Temperature contact measurement

Contact temperature measurement is based on a sensor or a probe, which is in direct contact with the fluid or material. A basic factor to understand is that in using the contact measurement principle, the result of measurement is the temperature of the measurement sensor itself. In unfavorable situations, the sensor temperature is not necessarily close to the fluid or material temperature, which is the point of interest. The reason for this is that the sensor usually has a heat transfer connection with other surrounding temperatures by radiation, conduction, or convection, or a combination of these. As a consequence, heat flow to or from the sensor will influence the sensor temperature. The sensor temperature will stabilize to a level different from the measured medium temperature. The expressions radiation error and conduction error relate to the mode of heat transfer involved. Careful planning of the measurements will assist in avoiding these errors. 

With contact temperature measurement, placing the measurement probe in contact with the object of measurement (duct, surface, etc.) produces an additional route for heat conduction to or from the object. This perturbation error changes the initial temperature field in the vicinity of the contact point and creates measurement errors. 

The first clinical IRET used thermopile sensors to achieve non-contact temperature measurement in the ear. In 1991 a tympanic thermometer for home use was first introduced to the consumer market (Thermoscan HM 1). It utilized a pyroelectric sensor which requires the use of a suitable mechanical shutter or chopper mechanism, since it is only sensitive to temperature changes [3]. The main advantage of the pyroelectric sensor unit was its lower cost. However, prices for thermo-... 

Thermopiles have been used for non-contacting temperature measurement in hairdryers, to prevent damage to the hair and to speed up the drying process. 

Non-contact temperature measurement inside microfluidic channels was achieved by using fluorescence quenching of a rhodamine dye. The intensity of the dye fluorescence is temperature-sensitive in a range temperature of 5-95°C [795], Another on-chip temperature measurement method was achieved by measuring... 

Slyadnev, M.N., Tanaka, Y., Tokeshi, M., Kitamori, T., Non-contact temperature measurement inside microchannel. Micro Total Analysis Systems, Proceedings 5th [lTAS Symposium, Monterey, CA, Oct. 21-25, 2001, 361-362. 

Temperature is undeniably the most important property for all calorimetric measurements, because it is the common denominator. Two different techniques for temperature measurements are used for pulse calorimetry contact thermometry (e.g. thermocouples) and radiation thermometry or pyrometry. Because pulse calorimetry is often used to handle and measure liquid materials, non-contact radiation thermometry is far more common in pulse-heating than contact thermometry. Other reasons for non-contact temperature measurement methods include the fast heating rates and temperature gradients (inertia of the thermocouples), difficulties mounting the contact thermometers (good thermal contact needed), and stray pick-up in the thermocouple signal because the sample is electrically self-heated. 

A pyrometer is a non-contacting temperature measurement instrument that is usually used for temperatures above 500 °C, although with some modifications it can measure temperatures below room temperature. The word pyrometry comes from the Greek words pyro (Are) and meter (measure). The basic principle relies on the notion that all bodies emit thermal radiation proportional to their temperature. Pyrometers detect this thermal radiation and through Planck s law the temperature can be determined. 

For temperature measurements on the emerging extrudate, contacting-t)rpe measurements are not suitable because of damage to the extrudate surface. For non-contacting temperature measurements, infrared (IR) detectors can be used. The intensity of the radiation depends on the wavelength and the temperature of a body. Non-contact IR thermometers can be used to determine the temperature of the plastic after it leaves the die. IR sensors can also be used to measure the melt temperature inside the extruder or die see Section 4.3.3.2. 

The Ferranti-Shidey viscometer was the first commercial general-purpose cone—plate viscometer many of the instmments stiU remain in use in the 1990s. Viscosities of 20 to 3 x 10 mPa-s can be measured over a shear rate range of 1.8-18, 000 and at up to 200°C with special ceramic cones. Its features include accurate temperature measurement and good temperature control (thermocouples are embedded in the water-jacketed plate), electrical sensing of cone—plate contact, and a means of adjusting and locking the position of the cone and the plate in such a way that these two just touch. Many of the instmments have been interfaced with computers or microprocessors. 

Example. A trap on a 150 psi steam line has been found to be blowing live steam on the basis of contact pyrometer measurements taken immediately upstream and downstream of the trap. The catalog rating of the trap is 5,000Ib/hr at saturation temperature (°F sub-cooled) at 150 psi. 

The noncontact measurement principle, usually called optical or radiation temperature measurement, is based on detecting electromagnetic radiation emitted from an object. In ventilation applications this method of measurement is used to determine surface temperatures in the infrared region. The advantage is that the measurement can be carried out from a distance, without contact with the surface, which possibly influences the heat balance and the temperatures. The disadvantages are that neither air (or other fluid) temperature nor internal temperature of a material can be measured. Also the temper-... 

