The measurement of mass flow

One class of flow measurement which is becoming of increasing importance (particularly in the form of sensors for control systems) is the monitoring of mass flow. This is rapidly superseding the measurement of volumetric flow—especially where it is required to determine accurately the transfer of large quantities of gas and liquid in the oil, gas and water industries. Two principal approaches are employed to measure mass flow. One is indirect and uses a combination of volumetric flow and density and the other is direct in that it involves the measurement of properties which are sensitive to variations in the mass rate of flow itself. 

Any combination of flowmeter and suitable specific gravity (density) meter (Section 6.6) can in theory be employed to produce a mass rate of flow. However, these so-called indirect meters have a limited range and accuracy and require careful and frequent calibration and maintenance. An instrument based on the turbine flowmeter (Volume 1, Section 6.3.9) and acoustic density meter (Section 6.6.1) has been employed with good results 8) (see Fig. 6.5). 

The turbine meter produces a pulsed output of frequency at, such that  

The microprocessor measures att and at2 by counting pulses over a fixed time period and calculates the mass m of fluid which has passed in time t from  

These instruments can be divided into meters which employ thermal effects and those which are energised by variations in the momentum of the fluid passing through the meter. The former rely upon the rate of loss of heat from a heated wire immersed in a fluid and this rate of loss of heat is a function of the mass flowrate. This type of instrument is described in Volume 1, Section 6.3.9. A discussion of two of the more common types of momentum transfer meter follows. 

---

The measurement of mass flow can be obtained by multiplying the volumetric flow with density or by the direct measurement of Coriolis, thermal, impact, and angular momentum effects. 

In general practical situations the mass and energy balances do not yield always linear expressions like equation (2). For example, one device serves for the measurement of mass flow rate and separate analyzers are used to determine the compositions. Consequently the balance may result in bilinear forms as shown in equation (5). 

We now assume, as suggested in the text, that the error in each of the measurements of mass flow is simply proportional to the measm-ement itself, so that ... 

---