Use the inside calipers to find the internal diameter of the pipe by holding one leg of the caliper on one side of the inside wall of the pipe. Gradually increase the distance between the two legs of the calipers until the other leg touches the inside wall on the opposite side of the first. Ensure the two legs are approximately parallel to each other, then transfer the vernier calipers with the legs still apart to a scale rule. Record the confirmed distance between the legs of the calipers, which is the internal diameter of the pipe. Halve this value to find the radius of the pipe.
Take the scale rule and place it in one edge of the pipe. Ensure that your eye level is orthogonal to the edge of the pipe to eliminate parallax error which may lead to bad measurements. Measure to a considerable length of no less than 40 centimeters for more accurate results. Record the measurement.
Find the cross section of the pipe using the conventional mathematical formula (area = pi x radius^2) using the value you had earlier recorded. You can determine the volume of the pipe by multiplying the cross section area (C.S.A) by the length of the pipe. This will give the volume of a fluid passing through the particular section of the pipe.
Determine the rate of the flow of the fluid -- for example, water -- by multiplying the density of the water to the value of the volume and then divide the resultant value by the time the water moves from one end of the pipe to the other. This rate of the flow of water can also be calculated by dividing the mass of the liquid by the time it takes to move from one end to the other because the result of multiplying density to volume gives mass.