Temperature measurement is a case in point. A large transducer in close contact with the body whose temperature is being measured will act as a heat sink and consequently produce a localized reduction at the point where the temperature is being measured. On the other hand, if an air gap exists between the transducer and the hot surface then the air (rather than the surface) temperature will be measured. 

When selecting a lubricant, both the temperature at the contact area and the ambient temperature at important factors to be considered. Measuring the peak contact temperature is very difficult. The maximum rise in temperature of the oil leaving the gears and the maximum oil temperature are specified for various types of gears. For spur, bevel, helical and spiral level gears, the temperature rise should not normally exceed 30°C (86°F) with a maximum oil temperature of70°C (158°F). 

For correlating relative Eamo values with values in the UHV scale (0 values), two quantities must be known 0 and A0. Contact potential measurements at metal/solution interfaces can be measured.4 In that case the interfacial structure is exactly that in the electrochemical situation (bulk liquid phase, room temperature). However, 0 to convert E into 0 must be independently known. It may happen that the metal surface state is not exactly the same during the measurements of 0 and A0. 

Fig. 9.9 Experimental set-up 1 test module, 2 heater, 3 electrical contact, 4 micro-channel, 5 Pyrex, 6 peristaltic pump, 7 and 8 pressure and temperature measurements, 9 cooler, 10 reservoir, 77 IR camera, 72 microscope, 13 high-speed video camera, 14 PC, 75 synchronizer, 16 video recorder. Reprinted from Peles et al. (2001) with permission...

Contact angle measurements were obtained using a goniometer, measuring the advancing angle from 2 to 20 microliter drop sizes, of purified water upon polymer films at room temperature. Films were cast on metal plates and allowed to dry slowly from chloroform solutions. Several spots were measured on each film and the results averaged. 

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... 

In clinical settings core temperature measurements, including pulmonary artery and esophagus measurements, are often required. In 1959 Benzinger [1] first proposed the human tympanic membrane as the ideal site for core temperature measurements. The tympanic membrane is ideal, because it is located near the carotid artery and shares its blood supply with the hypothalamus, which controls body temperature. First temperature measurements in the ear were performed with thermistor sensors in direct contact with the tympanic membrane. The invasiveness of this method limited its use mainly to anaesthetized patients. 

A thermopile sensor generates an output voltage that depends on the temperature difference between its hot and cold contacts. For infrared temperature measurement, the hot contacts are normally thermally insulated and placed on a thin membrane, whereas the cold contacts are thermally connected to the metal housing. Infrared radiation, which is absorbed by the hot contacts of the thermopile, causes a temperature difference between hot and cold contacts. The resulting output voltage is a measure for the temperature difference between radiation source and cold contacts of the thermopile sensor. It is therefore necessary to measure also the temperature of the cold contacts by an additional ambient temperature sensor in order to determine the temperature of the radiation source. 

The infrared ear thermometer is a major step in the development of thermometers for body temperature measurements. Compared to traditional mercury-in-glass or electronic contact thermometers it is more convenient, safer and faster. During its 10 years in the consumer market it has been gradually replacing conventional thermometers, especially for temperature measuring in children. 

In Fig. 6.1, an attempt is made to show to what extent sensors have been penetrating the appliance market over the past years, a trend which is set to continue in the next decade. In the beginning, there were relatively simple sensors for temperature, pressure, flow, etc. Over the last years, non-contact measuring devices have attracted much attention, such as non-contact temperature monitoring for toasters or for hair blowers. The introduction of more complex sensor systems, such as water quality sensors or multi gas sensing artificial noses is imminent. 

A TMA analyser will need to measure accurately both the temperature of the sample, and very small movements of a probe in contact with the surface of the sample. A typical analyser, as illustrated in Figure 11.20(a) and (b), uses a quartz probe containing a thermocouple for temperature measurement, and is coupled to the core of a linear variable differential transformer (LVDT). Small movements at the sample surface are transmitted to the core of the LVDT and converted into an electrical signal. In this way samples ranging from a few microns to centimetre thicknesses may be studied with sensitivity to movements of a few microns. For studying different mechanical properties the detailed construction of the probe will vary as is illustrated in Figure 11.20(c). 

The sample is used in finely powdered form and spread on a fine platinum wire mesh which is mounted in the sample holder frame (Pt). The thermocouple (PtRh 10%) for temperature measurement is directly in contact with the sample holder. 

---