wire temperature and thus restoring the balance in the Wheatstone bridge. Thiscurrent is measured and from it the gas flow rate is determined. To compensate for changes in the gas temperature, a second wire is usually incorporated, which ismaintained at ambient temperature. Minor corrections are also made according tothe gascomposition, to accommodate the variation in specific heat capacity, but hotwire anemometry is generally extremely accurate.This cooling effect occurs with flow in either direction, and so to measure exhaledtidal volume the hot wire anemometer is placed in the expiratory limb of thecircuit. It can be modified to provide information about the direction of flow byusing an additional heated wire placed just downstream from a small bar, as shownin Figure 5b. This bar shelters the wire from the full cooling effects of flow in onedirection but not the other, and thus inspiratory and expiratory flows can becalculated separately. For this purpose the sensor must be placed in the Y-piece of the circuit. This technique is particularly useful for neonatal ventilation.
2.2 Fundamental Concepts
:Thermal anemometer (hot wire anemometer) is a device for measuring the mass flow rate with the help of heat and mass transfer concepts.The thermal anemometer measure the mass unit area flow rate, so itmeasures the true velocity of the fluid, but the process must be done at fixedtemperature above the ambient temperature, so it will respond to the convectiveheat transfer, so by the forced convection, the output will be proportional tosensors Reynolds number, and because Reynolds number measures the density, notthe volumetric flow rate, which is known as (standard flow rate).
2.2.1 Reynolds Number
Reynolds Number Is a dimensionless number which gives the ration between the inertial forces (
) to the viscous forces (
= Actual Density of the flow.V = Actual Velocity.