Pressure Measuring instruments

Welcome friend's today will talk about pressure measuring instruments in this week measurement of pressure is a very common requirement in chemical process industries their process operations which take place at moderate pressures in fact we can say that most of the operations will take place at moderate pressures but there are process operations which will take place at high pressures and their operation which take place at low pressures or vacuum for example there are several chemical reactions which takes place at high pressure for a sensitive material we can carry out a  vacuum distillation for separation so it happens at low pressures lower than atmospheric pressure for multiple effect evaporators the last effect we can operate under vacuum so in a chemical process industry we will be require to measure moderate pressures high pressures low pressures pressures at all levels so in this week as well as in the following week we will be talking about several pressure measuring instruments so today we start our discussion on pressure measuring instruments so in todays Article we will concentrate on classification of various pressure measuring instruments and we will also talk about manometers which are very common pressure measuring instruments very commonly used in laboratories so before we start talking about pressure measuring instrument lets be familiar with certain terms associated with the pressure measuring instruments so will be familiar first with various units of pressures and we will also be familiar with various terminologies such as absolute pressure gauge pressure so and so forth so pressure is nothing but force per unit area common units for pressure include pounds per square inch which is briefly known as p s i square metre newton per metre square which is pascal and s i unit of pressure you may also have kilopascals one pascal is equal to one kilo pascal is equal to one thousand pascals there are other units such as inches of water or inches of mercury millimetre of water or millimetre of mercury bar atmospheric pressure you know one atmospheric pressure is equal to seven sixty millimetre of mercury at sea level torr is another units of pressure one torr is equal to one millimetre of mercury for instrumentation and control we use pressures in the range of three to fifteen p s i so this is used for pneumatic control systems three p s i is about twenty one kilo pascal and fifteen p s i is above hundred and five kilo pascal one pascal is one newton per metre square which is equal to one kg metre per second square which is equal to ten dyne per centimetre square one atmosphere is one point zero one three two five bars is equal to seventy five point nine seven centimetre of mercury commonly say as seventy six centimetre of mercury is equal to twenty nine point nine two inch of mercury we commonly say thirty inch of mercury which is equal to one zero three two two kg per metre square so please be familiar with these various units of pressures now lets define the terms gauge pressure absolute pressure atmospheric pressure so on and so forth please look at the diagram lets say this is the atmospheric reference meaning this is standard atmospheric pressure so this pressure is positive gauge pressure equal gauge pressure so gauge pressure is above atmospheric pressure below atmospheric pressure is negative gauge pressure or reduced pressure also known as vacuum if i now consider absolute zero as reference so this pressure will be my absolute pressure note this pressure is this much gauge pressure but this much absolute pressure similarly this pressure which is this much vacuum with respect to atmospheric pressure but this much absolute pressure because now zero is my reference so pressure above atmosphere is called positive or gauge pressure expressed as p s i g pressure below atmosphere is vacuum or negative gauge pressure absolute pressure p s i a is measured from a perfect vacuum that means zero pressure differential pressure which is the difference between two pressure sources has no reference to either absolute vacuum or atmospheric pressure will also define three more terms known as static pressure dynamic pressure and impact pressure static pressure is the pressure of fluids or gases that are stationary so imagine a fluid flowing through this pipe so static pressure is the pressure of fluids of gases that are stationary so at point a the pressure i measure will be known as static pressure dynamic pressure is the pressure exerted by a fluid or gas when it impacts on a surface or an object due to its motion or flow so the pressure at point b minus the pressure at point a will be the dynamic pressure impact pressure or the total pressure is the sum of the static and dynamic pressure on a surface or object so point b we will have the impact pressure or total pressure which involves both static pressure and dynamic pressure now how do we measure pressure see pressure can be very easily be converted to force because if i allow pressure to act on a on a surface area on a surface of known area a force will be developed now this force can be measured or compare against unknown force or standard calibrated force so that way by measuring a force i can measure pressure so one idea of pressure measurement maybe you allow the pressure to act on a surface area whose area we know accurately so a force will be developed now that force can be compared or measure against a known force now since we know the surface area