 Hi friends, I am Dr. Prakash Sarunke working as associate professor in mechanical engineering department of Walshchian Institute of Technology. Today continuing with our discussion on comparators, today we shall be discussing pneumatic comparators. At the end of this particular session students will be able to explain principle and working of pneumatic comparator and will describe merits and demerits of them. A comparator is a precise instrument used to compare the dimensions of given components with the standard and measuring the deviation. The comparator which uses gas or air under pressure for their operations are called as pneumatic comparators. Now the technique of using air in metrology that is for measuring purpose was first developed by the Solex company in France. Pneumatic gauging became popular in recent years due to the higher magnification range of around 30,000 to 1 coupled with other advantages that we shall be discussing later. Now pneumatic principles of pneumatic comparator. Pneumatic gauging or that pneumatic comparators are based on Bernoulli's theorem. Can you tell me what is Bernoulli's theorem related with the fluids? You have studied it, isn't it? Okay. Yes, who can tell? Bernoulli's theorem states that for a fluid system total head or total energy remains constant. And for the fluid system normally there are three heads, one is pressure head, another is velocity head or it is sometimes also called as flow rate head and potential head. In pneumatic comparators, deviation is shown by a variation in either a pressure head or a velocity head provided potential head remaining constant. So potential head is maintained constant and if there is variation in one head so that will vary the another head by equal amount but in opposite direction. So figure one illustrates the principle of pneumatic comparator. So let us see this figure, yes so this is the figure. So this shows that this compressed air is coming through the filter. Here there is a pressure regulator unit, filter and pressure regulator units are common for any fluid system especially pneumatic system. And say here there is a control orifice, control orifice and it is named as CO and area C. So this is a chamber, this is a chamber whose pressure can be measured with the help of this pneumatic gauge. So there is one more orifice so this is known as measuring orifice and through which measuring jet emerges and air is going to the atmosphere. So here is a restricting surface, restricting surface and distance between the restricting surface and this MO that is measuring orifice is designated by L. Now let us try to see the working. Air at constant pressure P is allowed to flow through a control orifice CO as we have seen in the figure into an intermediate chamber and then to a measuring orifice through which air will escape to the atmosphere. So when there is of course no restriction so air will be escaping to the atmosphere. Initially there is no restriction to the measuring jet and area of the CO that is control orifice and measuring orifice MO are such adjusted that incoming air flow is equal to outgoing one. Hence there will not be any change in the pressure inside the intermediate chamber so that is PI and it will be equal to atmospheric. However if the pre-flow to the atmosphere through the measuring orifice is restricted then there will be rise in the pressure inside the chamber. Now according to Bernoulli's theorem say the pressure rise will be equal to decrease in velocity head. So there will be decrease in velocity head because there is restriction, there is restriction and so that will result in increase in pressure. So pressure rise and increase in pressure rise will be proportional to the decrease in the velocity head. For a small portion in the vicinity of measuring orifice the outgoing air jet can be assumed to be cylindrical as shown in the figure. So if you refer the figure so you can see so here so this is highlighted so this is the measuring orifice so this is the restricting surface. So this air jet which is emerging from measuring orifice and going to the atmosphere so in this small this viscerity so it is almost cylindrical so it is assumed to be cylindrical. And say with the diameter D which is equal to the diameter of the measuring orifice. Flow rate will be equal to volume of cylindrical jet so that is PI by 4 into D square into L while D being when D is constant because diameter of the measuring orifice will constant the flow rate will be proportionate to L so that is the distance of the restrictor from the orifice that we have seen in the figure. While using the comparator initially a standard will be set to restrict the flow through the measuring orifice and a zero reading of pressure may be set for this particular position for this position a standard component will be there. Now then the standard will be replaced by different components for restricting the flow. If there is any variation in the dimension of the component from the standard then L will change L will change if the component is of larger diameter L will decrease if it is of smaller dimension L will increase and because of will say there will be reflection in change of flow rate and consequently change in pressure. So there are two methods to measure the deviation with pneumatic comparator. One is by measuring the air pressure another is by measuring the velocity or flow rate of the air principle remaining the same. Now working of pneumatic comparator which is also commercially known as solex gauge or solex pneumatic gauge the instrument was first developed and produced by solex air gauge limited France. So we can refer figure two the different components of this solex gauge are one is air compressor then air filter and regulator unit water tank and dip tube manometer control orifice measuring head with measuring orifice. So let us see the figure yes so this block so that represents the compressor. So this is the filter and regulator unit this is the control orifice so this is the water tank and this is the dip tube dip tube. So this is the manometer arrangement manometer arrangement and say this is the measuring head or measuring gauge. So this is the component so that is being measured so this is the I mean this measuring component and operation is highlighted. So let us first try to understand the some in working if the supply pressure is higher than the desired some air will bubble out from the dip tube so which is shown in this figure so this bubbles so the pressure desired pressure will be maintained by the water level in this particular tank if the supply pressure is excess of that say then that pressure I mean air will be bubbling out okay. So air at desired pressure will pass through this control orifice control orifice. So one say this is the manometer so manometer end is attached here and one end is here so it will measure the difference here and here the manometer. So this air will be going to the measuring head so this is the measuring head say that is for checking the bore it is also called as air plug. So air through this bore will be passing through the atmosphere and so this component say will act as a restrictor. Now what happens the measuring head will be aligned with the standard component initially which will indicate the pressure rise due to restriction of the flow. So this is the reference level this is the reference level. Now the different components are checked for deviation from this standard reference level while measuring the bore roundness can also be checked by rotating the measuring head so that is our air plug and taper is checked by moving the air plug through the length through the length. Now advantages and limitations of pneumatic comparators. Now advantages are very high magnification and because of this say that comparator is very sensitive to even a minor deviation so that can be checked. So the range will be magnification range will be 30,000 to 1 almost no moving parts in the comparator hence very high accuracy because naturally there will not be any friction there will not be wear and tear so over the period also there will not be loss of accuracy. References from textbook of meteorology by Mahajan, Dhanpatra and company privately made I thank to them and I thank you.