Barometric temperature measurement

The equilibrium vapor pressure (ps) can be measured by the barometric temperature measurement (BTM) and be converted into temperature by the water vapor pressure diagram (see Section 1.2.3). 

By barometric temperature measurements (BTM) and the measurements of the desorption rate (DR) the influence of varied drying conditions can be seen and analyzed. Figure 1.71 compares four different test runs ... 

Fig. 1.77. Schema of the barometric temperature measurement (BTM) and plot of the water vapor pressure of ice (Fig. 4 from [1.108]).

The most important data during main drying is the temperature at the moving sublimation front which cannot be measured by Ths or RTDs. In 1958, Neumann and Oetjen 11.651 showed that the barometric temperature measurement (BTM) measures exactly this data. In Fig. 1.77 this is schematically shown if the drying chamber is separated from the condenser by a valve for a short time the pressure in the chamber rises to the saturation vapor pressure (ps) corresponding to the temperature of the sublimation front. ps can be converted into the ice temperature by the water vapor- temperature diagram (e. g. 0.3 mbar = -30 °C). Data for accurate conversion are given in Table 1.11 the temperatures between -100 and -1 °C. 

The decisive data during main drying (MD) is the temperature of the ice at the sublimation front, Tice, which can only be measured by barometric temperature measurement (BTM). Figure 1.77 shows the principle the valve between the chamber and condenser is closed for less than 3 s, to fill the chamber with water vapor of saturation pressure p., corresponding to Tke as shown in Table 1.11. The two conditions for the use of BTM a leak rate of the chamber below a certain limit and enough ice subliming during the time the valve is closed, are described in detail in Section 1.2.1. 

Ice temperature is determined during main drying by the barometric temperature measurement. 

Another method used is to measure the pressure rise in the chamber after closing the valve between chamber and condenser, called barometric temperature measurement (BMT). The BTM, when automatically done, is very sensitive to control the transit from MD to SD. In Table 6 the data of one run are listed. Under... 

The temperature of the advancing ice front can be determined using a barometric temperature measurement, via a pressure rise test (PRT) (see later), using a water vapour temperature diagram, as shown in Figure 5. In practice, this requires an interruption of the cycle and lacks the convenience of the continuous output from a thermocouple. 

The first automatic control systems proposed date back to the early 1960s, and were based either on the barometric temperature measurement of the batch or on the monitoring of the resistivity of one or more sampled vials. Even if they had many limitations, and up to recent times were never really applied in industrial applications, especially those of the second kind, they introduced the concept of modem closed-loop control, and some control strategies that are still vahd (Nail and Gatlin, 1985 Jennings, 1999). 

Figure 14 Barometric Temperature Measurement (BTM) with evaluation of the process on the recorder strip (schematic representation).

Barometric Temperature Measurement (BTM). The time during which the flow of water vapour is interrupted must be so short, that the ice temperature itself cannot increase noticeably. 

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