we can find out now pressure which is force per unit area so pressure can be measured by balancing it against the column of liquid of known density we have various manometers which are based on this principal so in manometers we balance the pressure against the column of liquid of known density pressure can also be measured by balancing against a known force so pressure measuring instruments such as piston type ring balance bell type gauge are all examples of instruments which use this principle pressure can also be measured by balancing the force produced on a known area against the stress in an elastic medium this is an important class of pressure measuring instrument so here we balance the pressure or balance the force that is produced on a known area against the stress in an elastic medium this class of instruments are also known as elastic pressure transducers examples are bourdon tubes diaphragm types pressure gauge bellows so this is one possible classification of pressure measuring instruments here we classify pressure measuring instruments based on how they measure pressures but of course high vacuum measurement or instruments that are used for measuring very very low pressures or high vacuum will require specialised instruments so high vacuum requires specialised instruments so that was one possible classification another possible classification is based on the range of pressure measurement whether i am measuring high pressures whether i am measuring low pressure or moderate pressure based on that it is possible to classify various instruments so we can classify all pressure measuring instruments based on moderate pressure measurement very high pressure measurement and high vacuum or very low pressure measurement so we can classify all pressure measuring instruments into three categories pressure measuring instruments for measurement of moderate pressure pressure measuring instruments for measurement of very high pressure and pressure measuring instruments for measurement of high vacuum or very low pressures in this Article let us follow this classification so we will classify all pressure measuring instruments into three categories moderate pressure measurement very high pressure measurement and low vacuum or very low pressure measurement so let us now look at closely on moderate pressure measurement so what are the various pressure measuring instruments that are available under moderate pressure measurements most of the chemical process operations happen at moderate pressure measurement most of the chemical process operation occurs at moderate pressures so there must be various pressure measuring instruments which were able to measure moderate pressure so again there are various classifications based on the principle on which they work we called liquid column elements examples are barometers and various manometers mechanical displacement type ring balance manometer or bell type manometer so this is essentially pressure measurement by balancing the force by balancing it against the known force elastic pressure transducer so the force developed in the pressure measuring instrument is balanced by the space developed in the elastic medium so elastic pressure transducers such as bourdon tube diaphragm type bellows gauges and electrical pressure transducer resistance type potentiometer devices inductive type capacitive type piezoelectric types so these are various instruments under moderate pressure measuring instruments so liquid column elements examples barometers various manometers mechanical displacement or the instruments that balances force arraying balance manometer bell type manometers elastic pressure transducer bourdon tube diaphragm type bellows gauges and electrical pressure transducers resistance type potentiometer devices inductive type capacitive type and piezoelectric type some of these electrical pressure transducers we have talked about when i discussed transducer elements apart from these we will talk about these pressure measuring instruments apart from this only talk about ring balance manometer or bell type manometer we will also talk about an instrument known as dead weight tester for piston type pressure gauge which is also used for calibration of various pressure measuring instruments so that was moderate pressure measuring instruments very high pressure measurements very high pressure measurements is done by electric gauges based on change of resistance of manganin or gold chrome where very high vacuum measurement or very low pressure measurement will discuss wire very high vacuum measurement or very low pressure measurement we will discuss instruments known as mcleod gage thermal conductivity gage ionization gage and knudsen gage so in this week and the following week we will be talking about all these various pressure measuring instruments now lets talk about some indicative range of instruments so we want to know what are the ranges of pressures over which i will use say monometer what are the ranges of pressures over which i will use bourdon tubes and so on and so forth now the numbers that are going to present are some indicative rangers for example below one millimetre of mercury we use manometers or low pressure measuring instruments between one millimetre of mercury to one thousand atmosphere we use elastic pressure transducers such as bourdon tube diaphragm gauges bellows etc for measurement of high vacuum or very low as low as up to ten to the power minus millimetre of mercury or ten to the power minus