Overview

Calibrating flow is essential for any transducers or measurement systems that provide information on the amount of material transferred in a given time. These measurements will usually be made in units of mass per unit time (mass flow) or volume per unit time (volumetric flow). The two types of measurements are connected by the density (mass per unit volume) of the flowing material. The major factors that affect density are pressure and temperature, and these measurements are usually encountered in any flow calibration work. The majority of flowmeters in use today are mounted in circular pipes and calibration facilities have to be able to match the field installation of the devices.

The accuracies associated with flow calibration are not nearly as good as those associated with the calibration of temperature, pressure, mass, or electrical equipment. National standard and primary standard laboratories can typically calibrate temperature, pressure, mass, and electrical equipment with uncertainties ranging from 1 to 10 parts per million. In contrast, the uncertainties for NIST gas flow calibrations range from 200 to 2000 parts per million, while liquid flow calibrations are in the 800 to 1000 parts per million range.

The equipment needed for flow calibrations can be very bulky and expensive to operate. The water flow calibration facility at NIST contains an 8,000-cubic-foot water reservoir tank, 4 pumps ranging from 100 to 150 horsepower, weigh scales up to 50,000 pounds, and 30- to 40-foot piping runs with diameters ranging from 1 inch to 16 inches.

The reservoir...