This chapter describes the important types of flowmeters commonly used in measuring pipe flow. Flowmeters play a central role in pipeline instrumentation and monitoring. Calibration is usually done in the manufacturer's laboratory before the instrument is shipped to the customer. The constant-head tank is needed in order to maintain a steady flow of water through the flowmeter for calibration. Generally, for cases in which a magnetic flowmeter is applicable and costs about the same as an acoustic flowmeter, the former is likely to be a better choice because it is more reliable and accurate. A meter prover is a specially designed short loop of pipe used for calibrating or checking the accuracy of flowmeters used in pipe. An elbow flowmeter uses an existing elbow or bend of a pipe to measure the discharge through the pipe. Vibratory flowmeters have a loose part exposed to the flow, vibrating at a frequency proportional to the fluid velocity and the discharge.