For total head tubes that have reasonably long horizontal legs relative to tube diameter, the tube coefficient is commonly unity. However, pitot tubes with damaged tips, short tips, tube burrs, and short tips need calibration checks to determine correct tube coefficients that may deviate considerably from unity. A more complex form of pitot tube is known as the pitot-static tube , which consists of the total head tube threaded through the center of a larger tube. Static ports are drilled perpendicular to and around the circumference of the horizontal part of the outer tube. These distances protect the static ports from stem and tip disturbances, which would cause tube coefficients to deviate from unity.
Some special instruments have coefficients that differ considerably from unity.
Appropriate coefficients should be applied as specified by manufacturers. Manometers are commonly used to measure heads separately or differentially in a U-tube. The velocity head is obtained by subtracting the pressure head from the total head. More conveniently, a differential pressure cell that senses velocity head directly can be used. A recording digital voltmeter attached to the pressure cells can provide continuous records of velocity.