Bridge Network

The sensor usually consists of a coil of wire made from the material that is wound on a former and the whole sealed to prevent oxidization, although a film of the metal deposited on a ceramic substrate can also be used. The resistor is connected in a Wheatstone bridge network (Figure 17.17), using fixed resistors in the other three arms. The instrument connected across the bridge is calibrated directly in terms of temperature. The range is limited by the linearity of the device and the upper temperature, which can be measured, must be well below the melting point of the material. 

The same applies to the historic gas-hydrates (hydrate clathrates, Fig. 5)17,18). However, on principle, only such molecules are suited for inclusion into the complicated H-bridge networks of gas-hydrates which do not interfere with the H-bridges of water, but have a hydrophobic nature. More recent hosts related to this inclusion principle are given in Chapter 3 of this book. 

These measure the change in thermal conductivity of a gas due to variations in pressure—usually in the range 0.75 torr (100 N/m2) to 7.5 x 10"4 torr (0.1 N/m2). At low pressures the relation between pressure and thermal conductivity of a gas is linear and can be predicted from the kinetic theory of gases. A coiled wire filament is heated by a current and forms one arm of a Wheatstone bridge network (Fig. 6.21). Any increase in vacuum will reduce the conduction of heat away from the filament and thus the temperature of the filament will rise so altering its electrical resistance. Temperature variations in the filament are monitored by means of a thermocouple placed at the centre of the coil. A similar filament which is maintained at standard conditions is inserted in another arm of the bridge as a reference. This type of sensor is often termed a Pirani gauge. 

Fig. 6.31. Level-sensing capacitor and associated bridge network (a) concentric cylinder capacitator (6) capacitance bridge...

Conductivity yes yes liquids Tank wall can be used as one electrode if metallic. Liquid must be electrically conductive. Electrodes form part of a conductivity bridge network. 

A katharometer is employed to determine the concentration of H2 in a H2/CH4 mixture. The proportion of H2 can vary from 0 to 60 mole per cent. The katharometer is constructed as shown in Fig. 6.54 from four identical tungsten hot-wire sensors for which the temperature coefficient of resistance ft, is 0.005 K. The gas mixture is passed over sensors R, and R whilst the reference gas (pure CH4) is passed over sensors R2 and R,. The total current supplied to the bridge is 220 mA and it is known that the resistance at 25°C and surface area of each sensor are 8 Q and 10 mm2 respectively. Assuming the heat transfer coefficient h between gas and sensor filaments to be a function of gas thermal conductivity k only under the conditions existing in the katharometer and that in this case h = k x 10 (h in W/m2K and k in W/mK), draw a graph of the output voltage V0 of the bridge network as a function of mole per cent H2. 

Figure 8. Hyperbolic Penning cell imbedded in a balanced bridge network.

The block consists of a temperature detector (RTD) that measures the temperature. The detector is felt as resistance to the bridge network. The bridge network converts this resistance to a DC voltage signal. 

---