torr we use high vacuum measuring instruments such as mcleod gage thermal conductivity gauge ionization gauge etc for high pressure measurements such as one thousand atmosphere and above we use electrical resistance type this electrical resistance type depend on the principle that at high pressure there will be changes in the resistance this one so electrical gauges based on change of resistance of manganin or gold chrome wire so now lets talk about moderate pressure measurements so we start our discussion with barometer you are all familiar with barometer since your school days is a simple instrument which measures atmospheric pressure so the simplest mercury barometer consists of a long glass tube of uniform cross section with one end sealed so a tube with uniform cross section and one end is sealed other end is open this tube is filled with mercury and then inverted and placed vertically in a reservoir of mercury so this is the reservoir of mercury mercury in the tube will adjust itself until the weight of the mercury column balances the atmospheric force exerted on the reservoir the mercury in the tube will have a vacuum above it so the weight of the mercury column has balanced the atmospheric pressure or atmospheric force exerted on the mercury level in the reservoir the height of the mercury indicates the atmospheric pressure at sea level this height of mercury column is seventy six centimetre or thirty inches of mercury that is one point zero three three kg per centimetre square or fourteen point seven pounds per square inch next lets talk about u tube manometer it consists of u shaped glass tubes partially filled with a liquid known as manometer liquid so u tube manometer is u shaped glass tube and this is partially filled with a liquid known as manometer liquid mercury is a very common manometer liquid now these are known as two limbs of the manometer so this is one limb this is another limb if both the limbs are connected to the same pressure source the level of mercury in both the limbs will be same but if these two limbs are connected to two different pressure sources level of mercury in both the levels will be different in the diagram pressure p one is greater than pressure p two so the mercury level in this limb is pushed down compared to this limb now this difference between the mercury level in this limb and this limb which is shown here by h is the measure of difference of pressures between this limb and the other limb so if you if you make a balance if you right a balance so take this two points so here the pressure is p one if i neglect this column it may be filled with air here the pressure will be p two plus the pressure due to this column that is h which can be considered as rho g h which is density of the mercury times acceleration due to gravity times h so p one equal to p two plus rho g h so which gives you p one minus p two equal to rho g h so when two sides are connected to two different pressure sources the liquid rises higher in the lower pressure side so that the difference in the heights of the two columns of liquid compensates for the difference in pressure a scale graduated in pressure units is attached to read h so a scale graduated in pressure units will be attached from which you can read the h it may be difficult to read the meniscus meniscus level of mercury so that may bring in some error in the measurement manometer can be used to measure gauge pressure differential pressure as well as absolute pressure if both the limbs are connected to two different pressure sources such as p one and p two this is measuring differential pressure if one limb is open to atmosphere and another limb is connected to the pressure source i am measuring gauge pressure and if on limb is connected to vacuum so this is lets see this is connected to a sealed chamber and i completely evacuate it equated then i am measuring absolute pressure so manometer can be used to measure gauge pressure differential pressure as well as absolute pressure when you measure gauge pressure one end is connected to the pressure source the other limb is open to atmosphere when you are measuring differential pressure both the limbs are connected to two different pressure sources and when you are measuring absolute pressure one end is completely evacuated and sealed and the other end is connected to the pressure source we are measuring so that way you can measure gauge pressure differential pressure as well as absolute pressure so now there should be certain desirable properties of manometer liquid manometer liquid must not wet the wall monometer liquid must not absorb gas or it must not react chemically with the fluid whose pressure i am measuring manometer liquid or manometer liquid should have low vapour pressures if we use a liquid with high vapour pressure there will be loss of liquid and their will be error manometer liquid should move freely some common manometer liquids are water but you have to remember that there will be evaporation loss aniline carbon tetrachloride bromoform mercury transformer oil so these are some common manometer liquids lets now talk about various types of manometers various types of manometers are u tube manometers we just talked about u tube manometers then well type or reservoir manometer which is a modified manometer which is a modified u tube manometer then we have inclined manometer and then you also have float type manometer well type manometers in a well type manometer one leg is replaced by large diameter well