OIV-MA-BS-06 Density of alcohols and alcoholic beverages method for determining electronic densimetry (principle based on measuring the period of oscillation)

 

Type IV method

 

1. Introduction

This method for determining the density of neutral alcohols and alcoholic beverages is based on the change in oscillation frequency in relation to the change in mass based on calibration with two fluids of known density.

Electronic densimeters with digital displays are commercially available to perform this determination.

2. Object and scope of application

The purpose of this document is to describe a method for determining the density of alcohols and alcoholic beverages at atmospheric pressure.

The application of the method is restricted to products with a vapour pressure of less than 800 hectoPascal (600 mmHg) and a viscosity of less than approximately 15,000 m2/s (I m/s = 1 cSt) at the test temperature.

With reference to the currently applicable regulations, the test temperature is set to: 20°C.

3. Density

The density of a liquid at a given temperature is equal to its mass divided by its volume:

It is expressed in kilograms per cubic meter (kg/m³) at a temperature of 20 degrees Celsius (°C) for alcohols and alcoholic beverages.

Note: electronic densimeters display results expressed in grams per cubic centimetre which may be converted into kilograms per cubic meter.

4. Principle

4.1. A liquid sample of a few millilitres is introduced into a vibrating measuring tube.

Measuring the period of oscillation of the tube containing the sample determines the density of the sample at the test temperature, using a previously calibrated apparatus.

4.2. Principle of vibrating measuring tube.

Figure 1: Vibrating measuring tube

Electronic densimeters operate according to the vibrating measuring tube principle (fig. 1): the fluid is introduced into a U-shaped tube and subjected to electromagnetic excitation (fig. 2 and fig. 3).

 

Figure 2: Reactive forces exerted by the fluid

 

The induced period of oscillation is thus proportional to the total mass subject to vibration and can be used to determine the density of the sample based on the following equation:

Figure 3: Resulting torsion

• where T = induced period of vibration
• M = mass of the empty tube
• V = volume of the oscillated sample
• C = spring constant
• p = density of the sample

4.3. Detailed principle.

The density of the liquids is determined by the electronic measurement of the oscillations of a vibrating U-shaped tube.

To carry out this measurement, the sample is introduced into an oscillating system, whose specific frequency is thus modified by the mass of the substance introduced.

The system comprises an undamped U-shaped vibration tube, subject to electronic excitation. The two straight sections of the U-shaped tube act like a spring mechanism. The oscillatory movements occur perpendicular to the U. The filling capacity, V, is delimited by the two fastening points. If the oscillator contains the volume established as V, the latter vibrates within and with the tube. It is accepted that the mass is proportional to the density. Since filling the tube beyond the fastening point does not affect the measurement, it is possible to perform a continuous flow measurement.

By maintaining a constant temperature over the entire system, the density will be calculated based on the period assuming a hollow container having mass M suspended by a spring, with a spring constant c. The hollow container will be filled with a volume of liquid of density p. The natural frequency of this vibratory system is:

F=1/2

• A = (c/4)²  V
• et B=M/V
• which gives p = A T2 - B
• (p = rho).

Constants A and B are the oscillator's spring constants, i.e. the mass of the empty tube and the tube's filling capacity. A and B are therefore system constants specific to each oscillator. They can be deduced from the measurement of two periods when the oscillator is filled with substances of known densities.

5. Apparatus

5.1. Digital display densimeter.

This appliance comprises the following elements:

• a glass measuring cell containing the measuring tube, a constant temperature chamber to be connected to an external circulating thermostatic bath, and a thermowell. The chamber can also be thermostated using an integrated device with a semiconductor element which uses the PELTIER effect,
• a system for oscillating the measuring tube and for measuring the period of oscillation,
• a clock,
• a calculator and digital display.

5.2. Temperature control.

The densimeter's performance standards can only be met if the measuring cell is connected to a thermostatic bath, ensuring a temperature stability better than ± 0.02 °C, or if the densimeter has an integrated thermal control device which can achieve the same temperature stability.

5.3. Sample injection syringes.

At least 2 ml polypropylene or glass syringes with tips that fit to the cell inlet. An adapter with PTFE cones is required in order to avoid the deterioration of the tip of the measuring cell.

The electronic densimeter can also be equipped with the appropriate autosampler for the apparatus.

5.4. Temperature measurement.

Temperature measurement is carried out on the cell with a temperature probe whose sensing device (a platinum resistance probe compliant with class A of standard NF C 42.330), which is mounted with 4 wires, is introduced into the thermowell provided for this purpose in the cell. The probe is combined with an electronic temperature transmitter whose readout has a resolution of 0.01 °C. The probe and transmitter must first have been be calibrated in an approved calibration centre in order to ensure temperature measurement with an uncertainty less than or equal to ± 0.05°C.

