Comparison between Digital Densitometer and Classical Methods in Measuring Density and Concentration 

（Technical Center of Hainan Entry-Exit Inspection and Quarantine Bureau Hannan Haikou）

Overview 

In the inspection of import and export commodities, density(specific gravity) and concentration(alcohol ,sugar) value are closely related to quality and weight. Especially in the weight identification service, the value of the goods is determined by weight, which transformed by density. Hence, accurately determine the density and concentration value matters the fair dealing in international trade. At present, methods of determination commonly used in China and international standards are densitometry method, pycnometer method and alcohol meter method. Densitometer method simple and quick, but the accuracy is poor. Commonly, densitometer and alcohol meter precisely to the two decimal places. Special densitometer precisely to the third decimal places at most and need to look up the table to transform the temperature coefficient. Pycnometer method can be precisely to the fourth decimal place, but has the drawbacks of long operation time and severely demanded by the environment temperature, it is not convenient to determine the value accurately and quickly. Author measures density and concentration value of a variety of liquid by imported digital densitometer. The instrument is controlled by a microcomputer and can be determined at one time by different settings. It can display and print density values , specific gravity values alcohol degree and sugar degree at different temperatures. The method is fast and simple. Each sampling quantity is as low as 1~2mL.Compared with classical method and analyzed data by mathematical statistics principle we find there is no significant difference between the results of these two methods.

1. Instrument and Principle 

1.1 Instrument and Utensil

       Digital densitometer , petroleum densitometer , alcohol meter ,sugar meter;250ml 500ml 1000ml glass measuring cylinder; constant temperature water bath thermometer.

1.2 Method and Principle 

1.2.1 Instrument method 

The instrument is designed by the principle of vibration of U type oscillator tube. It measures the density and specific gravity value of liquid by measuring the resonant frequency of liquid. And calculate the concentration of liquid by looking up the built-in density-concentration (sugar degree alcohol degree) table. The instrument operates simply. Inject 1~2 ml sample by injector. Keep it in the constant temperature sink for about 5 minutes and then the screen will display the density and concentration needed. The instrument will calibrated to target-temperature in the temperature range.

1.2.2 Pycnometer method 

       According to the principle of specific gravity. Generally refers to the specific value of the liquid mass and water mass under the same temperature and have the same volume.

1.2.3 Densimeter method 

       Commonly densimeter and special densimeter which designed according to the Archimedes principle. For example petroleum densimeter designed for petroleum products. Alcohol meter designed for measuring the concentration of alcohol and sugar meter designed for sugary fluid and juice products.

2. Experiment data and analysis 

2.1 Significance difference test between instrument method and classical method 

   1). Select 10 different liquid samples and measure them specific gravity value by instrument method and pycnometer method respectively. Select 10 different petroleum products and measure them density value by instrument method and petroleum densimeter method respectively. Then use paired t test to statistic process experimental data. The results are shown in table 1 and table 2 and it proves the instrument method accords well with the pycnometer method and densimeter method.

Table 1   Comparison of measuring specific gravity value by instrument method and pycnometer method  

Table 2 Comparison of measuring density value by instrument method and petroleum densimeter method 

2). Results of measuring alcohol degree in different wine and edible alcohol by instrument method and alcohol meter method are shown in Table3. We measure crude wine sample and distilled wine sample. The experiment result shows that the same as the alcohol meter method, when measuring alcohol degree of wine by instrument method, only alcohol degree of distilled wine can be measured accurately. With regard to fermented wine, because it contains a lot of sugar or pigment and specific gravity of aqueous solutions of these contents are greater than 1. The effect is lower the alcohol degree value in actual measurement of alcohol. So when we measure fermented wine ,mixed wine and colored wine ,we should distill it before measure. There are no significant difference between instrument method and alcohol meter method when measuring alcohol degree in distilled wine ,mixed wine and fermented wine.

Table 3 Comparison of measuring alcohol degree by instrument method and alcohol meter method

Note: Liquor sample are measured in 20℃,1# represent undistilled liquor sample and 2# represent distilled liquor sample.

3). Results of measuring sugar degree in soft drinks by instrument method and sugar meter method are shown in Table4. The main ingredients in soft drinks are cane sugar and water. The content of other additives is very low and it has little effect on the determination. Generally a little higher than sweet water. Even if it does, it is a systematic error. In other word, the effect is the same for both methods. Statistical results show that there are no significant differences between these two methods when measuring the sugar degree in white gourd tea, coca cola, grape soda and Red Bull.

Table 4    Comparison of measuring sugar degree in soft drinks by instrument method and sugar meter method

2.1 Comparison of experimental precision of the method

Select 8 pairs parallel test data of density value and specific gravity value. Compare the precision of instrument method and pycnometer method, petroleum densimeter method and the results meet the requirements of various methods. Precision is expressed as the relative deviation of parallel tests. Details are shown in Table 5 and Table 6. Relative deviation of instrument method is in 0-0.0070%, pycnometer method is in 0.0055%~0.021%, petroleum densimeter is in 0.011%~0.025%. This shows that the precision of instrument method is high on account of its high degree of automation.

Table 5 Comparison of measuring precision between instrument method and pycnometer method

Note: sample are measured in 20℃ 1# 2# are the parallel results

Table 6 Comparison of measuring precision between instrument method and petroleum densimeter method

Note: sample are measured in 20℃ 1# 2# are the parallel results

3. Conclusion

Experimental proof digital densitometer has the remarkable superiority in contrast to classical method:

1. Easy and fast operation

2. Accurate measurement results

3. Widely used , one instrument can measure density, specific gravity, alcohol degree, sugar degree of variety samples of chemical and food products.

4. The measurement is not affected by the environment and can auto-calibrated to the target temperature in temperature ranges.

Hence, in the daily commodity inspection work, used in lieu of above classical methods can greatly improve the efficiency of work.

References:

[1]   GB/T 1884 - 1992.石油和液体石油产品密度测定法(密度计法)[S].

[2]   GB/T 2540 - 1981.石油产品密度测定法(比重瓶法)[S].

[3]   ISO 279一1981.香油精在20℃时相对密度的测定[S].

[4]   GB/T 10345.3 - 1989.白酒酒精度的试验方法[S].

[5]国家进出商品检验局数据统计专业委员会.实验室数据处理[M].北京:科学普及出版社，1992.125 -140.