usually located on the top of the cylinder) and allow the liquid to gently boil and change to its vapour state. The vapour will displace any air or stagnant gas vapours that may be present and evacuate them out of the purge valve.
  4. Carefully purge at a slow controlled rate so that the temperature does not drop by more than 5 °C and the boiling does not cause the hydrometer or thermometer to bounce around in order to prevent damage from occurring.
  5. Continue to boil the product off for 10 to 15 seconds, and then close the purge valve. Do not allow the product supply in the pressure jar to become depleted.
  6. Repeat as necessary.

Appendix 2—Method of using a pycnometer

Density is defined as the mass of a unit of volume. There are various methods of determining the density of a product, but for most low vapour pressure products, Measurement Canada recognizes and uses stainless steel pycnometers, usually of a nominal size of 500 mL or larger. This procedure addresses the use of this type of density determination equipment.

The following procedure should be read in conjunction with the density determination procedure above.

Preparation

Pycnometers are vessels of known volume. They are available in several standard nominal sizes. For our purposes, a 500 mL pycnometer is the most common one used. A pycnometer will be supplied with an appropriate calibration sheet. This sheet indicates the actual true volume of the pycnometer as well as the reference temperature for this volume. Both pieces of information are critical for accurate use. Pycnometers are supplied with matching lids and sometimes locking rings. All parts must be from a matched set. All parts must be clean and dry before use and during use, any product on the outside of the vessel must be carefully removed. Properly placing the lid on the pycnometer is a skill that must be practiced in order to achieve repeatable results. Care must always be taken to ensure that the lid is fully seated and any displaced product is removed from the top before weighing the full vessel.

Weighing

Because a pycnometer is used to determine density, its weight must be accurately taken both empty (tare) and when filled with product (gross).

Ensure the pycnometer is clean and dry when weighed empty and completely full of product when weighed full.

Ensure there is no residual product on the outside of the pycnometer or its lid when weighing full.

Carry out weighing on a high resolution scale of known accuracy.

Ensure test standards are available to check the scale throughout its range paying particular attention to any errors at both the tare and gross weight ranges.

Filling

Fill the pycnometer to capacity taking care not to entrap any air in the vessel, particularly with viscous products.

Once full, place the lid on to the vessel and ensure it is seated completely. This action will force excess product out through the vent hole in the lid.

Carefully wipe away excess product before the gross weight is taken.

Temperature stability

Temperature stability of the product and vessel is critical to obtaining accurate measurements. For this reason, a full pycnometer must always be allowed to sit in order to obtain thermal equilibrium before taking any measurements.

Note the temperature and compare it to a second temperature taken approximately 10 minutes after the first.

If the temperature has not changed by more than ± 0.2 °C, weigh the vessel and calculate the density.

Temperature correction

The stated volume of the pycnometer is only correct at the given reference temperature. For this reason, if the actual stabilized temperature of the product and pycnometer is different from the reference temperature stated on the certificate for the pycnometer, a correction for the effects of temperature will have to be made. The cubical coefficient of expansion /°C of the pycnometer, sometimes referred to as Gamma (γ), will also be listed on the certificate and is used to make the necessary corrections. The actual calculations are shown in the STP above.

Worksheets

Request the Measurement Canada-authorized spreadsheet (Density_by_pycnometer.xls) for automated calculations.

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