DEVELOPMENT AND TESTING OF AN AIR PYCNOMETER. U.A.Garba1, Z. Mustafa1, A. Ja’afaru2 Department of Agricultural Engineering, College of Engineering, Kaduna, Polytechnic, Kaduna,2Department of Mechanical Engineering, College of Engineering, Kaduna Polytechnic, Kaduna 1 ABSTRACT Foods and crop processing machines design is dependent on the correct analysisand determination and determination of their physical properties. An air pycnometer that can be used to determine the volumes of solids that are powder,dried fruits and vegetables was developed. The pycnometer was tested using fourdifferent samples each of 300g per replicate, namely Cassava and Wheat flour, dried and grounded spinach leaves and dried grounded tomatoes. Five setsfor each sample was used such that there are five replications for each sample. Results obtained were used to compute volume of solid Vs and prosity Ps which were plotted against number of replication in comparison to a “linear Vs” line on each plot. It was concluded that the pynometer is functioning well especially for dried and grounded vegetable crops. Key words: Pycnometer, porosity, replicates, valves, Chambers. INTRODUCTION The design of food and crop processing equipment depends on the correct analysis and determination of their physical properties. Properties such as volume, porosity, specific gravity, density etc. determine the design specifications of these machines to ensure efficient processing. For example, Seed (or particle) density is one of the most important physical properties of agricultural materials because it is needed in process design calculations and it is used in the characterization of these materials (Fashina, 2010). Also, the porous structure of dehydrated plant materials is a key parameter that affects the transport properties and a number of quality characteristics of these materials; however this porous structure may be determined by the bulk porosity, pore sizes and pore size distribution parameters (Karanthanos et al, 1996). The volume and porosity of objects or materials such as powders, soils, puffed cereals, pepped popcorn or dried fruits and vegetables that are highly hygroscopic (readily take up and retain moisture) cannot be measured by water displacement; however they can be determined using Air Pycnometer method ( Ma et al, 1998). An air pycnometer is an equipment that uses air displacement to determine volume and porosity of materials mentioned above. Different type of Air Pycnometer exists. However, the application of Air Pycnometer varies in fields where the need for specific weight calculation is desired. The need for the determination of densities and porosity of substances such as soil, powder and food commodities cannot be overemphasized. The determination of density, porosity, specific weight is required in agricultural research, powder and granulates investigations for pharmaceutics industry, road building, food industry etc. Also, Commercial Pycnometers are used for process and manufacturing control in a wide range of applications (Geddis et al, 1996). Air Pycnometer also referred to as Gas Pycnometer has been used for many years to measure the volume of solid materials sealed in a gas tight system (Geddis et al, 1996). It provides indirect Air space measurements by relating a system’s pressures and volumes using Boyle’s Ideal Gas Law. (Agnew et al, 2003). This paper discuss the design and fabrication of an Air pycnometer carried out at the department of Agricultural Engineering, Kaduna Polytechnic Kaduna State of Nigeria. MATERIALS AND METHOD Pycnometry is often thought of as the liquid method described by Blake and Hartage (ASTM, 1986). Air Pycnometer is a device that uses Air displacement to determine volume. The system makes use of pressurized chambers: one empty and one containing the material for which the volume is being determined. The volume of Air displaced by the sample is determined from pressure-volume relationships based on Boyle’s Law. Figure 1 below gives an illustration and description of how the pycnometer developed operates. Chamber #2 + solid Chamber #1 pressure gauge valve #1 valve #2 valve #3 air out air in Fig 1: Schematic diagram showing how the pycnometer operates. The pycnometer was constructed using locally available materials. It comprises a wooden frame made up of two 15mm thick plywood which are fitted at right angle to each other to form a wall of 3.5m2 and base of 3m2in area respectively. Two air tight containers of equal volume of 1809.79cm3 serving as chambers 1 and 2 were fitted 50cm apart on the wall with an air inlet fitted at the top of each chamber. A 12kg gas cylinder was filled with air and used to supply air via a hose of diameter 0.5cm and length 120cm to chambers 1 and 2. The hose was fitted with 3 control valves with valve 1 fitted 60cm from the cylinder outlet, valve 2, 30cm from valve 1, and valve 3, 30cm from valve 2. Two other hoses of lengths 30cm were fitted in between valves 1 and 2, and valves 2 and 3 to supply air into chambers 1and 2. A SCOTT pressure gauge model no.PT 28 measuring up to 11kPa was fitted in between valves 1 and 2. Plate1 below shows The diagram of the constructed pycnometer. PLATE 1: Picture of the constructed pycnometer. According to the well-known perfect gas law, pressure and volume are related by the following relationship: PV nRT Where P is the absolute pressure in the closed system, V is the volume, n is the number of moles of Air in the closed system, R is the universal gas constant and T is the absolute temperature of the system. For the pycnometer developed the pressure to volume relationship in the two chambers at constant temperature is defined as; P1 PV1 P2 PV2 