Thermal mass type

General Principles There are two main types of mass flowmeters (1) the so-called true mass flowmeter, which responds directly to mass flow rate, and (2) the inferential mass flowmeter, which commonly measures volume flow rate aud flmd density separately. A variety of types of true mass flowmeters have been developed, including the following (a) the Maguus-effect mass flowmeter, (b) the axial-flow, transverse-momentum mass flowmeter, (c) the radial-flow, transverse-momentum mass flowmeter, (d) the gyroscopic transverse-momentum mass flowmeter, aud (e) the thermal mass flowmeter. Type b is the basis for several commercial mass flowmeters, one version of which is briefly described here. 

Blanket-type electrical heaters are often used in combination with low thermal mass GRP tools (e.g., in the tooling concept advocated by Plastech TT, UK). The drawback with electrical heaters is that locally very high temperatures can occur. This can damage the tool and give an unacceptable variation in the curing process. 

Sensors can be based on a variety of transduction mechanisms, including electrochemical, optical, mass, and thermal. Different types of sensors, along with their transduction mechanisms, will be discussed later in this report. All sensors possess a transduction element connected to supporting instrumentation. Selectivity is achieved via the transducer. The qualities of an ideal sensor obviously depend on the application, with different qualities necessary for sensors used in shipboard laboratories, towed sensors, or sensors deployed on long-term remote moorings. Depending upon the specific application, sensors should possess the following qualities. 

Firing of Enamels. Firing can be carried out in intermittent box-type furnaces or continuous furnaces. The dryer and the furnace form one continuous unit or function as separate units in the continuous firing process. Most industrial furnaces are fiber-lined (low thermal mass), which lowers cost and downtime between firing schedules. 

Fig. 8.4 Schematic diagram of a flow-type photochemical system, (a) Quartz glass cylinders on which Ti02 was coated, (b) outer cylinder (35mm o.d. x 1200mm), (c) 60W black light, (d) reactant gas mixture (nitrogen), (e) nitrogen, (f) oxygen, (g) air, (h) thermal mass flow controller, (i) humidifier, (j) gas-bubblers, (k) pump.

In the thermal-electrolytic-type AgCl electrode, silver is formed by heating a paste of silver oxide coated on a spiral of platinum wire. Part of the spongy silver mass in converted to AgCl by the electrolysis process described above. Because the silver is porous, the electrodes tend to be rather sluggish, but they can be prepared with very small bias potentials ( 20 jtV). 

Fuel flow control in low-temperature cells is relatively simple. The control valves used are servomotor-driven quarter-turn valves, and the flow sensors can be the types used on gas furnaces, such as thermal dispersion type mass flow meters. 

Venturi-type thermal mass flowmeter. (Courtesy of TSI Inc.)... 

OTHER INSERTION METERS. Modified forms of magnetic meters, turbine meters, ultrasonic meters, thermal mass flowmeters, and other types are available as insertion meters. They all have advantages for certain services. Insertion meters are generally cheaper than full-bore meters and are usually the most cost-effective method of measuring flow in large pipes. 

A typical thermal mass meter is shown in Figure 4.20. The operation of these meters is based on the principle that the rate of heat absorbed by a flowing fluid is directly proportional to the mass flow rate [23]. In general there are three types of thermal mass meters constant temperature, constant power, and energy balance. 

Energy balance type thermal mass meter. (Adapted from "Flow Measurement," http //en.wikipedia.org/wiki/Flow Measurement.)... 

There are many other types of flow meters that can be used such as venturi meters, vortex shedder flow meters, and Coriolis flow meters. Any of these may be preferred depending on the application. Fluid flow can also be measured indirectly using chemical analysis and mass and energy balance calculations. Air flow measurements are commonly made this way since an oxygen analyzer is generally more accurate than a thermal mass flow meter. 

Some indication of the rates of heat and gas production can be obtained from the results of the screening tests (Section 3.4, page 32), but getting accurate data requires the use of adiabatic calorimeters (Sections 3.6.2 and 3.6.3). A different type of calorimeter, developed specially to provide data for the design of venting systems, uses a pressure equalization system and a weak test cell with a low thermal mass (Section 3.6.3). 

Thermal mass-flow meters directly measure the mass-flow rate utilizing the thermal transfer process between a heated element and the flowing mass. According to the thermal transfer mechanism, they can be classified as calorimetric- or boundary-layer-type flow meters. Calorimetric flow meters utilize the simple principle that as the mass flows through the device, power has to maintain the temperature of the incoming mass at a known level. By measuring the supplied power P and the temperature above ambient, AT, the mass-flow rate M can be calculated as the expression... 

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