PITOT TUBE Velocity Measuring Instrument

The Pitot tube is a precision instrument designed to measure the difference between impact and static pressures in a fluid. Its design is rooted in careful engineering, consisting of two concentric tubes aligned parallel to the fluid's flow direction. The inner tube's open end is carefully positioned to capture the impact pressure, while the outer tube's sealed end features a series of strategically placed orifices. These orifices provide an accurate reading of the static pressure, but their placement requires careful consideration.

Two potential disturbances can compromise the accuracy of the static pressure reading. Firstly, the instrument's head can disrupt the fluid flow, leading to incorrect readings. Secondly, the stem's perpendicular portion can also create turbulence, affecting the measurement. By carefully positioning the orifices and minimizing these disturbances, the Pitot tube can provide reliable and accurate pressure readings.

These two disturbances cause errors in opposite directions, and the static pressure should therefore be measured at the point where the effects are equal and opposite.

If the head and stem are situated at a distance of 14mm diameters from each other as on the standard instrument. The two disturbances are equal and opposite at section 6mm diameters from the head and 8mm from the stem. This is, therefore, the position at which the static pressure orifices should be located. If the distance between the head and the stem is too great, the instrument will be unwieldy; if it is too short, the magnitude of each of the disturbances will be relatively great, and a small error in the location of the static pressure orifices will appreciably affect the reading.

For Reynolds numbers of 500-300,000, based on the external diameter of the Pitot tube, an error of not more than 1 percent is obtained with this instrument. A Reynolds number of 500 with the standard 7.94 mm pitot tube corresponds to a water velocity of 0.070 m/s or an air velocity of 0.91 m/s. Sinusoidal fluctuations in the flow rate up to 20 percent do not affect the accuracy by more than 1 percent, and calibration of the instrument is not necessary.

A very small pressure difference is obtained for low rates of flow of gases, and the lower limit of velocity that can be measured is usually set by the minimum difference in pressure that can be measured. This limitation is serious, and various methods have been adopted for increasing the reading of the instrument although they involve the need for calibration. Correct alignment of the instrument concerning the direction of flow is important; this is attained when the differential reading is maximum.

For the flow not to be appreciably disturbed the diameter of the instrument must not exceed about one-fiftieth of the diameter of the pipe; the standard instrument (diameter 7.94 mm) should therefore not be used in pipes of less than 0.4 m diameter. An accurate measurement of the impact pressure can be obtained using a tube of very small diameter with its open end at right angles to the direction of flow; hypodermic tubing is convenient for this purpose. The static pressure is measured using a single piezometer tube or a piezometer ring upstream at a distance equal approximately to the diameter of the pipe and measurement should be made at least 50 diameters from any bend or obstruction.

The Pitot tube measures the velocity of only a filament of fluid, and hence it can be used for exploring the velocity distribution across the pipe section.

If, however, it is desired to measure the total flow of fluid through the pipe, the velocity must be measured at various distances from the walls and the results integrated. The total flow rate can be calculated from a single reading only if the velocity distribution across the section is already known.

Although a single pitot tube measures the velocity at only one point in a pipe or duct, instruments such as the averaging pitot tube or Annular, which employ multiple sampling points over the cross-section, provide information on the complete velocity profile which may then be integrated to give the volumetric flow rate. An instrument of this type has the advantage that it gives rise to a lower pressure drop than most other flow-measuring devices

Pitot tube working procedure and calculation to find the velocity of a stream:

To measure local velocity in a pipe or to calculate a point velocity in an open-air stream Pitot tube is used. It works on the principle of using pressure difference to find the velocity between the pressure points of measuring. It is simple in construction and easy in application, let us make ourselves comfortable to understand how pitot tube can help in determining a velocity variable. A manometer which is filled with a manometric fluid such as mercury or ccl4 can find the pressure difference when connected to the Pitot tube outlet endpoints. The Pi, impact pressure, and Po, static pressure are pressures obtained from the pitot tube. The Impact pressure is the pressure which is exerted by the fluid when a tube is inserted in its flow stream and static pressure is the pressure exerted by the fluid on the side wall of the tube at the same position.

Velocity = C × √2g∆h = C× √2g(Pi-Po)ρ 

Where:
C= coefficient
g=dimensional constant
ρ= density of fluid

The coefficient C is about 1.00 (±0.01) for simple Pitot tubes and for Pitot-static tubes, it may be around 0.98 to 1.00.

Problem:
Calculate the velocity of water flowing in a pipe having installed with a pitot tube which has a coefficient of 0.98 and a stagnation pressure is 5.67 m and a static pressure head in the pipe is 4.73 m

solution:
Between two points A, B where B is the stagnation point where zero velocity is developed which is at the open end of the tube, by applying Bernoulli theorem form A to B where A is a point at which the static point is taken

(P_A + V _A ² / 2g +  0) – no loss = (P_B + 0 + 0 )

VA = √2g(PB-PA) 

 = 0.98√2g(5.67-4.73)

   =  4.21 m/s

 Pitot tube instrument's performance

Pitot Tube Calculator
Stagnation Pressure (P1):
Static Pressure (P2):
Fluid Density (ρ):
Units:	psi Pa bar
Fluid Velocity (V):