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 

1.2.1Instrument 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.

1.2.2pycnometer 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.

1.2.3densimeter 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.