since the cross sectional area of the well is much larger than the other leg when pressure is applied to the well the monometer liquid in the well lowers only slightly compared to the liquid rise in the other leg so what we do is in a well type manometer one leg is replaced by a large diameter well since the cross sectional area of the well is much larger than the other leg when pressure is applied to the well the monometer liquid in the well lowers only slightly compared to the liquid rise in the other leg as a result of this the pressure difference can be indicated only by the height of the liquid column in one leg so you do not have to measure the liquid level in both the legs you can measure the liquid level in only one leg because the change in the mercury level in the well which has a large diameter is very very low this makes the instrument easier to use then the u tube manometer so let us look at some calculations so this is the well time monometer this is the well it has large diameter lets say this is the cross sectional area of the well at two this is the cross sectional area of a one and a two is much much greater than a one now when this two links are connected to different pressure sources the level of the mercury in well goes down by a very small amount let us say delta h and the level of the mercury in the low diameter leg increases much and lets this reading be h so initially this was the zero level so the change in the mercury level in the well has gone down only by small amount delta h and the change in the mercury level in the vertical leg is h and p two is better than p one so the amount of mercury that has been pushed down in the well has only gone up in the vertical leg so i can write a two times delta h which is the volume of the mercury that has gone down in the well is equal to a one h which is the amount of mercury that is contained in this length from that i can rearrange delta h equal to a one by a two into h for static balance we will write p two is equal to p one so p two is equal to p one plus rho g into h plus delta h so make a balance on these line so here the pressure is p two and here the pressure is p one pressure is p one plus pressure due to this mercury level plus pressure due to this delta h so p two equal to p one plus rho g into h plus delta h if you now put delta h equal to a one by a two into h i get p two minus p one equal to rho g into one plus a one by two into h now if a one by a two is much much less than one because a two is much much greater than a one then i can neglect this and then p two minus p one can be written as p two minus p one equal to rho g into h if the area of the well is five hundred or more times larger than the area of the vertical leg the error involved in neglecting the area term is negligible next lets briefly talk about inclined tube manometer the inclined monometer or draft gauge is a variation on the well type manometer in which one leg of the tube is inclined to increase measurement sensitivity inclined manometers can measure low pressures the low pressure arm is inclined so that the fluid has a longer distance to travel then in a vertical tube for the same pressure change this gives a magnified scale and thus increases sensitivity of the manometer so this is inclined manometer you have the well and the other leg is inclined so what happens is mercury travels a long distance in this arm so when the limbs are connected to two different pressure sources mercury level goes down here only by this amount say r m but since this is inclined this much will be the increase in the length of mercury in the tube and the relationship between r m and r one is r m equal to r one sin alpha so now again i can write the balance equation for static balance p two minus p one equal to rho g into one plus a one by a two into r m this follows straight from the discussions on well type manometer r m is equal to r one sin alpha so p two minus p one equal rho g into one plus a one by a two into r one sin alpha if a two is much much greater than a one then a one by a two will be much much less than one so in that case p two minus p one will be equal to rho g r one sin alpha the scale of the manometer can be extended greatly by decreasing the angle of the inclined leg alpha to a small value this comes from the relationship r m equal to r one sin alpha if you make alpha small sin alpha will be small so we have to increase our one so that r one into sin alpha matches with r m so by decreasing the value of alpha i can increase the sensitivity further and finally lets talk about float type manometer this is another variation of well type manometer this can also be used as a recording type manometer this works as follows do you have this two legs of the manometer these two are connected by a flexible connection now the diameter of this leg is higher and you can place a float inside this as the mercury level here goes up or comes down depending on these two different pressures the float goes up or comes down this moment of the float can be used to move a pointer against this scale which can be directly calibrated in units of pressure so that you can measure the difference between these two pressures so that way this mercury float manometer can be used to measure differential pressure so the span of the measurement can be changed by changing the diameter of the leg a large float can be placed to generate enough force so that the pointer and scale movement is possible so we stop our Article twenty one here