Specifications

2.13.2 Pitot Tube

A Pitot (pee-toe) tube is an instrument for measuring fluid flow velocity. First

conceived in the early 1700’s by a French engineer by the name of Henri Pitot,

these devices are commonly used in aeronautical applications for determining

airspeed velocity, as well as industrial applications for measuring air and gas

velocities. These devices work by pointing a fluid filled tube directly into the path

of fluid flow, measuring the induced pressure of the fluid as it has no path to exit

the tube. This measurement, known as the Stagnation Pressure, is a factor in

Bernoulli’s Equation, which establishes a relationship between the static and

dynamic pressure of a fluid flow. The relationship is as follows.

Where V is the fluid velocity, p

is the stagnation pressure, p

is the static

pressure, and ρ is the fluid density. The static pressure is measured by

evaluating the fluid flow pressure perpendicular to the direction of the flow

velocity.

2.13.3 Piezoresistive Pressure Sensor

These types of sensors utilize what is known as the Piezoelectric effect, which

describes the ability of certain types of materials to generate an electrical field or

electrical potential in response to an applied mechanical stress. In a design that

closely mirrors the effective design of a Pitot Tube, these types of sensors can

detect differential changes in gas pressure within the specific flow channel of

particular interest. Because the structure of the material can be altered at the

time of chip fabrication with subtle changes to semiconductor deposition pressure

and temperature, these sensors can be designed to detect very particular ranges

of pressure change, from high pressure gas lines and containment chambers,

down to very small pressures such as those exhibited during forced human

respiration, or the difference in pressure between two rooms of a building.

Figure 2.13.3-1 shows an example of such a chip. The two tubes sticking out of

the top are capable of measuring differential pressure differences, meaning the

difference between a static and a stagnant air pressure, useful in the

determination of total air velocity when used to solve Bernoulli’s Equation (See

Section 2.13.2). These chips are the combination of a piezoresistive silicon

wafer with an integrated ASIC capable of compensating for temperature and

outputting either a digital signal over a serial device bus, or outputting an analog

signal that can be digitized and evaluated on a particular microcontroller.