Pressure measurement direct / indirect

There are two general procedures for pressure measurement direct and indirect. 

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Before proceeding further, we will take a closer look at the significance of enthalpy and internal energy, because these cannot be measured directly but are determined indirectly by measuring other properties such as temperature and pressure. 

Measurements of glass transition temperatures at high pressure were made indirectly using, in particular, creep compliance [95, 96] or directly using differential scanning calorimetric techniques [97, 98]. The measured depression reaches values as high as 60°C for poly(methyl methacrylate) and polystyrene. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

The integral permeability coefficient P may be determined directly from permeation steady-state flux measurements or indirectly from sorption kinetic measurements 27 521 activity is usually replaced by gas concentration or pressure (unless the gas deviates substantially from ideal behaviour and it is desired to allow for this) and a<>, ax (p0, Pi) are the boundary high and low activities (pressures) respectively in a permeation experiment, or the final (initial) and initial (final) activities (pressures) respectively in an absorption (desorption) experiment. 

All factors related to the arrangement of the polymer chain in space are classified as tertiary structure. Parameters measurable directly (the radius of gyration RG, the end-to end distance h, the hydrodynamical radius RH, and the asymmetry in light scattering intensity) or indirectly (interaction parameters, the second virial coefficient A2) are related to the dimensions, such as size and shape of the polymer chain in a specific solvent under given conditions of temperature and pressure. For the exact determination of the coil size of macromolecules, it is necessary to ensure that measure-... 

The vapour pressures of all lanthanide trifluorides have been measured, including P111F3. Various techniques have been used, which can be divided in indirect techniques (effusion and boiling point methods) which provide accurate data for the total vapour pressure, and direct mass spectrometric techniques, which are less precise, particularly in case of quadrupole mass spectrometry, but give information on the vapour composition and the presence of dimeric or polymeric molecules or dissociation products. 

There are two basic ways for a vacuum gauge to read a vacuum direct and indirect. For example, say that on one side of a wall you have a known pressure, and on the other side of the wall you have an unknown pressure. If you know that a certain amount of deflection implies a specific level of vacuum, and you can measure the current wall deflection, you can then determine the pressure directly. This process is used with mechanical or liquid types of vacuum gauges. On the other hand, if you know that a given gas will display certain physical characteristics due to external stimuli at various pressures, and you have the equipment to record and interpret those characteristics, you can infer the pressure from these indirect measurements. This indirect method is how thermocouple and ion gauges operate. 

To carry out the experiments in a meaningful and systematic way, it will be necessary, first, to consider one of the parameters as a variable while keeping the others constant and then to measure the corresponding pressure drop. The same type of experiment is carried out for the measurement of the heat transfer coefficient. Contrary to the mass transport pressure drop, which could be measured directly, the heat transfer coefficient is obtained indirectly by measuring the temperature of the wall and of the fluid at the entrance and exit of the pipe. The determination of the functional relationship between Ap/1, a and the various parameters that influence the process is illustrated in Fig. 6.1. 

Some attributes of a process, such as temperature and pressure, are directly measurable. Others, such as attitudes of employees or the public, may only be measurable indirectly. Much thought and consideration must be given to developing those metrics that will need to be compared across the organization and will drive correct behaviors without masking others. For example, overemphasis... 

Transpiration or gas saturation techniques have been widely used for the measurements of vapour pressures [90,91]. An inert gas is passed over the sample and the amount of material transported as a function of temperature is determined. Different analytical methods have been used to quantify the mass transferred. The main advantages of this method are the large temperature range accessible and the small amount of sample needed (-30 mg) for the experiments. Head-space analysis has also been used by various investigators. The vapour in equilibrium with a solid is either measured directly by an absorption technique [92] or indirectly [93]. Vapour pressures can be also measured indirectly by using a quartz resonator where the frequency of the quartz crystal changes as a function of the thickness of the material deposited on its surface [94,95]. 

The rotary dryers (direct-heat dryers and kilns) are controlled by indirect means, e.g., by measuring and controlling the gas temperatures in their two ends, whereas shell temperature is measured on indirect calciners, and steam pressure and temperature as well exit gas temperature and humidity are controlled on steam-tube dryers. It is not possible to achieve control by measuring the product temperature because not only this is difficult but also its changes are slowly detected, although the product temperature is used for secondary controls. 

Consideration should be given to alternative methods of pressure measurement that may provide greato safety than the direct use of pressure gauges. Such methods include the use of seals or other isolating devices in pressure tap hues, indirect obsa-vation devices, and remote measurement by strain-gauge transducers with digital readouts. 

Two basic methods of approach can be undertaken to determine the suitability of a coal or coking blend (1) indirect coking pressure measurements at laboratory scale and (2) direct coking pressure measurements by larger-scale tests, using a few hundreds of kilograms of coal while trying to reproduce industrial conditions. 

The most common meters are used to measure temperature, pressure, flow, and level. However, various physical principles allow us to measure, directly or indirectly, the density of a liquid, the concentration of a pollutant in a gas, or the volatUity of certain compounds, for example. Tables 3.1, 3.2, 3.3 and 3.4 present descriptions of some devices. 

Thermodynamic logic often seems to be complex. This apparent complexity arises because the fundamental equilibrium functions cannot be measured directly. Equilibria are governed by energy and entropy through the Eirst and Second Laws, but unfortunately there are no energy or entropy meters to measure these quantities. Instead, inferences about equilibria are indirect and drawn from observations of quantities that can be measured, such as temperature, pressure, work, heat capacities, concentrations, or electrical potentials. Thermodynamics is a business of making clever inferences about unmeasurable quantities from observable ones, by various means. 

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