Heat conductivity gauge

The principle of design of a heat conductivity gauge (TM) is shown in Fig. 2.30. Electrical energy is fed into the wire (2) in such a way, that the temperature of the wire is kept constant. This amount of heat per time is in the area 2 of Fig. 2.31 and is approx, propor-... 

If a heat conductivity gauge (TM) is offered, the additional price for a capacitance gauge, which is strongly recommended, should be requested. 

Heat is not only conducted by the gas but also directly by the clamped ends of the filament and by radiation. These portions are not pressure dependent and lead to a residual indication, the so-called zero signal or offset . When the pressure has reached the respective offset signal, the heat conductance by the gas is small compared to the offset and it becomes difficult to be separated with sufficient accuracy. Therefore the pressure equivalent zero signal determines the lower measurement limit of a heat conductivity gauge. Typically it is about 10 mbar. 

An increase of heat radiation induced by blackening of the filament in the process environment can also lead to a considerable increase of zero signal. After a prolonged operation of a heat conductivity gauge it is not unusual that its offset rises by deposition of hydrocarbons on the filament resulting in a constant indication of 10 mbar with the real pressure being much lower. 

Compensated heat conductivity gauge naturai output signai... 

Characteristic of compensated heat conductivity gauge after offset correction and linearisation... 

Figure 7.6 Output signal of a compensated heat conductivity gauge (bridge voltage vs pressure) (a) pressure-dependent output voltage without correction and (b) linearization by logarithmic plot of squared...

Figure 7.7 Calibration curves of heat conductivity gauge = indicated pressure and...

With the rotary and diffusion pumps in tandem, aided by a liquid-nitrogen trap, a vacuum of 10 Torr became readily attainable between the wars by degrees, as oils and vacuum greases improved, this was inched up towards 10 Torr (a hundred-billionth of atmospheric pressure), but there it stuck. These low pressures were beyond the range of the McLeod gauge and even beyond the Pirani gauge based on heat conduction from a hot filament (limit Torr), and it was necessary to... 

The pressure range for DR measurements is normally one decade below the above data, and this has to be considered in the specification of the plant. All measurements discussed above have to be carried out by capacitance vacuum gauge, because these instruments measure pressure independently of the type of gas. All vacuum gauges based on the change of heat conductivity as a function of pressure show a result which depends... 

Fig. 1.81. Plot of the pressure measured by heat conductivity vacuum gauge (TM) during SD. In addition pressure rises in 30 s and related RM data are shown.

The total pressure during freeze drying may be measured by several methods, though only two are mostly used heat conductivity, and the membrane pressure difference gauge. Their operating principles and their advantages and disadvantages are described below. 

The sensing filament in the gauge head forms a branch of a Wheatstone bridge. In the TFIERMOTRON thermal conductivity gauges with variable resistance which were commonly used in the past, the sensing filament was heated with a constant current. As gas pressure increases, the temperature of the filament decreases because of the greater thermal... 

A thermal conductivity gauge uses a constant electric current to heat an element whose temperature is a linear function of gas pressure over a limited range. The temperature is typically measured with a thermocouple. In the popular Pirani gauge, a single metal filament is substituted for a thermocouple, and filament resistance is monitored [19]. The range of pressures detected by thermal conductivity gauges is — lO -lO 4 torr, which makes them useful for... 

To insure an undisturbed water vapor transport (see Section 1.2.4) the leak rate of a freeze-drying plant must allow BTM with sufficient accuracy. This applies for vapor pressures with ice temperatures ranging between -50 and -10 °C corresponding to 0.04—2.5 mbar. The pressure range for DR measurements is normally one decade below the above data and this has to be considered in the specification of the plant. All measurements discussed above have to be carried out with a capacitance vacuum gauge, because these instruments measure pressure independently of the type of gas. All vacuum gauges based on the change of heat conductivity as a function of pressure show a result which depends not only on the pressure of the gas mixture but also on the type of gas. Leybold AG [1.67] indicate that for instruments based on heat con-... 

Fig. 1.81. Plot of the pressure measured by heat conductivity vacuum gauge (TM) during SD. In addition pressure rises in 30 s and related RM data are shown. 1, pch measured by TM 2, pressure rise in 30 s 3, RM in % of solids (Figure 5 from [1.62])...

A strain gauge of 1.6 mm gauge length was mounted 21.6 mm from the impact face in order to monitor the transient strain wave in the specimen. A smaller gauge could not be used since it did not dissipate enough heat due to the poor heat conductivity of the ceramic specimens. 

Analysis of hydrogen mixtures with a thermal conductivity cell is well established. The most accurate measurements are obtained by use of a thermal conductivity gauge with the walls immersed in liquid nitrogen and the wire heated to 160° K. This is the teniperature when the difference in the rotational specific heats of orthpara-hydrogen is a maximum . Various modifications of thermal conductivity gauges have been made to improve their convenience in use . A room temperature flow analyser based on a thermal conductivity cell has been developed by Weitzel and White which is claimed to be as sensitive as low temperature units. Bridge current and temperature must be controlled very carefully, but the unit is relatively insensitive to changes in pressure and flow rate. 

The determination of the heat flux through a surface is important in a variety of scientific and engineering applications. Noel, Turley, and Tobin, and Turley et al. pursued development of several thermal phosphor-based heat flux gauges.A standard expression relates the heat flux, q, to the insulator thickness, J, thermal conductivity, k, and temperature difference, AT, across an insulating barrier ... 

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