Contact thermometer

Contact thermometers of a maximum-minimum type are also manufactured these make contact both on exceeding an upper limit and on dropping below a lower limit. By means of a fine adjusting screw the temperatures may be set with an accuracy of 0.1 °C [23]. Temperature control may be arranged to follow a variable reference temperature by moving the thermometer contact with a synchronous motor. 

The distillation heads Fig. 22(F) and Fig. 22(G) can be fitted with thermometers having a ground-glass cone just above the bulb (Fig. 22(M)). These are expensive, and it is usually more convenient to fit a thermometer pocket (Fig. 22(N)) which consists of a small well , fitting as shown into the neck of the flask. A small volume of mercury is placed in the well just to cover the bulb of a conventional thermometer, and thus provides excellent thermal contact between the thermometer and the sides of the pocket. 

In both of these pieces of apparatus, isothermal operation and optimum membrane area are obtained. Good temperature control is essential not only to provide a value for T in the equations, but also because the capillary attached to a larger reservoir behaves like a thermometer, with the column height varying with temperature fluctuations. The contact area must be maximized to speed up an otherwise slow equilibration process. Various practical strategies for presetting the osmometer to an approximate n value have been developed, and these also accelerate the equilibration process. 

We put a contact thermometer on the bearing housing. The thermometer indicated 152° F. During all these years, this chief mechanic s hands had seen a lot of temperature, abrasion, and abuse. His hands and touch were much more re - w ant to heat than mine, or the PM inspector s hands. 

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

TABLE 12.3 Contact Thermometers and Useful Measuring Ranges... 

The measurement range is dependent on the instrument but can cover the range -50 to +500 °C. The accuracy is not as high as the best contact thermometers. One reason for this is that the emissivity of the surface has an effect on the measurement result, and an emissivity correction is necessary for most instruments. The positive features are noncontact measurement and very fast dynamics, which enable a rapid scan of surface temperatures from a distance this is convenient when carrying out, for example, thermal comfort measurements. 

Figure 7.4 Microcomputer programming of a hatch cooling crystallizer. A, crystallization vessel, B, control heater, C, control cooler. surrounding the draft-tube), D, contact thermometer, E, discharge plug and conical baffle), F, recorder, G, relay, H, temperature programmer, I, cooling water pump, J, cooling water reservoir, K, water inflow L, water outflow after Jones and Mullin, 1974)...

Wells should slope downwards into the pipe, so that they can be part filled with liquid to provide better thermal contact. Where a pipe temperature is a critical factor in the operation of a system, it is usually worth fitting a permanent thermometer. 

So let us measure the temperature of a sample of gas A by placing it in thermal contact with a sample of gas B (our thermometer). There will be heat flow between the two gas samples if they are initially at different temperatures. Energy is transferred from the hotter gas to the cooler gas. When heat flow ceases, the gases have reached thermal equilibrium. Then the gases have the same temperature. 

Fig. 4. Sectional view of one stage of a new type of gas-liquid contactor without pressure drop. (1) Vaned-disk impeller (2) stator (3) contacting tank (4) impeller shaft (5) gas inlet pipe (gas is self-induced through this pipe into the dispersion) (6) gas outlet thermometer (7) thermometer pocket (8) drainage tank (9) lid [after Gal-Or (G4)].

Reliable micro-scale measurement and control of the temperature are required in developing thermal micro-devices. Available measurement techniques can be largely classified into contact and non-contact groups. While the resistance thermometer, thermocouples, thermodiodes, and thermotransistors measure temperature at specific points in contact with them, infrared thermography, thermochromic liquid crystals (TLC), and temperature-sensitive fluorescent dyes cover the whole temperature field (Yoo 2006). 

The simplest and, despite its several drawbacks, the most widely used type of control is the on/off control system. An example is a contact thermometer, which closes or opens a heater circuit. The designation on/off means that the controller output, or the manipulated variable (electric current) is either fully on or completely off. To avoid oscillations around the setpoint, the real on/off controller has built into it, a small interval on either side of the setpoint, within which the controller does not respond, and which is called the differential gap or deadzone. When the controlled variable moves outside the deadzone, the manipulated variable is set either on or off. This is illustrated in Fig. 2.30. Such shifts from the set point are known as offset. 

