Projects:2014s2-78 Investigation the Design and Development of Miniature Specific Gravity Sensor

1. Project introduction

In this project, we plan to find a new and easy way to measure the specific gravity. For reaching the target, in this paper, we analyse the RC, RLC in parallel and series circuit. Thus we decide to make use of the property of resonance circuit. RLC circuit has a fixed frequency which is the resonance frequency. As the changes of input frequency, the reactance and impedance of the circuit will change, which is called frequency response. When the input frequency is equal to the resonance frequency which is depended on L and C only, the circuit will have some special appearances. The differences are different respectively for RLC parallel and series circuit. Thus we can get the resonance frequency, by this frequency; we can get the specific gravity through mathematical calculation. In this project, we will make a capacitor which can let the substance flow through the plates of the capacitor. The volume of the capacitor can be calculated. By the relationship between volume fraction and permittivity, we can get the volume of the components of the substance. With the help of density of different components, the total mass can be calculated. Thus the substance density can be calculated. At last, the density divides the reference density, the result is specific gravity.

1.1 Project motivation Specific gravity is widely used and measured on the substance property. Traditionally, if we want to measure the SG, we need to use balance scale or measurement cup. The principle is based on the Archimedes' principle.In this project, we hope to explore another way to reach the target, namely, get the specific gravity with other principle.

1.2 Previous study Traditionally, there are several ways to measure the specific gravity. For different materials such as solids, liquids and gases, there are different ways have been devised to measure. On solid materials, at first, the solid materials are weighted in air, and then immersed in water. The difference between two materials, air and water, is the weight of the water displaced by the volume of the solid. When the solid material is lighter than water, the material will be fully submerge in water. The specific gravity of the solid is the ratio of the weight in air to the difference between air and the weight immersed in water. There are two ways to measure the liquid materials specific gravity normally. The first method is the hydrometer, which is an instrument that gives a specific gravity directly. The second way is the bottle method. About the hydrometer, it is a device used to determine directly the specific gravity of a liquid, which consists of a thin glass tube closed at both sides. One side is enlarged into a bulb that has fine lead shot or mercury to cause the instrument to float in a liquid. The glass tube is a scale calibrated that the reading number on it level with the surface of the liquid in which the device is floating indicates the number of times heavier or lighter than water, which is the specific gravity of the liquid. Based on the Archimedes’ principle, the level at which hydrometer floats is up to the density of the liquid. Thus the level can be used to measure the density and specific gravity proportionally. The temperature also influences the specific gravity. The hydrometers are usually calibrated at room temperature 20°C (68°F). As the variation in the depth, one type of hydrometer is made for use in liquids denser than water, and another is for lighter than water. On the bottle method, a flask made to hold a known volume of liquid at room temperature 20°C. The bottle is weighted first then with the known specific gravity liquid and weighted again. The different weight is divided by the weight of same volume water to give the specific gravity of the liquid. In same approach, the bottle method can be used to measure the specific gravity of gas at standard temperature as well. Taking the salt water as example, we need to measure the mass of cup with salt water, m1. Then we pour the part of the salt water into measurement cup. We measure the weight of the cup with left salt water again, m2. Now we can get the mass in the measurement cup, m=m1-m2. At same time, we know the volume by measurement cup. According to ρ＝mV, we calculate the density of salt water. As stated as above, we can summary that we need to know the target liquid mass and volume in order to calculate the density of the liquid. Finally we use the density to divide the reference density and the ratio is the specific gravity.

2. Project result analysis For measuring the specific gravity of wine during the specific gravity, we must know the grape juice before fermentation, the process fermentation and the final production, wine.We can simplify the grape juice as sugar water because they do have little effect on the calculation of mass and volume.We can simplify the wine as water with alcohol or water with alcohol and sugar regardless the other components because they do have little effect on the calculation of mass and volume. In this project,making use of the property of resosnace frequency of RLC series circuit will measure the value of capacitor. Then by calculation we will find the volme fraction of the target fluid, thus we can get the density of target fluid, naturally the SG value can be calculated.

3. Conclusion Through the analysis and test above, in this project, the frequency response with RLC series resonance circuit can solve the specific gravity problem. In the project, taking the sugar water as example, the capacitor value is fixed. There is a fixed frequency in the circuit. By frequency response, when the input frequency reaches the resonance point, we can find this value by measuring the voltage across the resistor and LC. When we have the resonance frequency, we can calculate the mass and volume of the fluid flowing through the capacitor. In fact, through this approach, we can also measure the solid or gas materials’ specific gravity, though the components of the target materials cannot be too complicated and the density of the components is known.

4. Acknowledgements

  Dr Said Al Sarawi–Dr Said Al Sarawi is a very kind and friendly supervisor who give our project a lot of help and advice.With his help,I can complete the project finally.

  Damith Ranasinghe –Damith Ranasinghe is a good supervisor who is good at the project. With his help I can get some advises about the project.  

  Ian Linke –Ian Linke is a good workshop teacher who helps me a lot to make the capacitor and provide other help on my experiment. 

5. References

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