 We're now going to do a bit of an experiment and that experiment is going to involve the pedostatic tube. So in the last segment we talked about the pedostatic tube as being a device that you can use to measure velocity in a flow and what the pedostatic tube involves is basically a tube within a tube and so we have an inner tube and then we have an outer tube wrapped around that and in this outer tube there'd be another tap so that measures P-static and this one here measures P-tonal or P-naught and in the side of the tube itself there's going to be holes and that enables us to measure the P-static and then up here the flow is going to stagnate and so that's where we're measuring P-naught where velocity goes to zero along a streamline and we saw in the last segment that we said that we have a way to be able to calculate the velocity based on the pressure measurement or the difference in pressure between P-naught minus P-infinity divided by the density of whatever fluid we're looking at so that would be rho of whatever fluid if it's air that would be the density of air so what we're now going to do we're going to take a look at a video clip with a P-tostatic tube and then we're going to make a velocity measurement by measuring the pressure differential using an incline manometer so let's take a look at that video clip now so what we have that's an image of a P-tostatic tube I'm zooming in on the front of it and you can see the static port that's looking from the front where you see the total pressure port and then as we angle it you'll be able to see both the total at the front and the static pressure tap on the side here's another P-tostatic tube and there you can see the taps at the end of it and then when we look at the front of that P-tostatic tube again you'll see a total pressure tap and then the static pressure tap on the side and that's where we measure the total pressure and the static and here's an incline manometer we're going to hook the P-tostatic tube up to the incline manometer one tube goes to the static one goes to the total let's see what happens when we turn on a flow and put the P-tostatic tube into it we can see that the manometer is increasing increasing increasing I've sped this up because it takes a while with this fluid to react we're going to measure that we get point zero four seven five inches of water and with this incline manometer it's been calibrated at sea levels that's 850 feet per minute so what we're now going to do we're going to do some analysis of that so if you recall from the video it said 0.0475 inches of H2O measured by the incline manometer and it also had a calculation 850 feet per minute but I have to say that that is at sea level and the experiment that you just watched was performed in Calgary and the pressure was around 89 kPa so the 850 feet per minute will not be accurate and we'll see that as we go through the calculation so to begin with the incline manometer is giving us a pressure measurement and that was in inches of water so I'm going to convert that inches of water I don't like working with British units so I'm going to convert that to SI so we get our delta H we can then take that and equate that to the difference between the total pressure tap and the static pressure tap and in order to do that we have to convert inches of water into a pressure so that's row of water G delta H plugging in the values we get 11.84 Pascal so that's a low pressure but that's what we're getting now in the Bernoulli equation let's take a look at it again remember we have this density here that is row air at local conditions and as I mentioned this experiment was done in Calgary probably around standard atmosphere in terms of temperature so 15 degrees C so what we're going to do let's calculate the density of air and that will be P over RT where P 89,000 plus or minus a couple hundred Pascal's but not a big deal here 287 for the universal gas constant and then assuming about 15 degrees C 288 K with that we get 1.0768 kilograms per meter cube so I'm going to take this I'm going to take my delta P measurement and we're going to take it and we're going to put it into this equation here for the pedostatic tube enabling us to then determine the velocity so when we do this we get about 4.7 4.69 meters per second is what the pedostatic tube was exposed to now I'm going to convert that to British units because that's what we had on the incline manometer and I just want to take a look as a bit of a sanity check to see how close we are so we have feet per second but that was in feet per minute so I have to convert that to feet per minute and what we get is 923 feet per minute and the incline manometer had been calibrated and it was giving us a value of 850 feet per minute so there's an error there and the source of that error comes from the fact that when I calculated density here I used the pressure of 89 kPa versus 101.325 kPa so let's take a look at what happens if we make that correction and do this calculation at sea level which it really wasn't so that would be incorrect but if we had said if density of error was evaluated at sea level and at sea level the pressure would be 101.325 kPa we would then come up with a velocity measurement using Bernoulli's equation and this pedostatic tube equation of 865 feet per minute which you can see is quite a bit closer to the one that I read off of the manometer from the video but that would be if you were to do this experiment at sea level for where I did do the experiment the correct velocities are the ones that we have here so 923 feet per minute or 4.69 meters per second so that gives you an example of making a measurement using the pedostatic tube I showed you it using an incline manometer but you can use an electronic pressure transducer or any other type of pressure measurement in order to figure out what the delta P is on the pedostatic tube that gives you an application of pedostatic tube for velocity measurements