Indirect pressure measurement

The sensing element (usually a fine W or Pt wire) is heated to a temperature that is approx. 100°C above that of its surroundings by an electric current. Heat is lost from the wire according to the mechanisms indicated (Qgas, QTad and Qend). Qrdd and Qend are pressure-independent effects that simulate a heat loss even 

If the sensing wire is at a constant temperature and the ambient temperature also remains constant, the electrical power to the wire  

This gives a non-linear scale with a zero pressure at about 10 2 mbar. 

Using a more complex electronic circuit, the zero pressure (with the sensing head at p 10 3 mbar) may be compensated for with a zero-adjustment potentiometer. The required temperature of the wire (and hence S) is adjusted by another potentiometer (atmospheric pressure adjustment). By using these two potentiometers, adjustments can be made so that gauge heads are interchangeable and may be connected to any suitable power supply. 

Further, since Pirani gauges working with a constant sensing-wire temperature have short response times (a few 10s of milliseconds) and provide 0-10 V outputs, they have general use in pressure control. 

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Instruments with indirect pressure measurement. In this case, the pressure is determined as a function of a pressure-dependent (or more accurately, density-dependent) property (thermal conductivity, ionization probability, electrical conductivity) of the gas. These properties are dependent on the molar mass as well as on the pressure. The pressure reading of the measuring instrument depends on the type of gas. 

Keywords d rec methods, indirect methods, rock properties, coring, core barrel, standard core analysis, special core analysis, slabbed core, sidewall samples, direct indications, microfossils, sonde, logging unit, invasion, mudcake, formation pressure measurement, fluid sampling, measurement while drilling, formation evaluation while drilling. 

Vacuum gauges may be broadly classified as either direct or indirect (10). Direct gauges measure pressure as force pet unit area. Indirect gauges measure a physical property, such as thermal conductivity or ionisation potential, known to change in a predictable manner with the molecular density of the gas. 

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. 

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.

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

Barolo et al. (1998) developed a mathematical model of a pilot-plant MVC column. The model was validated using experimental data on a highly non-ideal mixture (ethanol-water). The pilot plant and some of the operating constraints are described in Table 4.13. The column is equipped with a steam-heated thermosiphon reboiler, and a water-cooled total condenser (with subcooling of the condensate). Electropneumatic valves are installed in the process and steam lines. All flows are measured on a volumetric basis the steam flow measurement is pressure- and temperature-compensated, so that a mass flow measurement is available indirectly. Temperature measurements from several trays along the column are also available. The plant is interfaced to a personal computer, which performs data acquisition and logging, control routine calculation, and direct valve manipulation. 

Both gas flow rates and liquid flow rates can be measured by a wide variety of devices such as bellow meters, Venturi nozzles, nutating disk meters, orifice meters, rotameters, weirs (for liquids), Pitot tubes, and magnetic meters among others. Some devices measure volumetric flow directly as with meters in which the space between rotating paddles incorporates small volumetric displacements of fluid. Other device measure the flows indirectly by measuring the pressure drop caused by an orifice between two different sites in the pipe, or the change in voltage of a heated wire. 

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. 

In almost all work done prior to 1960, Via was determined indirectly from pressure measurements by means of an equation of state. Howevo-,... 

Percutaneous transhepatic hepatophlebography involves a typical puncture of the liver with a needle from the right midaxillary line to enter a hepatic vein. Pressure measurement and CM injection ensure that the position of the needle or the catheter is correct. Thrombosis of the portal vein is an absolute contraindication, which is why indirect splenoportography must be carried out prior to percutaneous hepatophlebography. Serious complications occur in 1.5-4.0% of cases. 

The accurate information about the surface tension-surface coverage relationship which for soluble monolayers is contained in the e0—tt curves, can be used, for example, to interpret rate of adsorption measurements. For fluid-fluid interfaces, adsorption onto an initially clean surface can be assessed only indirectly by measuring the changing interfacial tension. An example is shown in Figure 9, giving the change in surface pressure... 

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. 

The pressure measurement, achieved by use of the Textilpress device, is an indirect measuring method, which is based on die Laplace Law. Ltq>lace Law has been widely used to calculate the pressure delivered to a cylindo of known radius by a bric under known tension. However, this Law was originally developed by Laplace in 1806 to explain the surface tension phenomenon in liquids and their ability to form droplets or soap bubbles [6]. The Laplace Law is illustrated in Figure 3 and by equaticm (1). 

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. 

An inhibition of the uptake of NA in nerve endings of the sympathetic innervated blood vessels by tricyclic antidepressants can be demonstrated accurately with blood pressure measurements. Potentiation of the NA pressure response and antagonism of the effects of indirectly acting sympathomimetic amines, such as tyramine, are characterized by a rapid onset of action [23]. With proper precautions, these effects can also be studied in man. It must be remembered that these actions are on the peripheral nervous system and provide no guarantee of CNS-activity. 

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