VS P3 PV1 V2 VS ……………………………….. (1) Where P 1 = Pressure reading (gauge pressure) when valves 1 and 3 are open and valve 2 is closed. P 2 = Pressure of chamber 2 after it is filled with solids while valve 2 remains closed but valve 3 is open. P 3 = Pressure reading (gauge pressure) when valves 1 and 3 are closed and valve 2 is open. P = Atmospheric pressure (1.013x105pa, under sea level conditions) V1 = Volume of chamber 1 V2 = Volume of chamber 2 VS = Total volume of specimens contained in chamber 2 Rearranging equation 1 and solving for VS the total volume of the specimen in chamber 2 is P P3 VS V2 V1 1 ………………………………………………………. (2) P3 Porosity (Ps) is the part of a solid occupied by air or the fraction of volume not occupied by solid. Based on this definition the porosity of granular materials can be calculated from the same experimental data generated using the pycnometer by the following equation; Ps P1 P3 …………………………………………………………….. (3) P3 TESTING AND RESULTS The pycnometer was tested using four different samples of 300g each, namely Cassava flour, Wheat flour, dried and grounded Spinach leaves, dried and grounded Tomatoes. Five sets of 300g for each sample was used to test the equipment such that there are five replications for each sample. This is done to monitor the consistency of readings of the pressure gauge. Air was released from the cylinder via the hose to the entire system with valves 1 and 2 open while valve 3 was closed for 30 minutes. This is to allow the entire system to reach a stable condition and also to check for leakages using mixture of water and detergent which was robbed all over the hose and chambers. Valve 2 was then closed and valve 3 was opened and chamber 2 was filled with 300g of the first sample. After 15 minutes pressure P1 was recorded from the gauge. Valve 1 and 3 were then closed while valve 2 was opened. After another 15 minutes pressure P3 after which valve 2 was closed. Chamber 2 was emptied and second replicate was placed inside chamber 2 and valve 3was closed. The procedure was then repeated. Results obtained from five replicates for each sample was used to calculate Volume of solid (Vs) and Porosity (Ps) for each replication using equations (2) and (3) for each sample. The result obtained was summarized as shown in the table 1 below. Also plots of Volume of solids (V s) and porosity (Ps) against number of replications for each sample were presented below as figures 2, 3, 4 and 5 so as to analyze the consistency of readings obtained during testing of the pycnometer. TABLE 1: Summary of results obtained. CASSAVA Vs. Ps REPLICATES 1 672.44 0.75 678.67 0.5 723.92 0.75 723.92 0.65 670.92 0.75 2 3 4 5 WHEAT SPINACH TOMATO Vs. Ps Vs. Ps Vs. Ps 0.7 376.2 452.44 5 2 0.88 0.7 381.6 440.81 3 3 0.83 0.7 387.8 433.44 9 1 0.79 0.7 389.0 467.82 4 5 0.82 382.4 458.9 0.8 4 0.83 500 452.44 450 467.82 440.81 458.9 433.44 400 VOLUME (cm3) 350 300 Vs 250 Ps 200 Linear (Vs) 150 100 50 0.75 0.73 0.79 0.74 0.8 1 2 3 4 5 REPLICATES 0 0 6 FIG 2: Variation in volume and porosity forCassava flour VOLUME (cm3) 500 450 400 350 300 250 200 150 100 50 0 452.44 440.81 467.82 458.9 433.44 Vs Ps Linear (Vs) 0 0.75 0.73 0.79 0.74 0.8 1 2 3 4 5 REPLICATES 6 FIG 3: Variation in volume and porosity for Wheat flour 700 598.87 603.26 603.26 600.99 599.89 VOLUME (cm3) 600 500 400 Vs 300 Ps Linear (Vs) 200 100 0.69 0.72 0.71 0.69 0.68 1 2 3 4 5 0 0 FIG 4: Variation in volume and porosity forSpinach 6 REPLICATES 700 598.87 603.26 603.26 600.99 599.89 600 VOLUME (cm3) 500 400 Vs 300 Ps Linear (Vs) 200 100 0.69 0.72 0.71 0.69 0.68 1 2 3 4 5 REPLICATES 0 0 6 FIG 5: Variation in volume and porosity for Tomato DISCUSSION OF RESULTS As shown in figures 2, 3, 4, and 5 a line referred to as “linear Vs” is indicated on each plot. Values of computed volume of solid (Vs ) for each replicate were also shown. The linear line of Vs means all the values of Vs per replicate are equal and therefore the pycnometer is functioning very well. From the plots presented it can be seen that deviation from the linear line of Vs for the replicates is more in cassava and wheat flour compared to dried and grounded spinach and tomato. However variation in computed porosities of all samples is negligible. But computation of porosity is dependent only on pressure gauge reading while that of the volume of solid is influenced by volumes of chambers 1 and 2. Also it can be observed that Cassava (a tuber crop) and wheat (a seed grain crop) flour have prominent variation compared to spinach and tomato which are vegetable crops. CONCLUSION Based on the result presented and discussed above it was concluded that even though there are variations of values the pycnometer constructed is functioning very well because the variations are not significantly high especially for spinach and tomato. Also the equipment may be considered more functional for dried grounded vegetables than grounded tuber and seed grain crops. REFERENCES Blake P. and Hartage .W (1986) American Society of Tractor and Machineries” Handbook U.S.A. Fashina O. (2010) “Technical Note: Measuring Grains and Legume seed density using pycnometer and envelop density analyser” Unpublished. Geddis. A.M, Guzman A.G, Basset R.L. (1996) “Rapid estimate of solid volume in large Tuff cores using a gas pycnometer” University of Arizona U.S.A. Karanthanos V.T., Kanallopoullos N.K, Belessiotis V.G. (1996) “Development of Porous structure during air drying of Agricultural plant products” University of Greece, Greece. Li M, Denny C.D, Leonard G.O, Gustavo V.B.C (1998) “Engineering Properties of Foods and other Biological materials” Washington American Society of Agricultural Engineers and Washington University U.S.A.