Thermometer placement

The thermometer should be placed so the bottom bulb is centered by the outlet to the condenser (Figure 3-25). Refer back to Figure 3-10, p. 26, and notice how the composition changes dramatically at the top of the column and how a small difference in thermometer placement can make a large difference in the purity of the final product. The measured temperature of the system is accurate only if the thermometer bulb is totally bathed in the condensing liquid. This is achieved by having the reflux line just slightly above the top of the bulb. 

Thermometer Placement. If a fhermometer is used, be sure that the bulh is placed in the stem of fhe Hickman head just below the well. If it is placed higher, it may not he surrounded by a constant stream of vapor from the material being distilled. If fhe fhermometer is not exposed to a continuous stream of vapor, it may not reach temperature equilibrium. As a result, the temperature reading would he incorrect (low). 

The discussion of the applications of DTA is divided into three parts. The first covers the more qualitative DTA, which was historically the first major DTA application. The term qualitative is chosen for these applications because no quantitative heat measurements are made. The temperature is, however, quantitatively fixed. Often transition temperatures can be fixed more precisely by such qualitative DTA than by the more quantitative DSC because of less stringent experimental restriction on sample mass and thermometer placement. Many of the thermometric measurements given in Chapter 3 can also be carried out by DTAand often with better results, so that Sect. 4.5 is an extension of the introduction to the thermodynamics of two-component systems given in Sect. 3.5. The measurement of heat is the... 

Instrumentation will include field instruments used in the manufacturing process and other instruments assoeiated with special tasks, such as that of monitoring laboratory or computer room environmental eonditions. The accessibility of instruments must be such as to permit their eleaning and maintenanee. Siting is also important, and instruments should be installed as close to the point of measurement as possible. The placement of flowmeters in piping dead-legs should be avoided. Careful consideration should also be given to the appropriate position of other instruments sueh as thermometers and thermocouples so that they, too, can fulfill their measurement and eontrol functions. Construction materials that come into direct contact with the pharmaceutical or healthcare production process stream must not contaminate or affect the manufactured product in any way. Instrument lubricants and coolants must not come into contact with in-process product or equipment. The reliability of instruments should also be considered for instance, a pressure transmitter that uses atmospheric pressure as its reference may suffer from poor reliability. A draft calibration schedule may also be prepared. 

Errors in measurement arise from calibration and reading of the thermometer and pressure gauge, inappropriate placement of the sensors of these instruments, failure to achieve equilibrium and impurities in the sample. Impurities may be present in the original sample or may arise from decomposition of the sample or other chemical changes that occur during the course of the measurement. 

The thermometer should extend precisely to the position shown in the drawing. Any other placement will give false temperature readings. Pushed down lower, it will give false readings on the high side. Placed higher up, it will read too cold. 

Calibration of a DTA involves adjustment of instrumental electronics, handling and manipulation of the data in order to ensure the accuracy of the measured quantities temperature, heat capacity and enthalpy [614,615,621]. Temperature sensors such as thermocouples, resistivity thermometers or thermistors may experience drifts that affect the mathematical relationship between the voltage or resistance and the absolute temperature. Also, significant differences between the true internal temperature of a sample with poor thermal conductivity and the temperature recorded by a probe in contact with the sample cup can develop when the sample is subjected to faster temperature scans. The important quantity measured in DTA experiments is the AT output from which enthalpy or heat capacity information is extracted. The proportionality constant must thus be determined using a known enthalpy or heat capacity - the power-compensated DSC requires lower attentiveness as it works already in units of power. The factors such as mass of the specimen, its form and placement, interfaces and surface within the sample and at its contact to holder, atmosphere... 

A platinum resistance thermometer or thermocouple in die gas stream is used to control the temperature. Placement of this sensor is important. Close to the edge of the sample yields better agreement between the controller set point and sample temperature. Close to the gas inlet gives less overshoot. 

A1.1.2.4 The collar for the cup-thermometer ferrule shall be set at an angle which permits placement of the thermometer with its bulb approximately in the horizontal center of the cup, at a depth prescribed in Table Al.l. 

Fit the temperature sensor through a snug-fitting device designed to mechanically center the sensor in the neck of the flask. In the case of a thermometer, the bulb is centered in the neck and the lower end of the capillary is level with the highest point on the bottom of the inner wall of the vapor tube (see Fig. 3). In the case of thermocouple/ resistance thermometer, follow the manufacturer s instructions as to placement... 

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