The probe and transmitter must be periodically checked.

6. Products
   1. Reference fluids.

During tests, the fluids must maintain their density characteristics; therefore, they must not be made of mixtures of products with different vapour pressures; their molecular composition and purity must be known. Their viscosity must be lower than 2 mm²/s.

Reference fluids must be chosen so that the densities encompass those of the products to be measured. The difference in density between the 2 reference fluids at the same temperature must be higher than 0.01 g/m³.

The reference fluid densities determined at a temperature of 20 °C with an uncertainty of less than ± 0.05 °C must be known with an uncertainty of less than ± 0.05 kg/m³.

When measuring the density of alcohols and alcoholic beverages, the following must be used, under the conditions previously described:

• hydro-alcoholic solutions whose density is exclusively determined using the pycnometric method (reference method).
• recently prepared, degassed double distilled water, or water of equivalent analytical purity,
• dry air.

6.2. Cleaning products.

• chromic acid,
• organic solvents: ethanol 96 vol%, pure acetone.
  3. Drying.

• Pure acetone, dry air

7. Period measurements in ambient air

Prior to the commissioning and calibration of the densimeter, it is essential to ensure the reproducibility of the measurement in the ambient air so that this measurement can be used to quickly check the cleanliness of the cell and consistency of the densimeter before every density measurement.

It should be possible to perform measurements of periods in the ambient air with a reproducibility of ± 10^-5 in relative values over the period for the same barometric pressure and the same temperature.

With some densimeters, the resonant period in the ambient air varies depending on the position of the temperature probe in the thermowell. For these densimeters, either the measuring cell must be replaced, or the temperature probe must be permanently attached, or its position in the thermowell must be accurately determined in order to achieve the reproducibility conditions described above.

 

NOTE: The use of polluted or excessively humid air may negatively influence the measurements. When these characteristics are combined in the test room, it is advisable to make the drying air flow through a purifier/dryer.

8. Apparatus calibration
   1. General.

The apparatus must be calibrated upon initial commissioning. It must be recalibrated if a deviation in the air measurement is observed (see section 9.20) and, in any case, every three months.

8.2. During initial calibration, it is necessary to calculate the values of constants A and B, determined by measuring the periods of oscillation (T1 and T2) respectively obtained using two reference fluids.

8.2.1.    Place the display selector on the period measurement (T) position. Rinse the cell with acetone. Dry it with dry air generated by the pump that is integrated to the densimeter. When the reading is almost stable, stop the air supply, wait for thermal equilibrium and record the period of oscillation (Ta) obtained with an ambient air temperature of 20 °C. This process helps to check the cleanliness of the cell and stability of the apparatus at every calibration or determination of sample density.

8.2.2.    Calibration measurement using the first reference fluid. Use a syringe to fill the cell through its bottom port with the standard liquid until it comes flush with the top port. Leave the syringe in place. While performing this, check the filling quality by visually checking for any air bubbles, even tiny ones. When thermal equilibrium has been reached, record the reading for the period of oscillation (T1). If the thermal control is compliant with the required accuracy, the value of T1 must not vary by more than ± 20 nanoseconds (2 resolution points).

8.2.3.    Calibration measurement using the second reference fluid. Empty the cell with the syringe by drawing from the bottom port. Rinse the cell with acetone. Dry it with dry air generated by the pump that is integrated to the densimeter. To do so, connect the air outlet to the top port of the cell, start the pump and allow it to operate until the reading for T2 is almost constant; stop the pump, and when thermal equilibrium is reached, record the reading for the period of oscillation (T2) which corresponds to the air. If the reading for T2 matches the value obtained during previous tests carried out with a properly cleaned cell and at the same temperature, the cell can be considered clean and dry.

Carry out calibration using the second standard by repeating the steps in paragraph 8.22 and record the reading for the period of oscillation (T2) corresponding to the second reference fluid.

8.2.4.    Based on the T1 and T2 values measured and the known values for the densities of both reference fluids, calculate constants A and B using the following equations:

• A = T1² - T2² / p1 – p2
• B= T2² – Ap2

where

• T1 is the observed period of oscillation with the cell containing the first reference fluid (in ms).
• T2 is the observed period of oscillation with the cell containing the second reference fluid (in ms).
• p1 is the density of the first reference fluid at the test temperature (in g/cm³),
• p2 is the density of the second reference fluid at the test temperature (in g/cm³).

