The main reasons for choosing pitot tubes or annubars are as follows:
• Very low pressure loss
• Ease of installation (i.e., they may be inserted into existing piping or duct)
• Relatively low cost
Pitot tubes and annubars can be inserted into process lines through either flange or threaded connections. Various connections and mounting techniques are usually available in vendor technical brochures and manuals.
The accuracy of a pitot tube is often lower than that of orifices and venturis. Typical accuracy is ±5% to ±10% on volume measurement. (The velocity measurement made by the pitot tubes may be accurate to within ± 1%, but this measurement is not often of interest.)
The accuracy of an annubar may be better than that of the pitot tube. Some vendors claim ± 1% accuracy for the primary element over a turndown of 10 to 1. With the transmitter, the combined accuracy may be between ±1.5% and ±2%, provided a uniform velocity profile is available.