To do this, the software must be aware of conditions within the reactor therefore, we shall arrange that it is fed with regular messages from a digital thermometer in contact with the fermenting medium. The thermometer, and any other probes in the vessel, send messages to the software in the form of binary strings, such as ... 

An instrument for measuring temperatures, in the rubber industry the term is usually applied to an instrument for determining the surface temperature of mill and calender rolls, moulds, etc. The instrument is usually based on thermocouples or, where higher accuracy is required, platinum resistance thermometers. Infrared (IR) techniques are now used which have the advantage of non contact but require careful calibration for the emissivity of the surface. 

Referring to Fig. 5.6, the 3He refrigerator [25] contains a pump P and an evaporator E. They are connected by a stainless steel tube T internal to the copper support C. The latter is in good thermal contact with the working plane B of a pumped 4He cryostat (for example that of Fig. 5.3) not shown in figure. The tube is connected to a charcoal cryopump P linked to the 4lie bath by a thermal connection L. A thermometer Th monitors the temperature of the pump. A thermal shield (not shown), at the temperature of the 4He bath, surrounds the refrigerator. 

In a resistance level detector, a resistive thermometer is powered with 0.1-1 mW. When the thermometer is inside the LHe, its temperature is about 4 K. If it is raised into the vapour phase, its temperature raises because of the weaker thermal contact. The sensing element is usually a NbTi wire (see e.g. ref. [31]). 

The zero-th law, which justifies the existence of the thermometer, says that two bodies A and B which are in thermal equilibrium with a third body are in thermal equilibrium with each other. There is no heat flow from one to the other, and they are said to be at the same temperature. If A and B are not in thermal equilibrium, A is said to be at a higher temperature if the heat flows from A to B when they are placed in thermal contact. The changes in temperature usually produce changes in physical properties like dimension, electrical resistance and so on. Such property variations can be used to measure the temperature changes. 

Ge resistors are specifically produced for low-temperature thermometry carbon and Ru02 resistors are commercial products for electronics. Pure carbon is not a semiconductor. The negative slope R(T) is due to the production process which consists in pressing and sinterization of carbon particles with glue. The resulting resistance is probably determined by the contact resistance between the particles. The cost of the carbon resistor thermometer is very low. Manufacturers such as Speer, Allen-Bradley and Matsushita have produced in the past carbon resistors for many years. Most of firms have now ceased manufacture, although their products may still be found in the storerooms of research laboratories. 

The small power needed to measure the resistance of the thermometer brings the thermometer at a temperature over that of the support surface. Such overheating is due to the contact resistance at the interface (see Chapter 4). A typical value of the contact resistance is ... 

For example, if we want a thermometer overheating AT/T less than 1%, with a contact surface A = 10 4m2, the power P(T) to be supplied to the thermistor must be ... 

Equation (9.15) is sometimes optimistic. Thermometer overheating as a function of temperature and contact resistance can be experimentally found, varying the power supplied to the thermometer while keeping constant the support temperature. This is shown in Fig. 9.9 for an RuOz thermometer. Note that a measuring power of 5 x 10 12W at 20 mK produces an overheating of 12mK ... 

The contact resistance may change when the thermometer is moved from a position to another. Hence the accuracy of resistance temperature measurements below about 25 mK... 

An advantage of this technique, instead, is the good thermal contact with the (metallic) radioactive material, and, more important, the good thermal coupling between lattice and nuclear spins. Last, the j ray energy is usually quite high, and no special window in the cryostat is needed. Nuclear orientation thermometer is a primary thermometer. 

The simplest experimental arrangement (two-probe method) uses two thermometers one on the thermal bath at Ts, the second on the warm end of the sample together with the heater (see Fig. 11.1). Such configuration can be used when one is sure that contact resistances are negligible compared with the sample thermal resistance. This is seldom the case at very low temperature. A sample bath (and sample support) temperature drop ... 

The thermal conductance of each glue spot below 150 mK was very low because of the two contact resistances Rc (Kapton-glue and glue-copper), and the power Ph delivered to the copper sample did not flow through the Kapton foil. To be sure of that, however, 1 mm large, 56 xm thick copper ribbon was internally glued around the upper end of the Kapton support. A heater Hk and a thermometer Tk (Fig. 11.6) were fixed on the ribbon and a power Pk was delivered to the Kapton support in such a way that T = Th. 

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