Depending on the procedure for the densimeter being used:

8.2.5.    Enter constants A and B using the digital display at the top of the apparatus. In order to make sure they have been correctly memorised, display the values by placing the selector on the "A" and "B" positions.

8.2.6.    Place the selector on the measurement position. The densimeter should immediately display the densities of the samples introduced into the measuring cell.

NOTE: Some electronic densimeter models automatically calculate the calibration constants.

8.3. Checking the calibration.

In order to validate the operation, measure a reference solution whose density value is within the calibration range used.

Reference substances, verified by a metrology agency, are commercially available.

The calibration is validated if the result of the measurement of the reference substance density complies with the accuracy class of the electronic densimeter being used.

9. Procedure
   1. Preparation of test apparatus.

• Place the densimeter on a perfectly stable support, isolated from any vibrations.
• Connect the densimeter to the circulating constant temperature bath using flexible rubber pipes or insulating tubes. Fill the water bath according to the manufacturer's instructions and add a product to prevent the formation of algae.

Set the bath temperature to reach and maintain the requisite test temperature on the densimeter.

• Accurately setting and controlling the temperature in the measuring cell are very important parameters, as a 0.1 °C error can result in a variation in density in the order of 0.1 kg/m³.
• The following rules must be observed:
• the measuring cell must be maintained at a constant temperature for 6 hours before the test.
• the maximum temperature variation measured by the temperature probe of the measuring cell must not exceed ± 0.02 °C.
• the pipe flow, length and insulation between the thermostatic bath and the cell are to be adjusted to ensure the stability of the cell temperature.

9.2. Checking period measurements in the ambient air.

• Clean, rinse and dry the cell.
• Carry out a measurement in the ambient air. Check that the period measured does not deviate by more than i0- in relative values from the period determined under the conditions described in section 7. If deviation occurs, clean the cell again with a lukewarm chromatic acid solution (warning: this product can cause serious burns), which is the most effective cleaning agent. If the deviation persists after several cleaning operations, repeat the calibration process.

9.3. Density measurement.

• Filter the sample first, if necessary.
• Illuminate the cell.

If only a small quantity of sample is available, use the syringe to introduce the quantity needed so that the liquid to be measured reaches the top port of the clean and dry cell. While filling the cell, make sure all air bubbles are completely removed; the sample must be homogeneous and must not contain any solid particulates.

• Leave the syringe in place on the bottom port of the cell.

NOTE: Introducing dark coloured samples into the cell does not help to establish the absence of air bubbles or solid particulates with certainty.

Switch off the lamp immediately after introducing the sample, as the heat it produces influences the measurement temperature.

9.4. Calculation and expression of results.

9.4.1.    Densimeter with an integrated calculator.

After a few minutes, the density value stabilises, indicating that the equilibrium temperature of the measurement has been reached. If the measurement temperature does not differ by more than ± 0.01 °C from a temperature of 20 °C, record the reading.

If needed, convert the obtained result into kg/m³.

9.4.2.    Densimeter without a calculator.

Allow the reading for the period of oscillation (T) to stabilise within one unit from the fourth decimal place. If the measurement temperature does not differ by more than ± 0.01 °C from a temperature of 20 °C, record the reading.

Calculate the density of the sample in kg/m3 using the following formula:

where

• T is the period of oscillation for the sample measured (in ms).
• A and B are the constants defined during the calibration prescribed in paragraph 8.

9.4.3.    Viscosity correction.

If the liquid whose density is being measured has a viscosity higher than 2 mm²/s, correction is required to take the viscosity into account, using the formula provided by the manufacturer of the densimeter.

10. Test report

The test report must indicate:

• the method used,
• the result and mode of expression of results,
• the specific details and any unforeseen events recorded during the measurement,
• operations not included in the method.

 

APPENDIX A

TABLE 1

AIR DENSITY

Air density, expressed in g/cm³, varies with the pressure P expressed in mbar and the temperature expressed in °C.

A t °C and p Ton-, calculate the density using the following formula:

Values are given for contents of 0.03 vol% of CO₂ in the air; values change by ± 1/19000 with each variation of ± 0.0001 of the CO₂ volume.

Composition of dry air at ground level:

	N2	O2	A	CO2	Ne	He	Kr	X	H2
Volume in %	78.09	20.95	0.93	0,03	0.0018	0.0005	0.041	0.068	0.045
Mass in %	75.52	23.15	1.28	0.05	0.0013	0.047	0.043	0.044	0.084