Resistance thermometer, metallic semiconductor

Temperature The level of the temperature measurement (4 K, 20 K, 77 K, or higher) is the first issue to be considered. The second issue is the range needed (e.g., a few degrees around 90 K or 1 to 400 K). If the temperature level is that of air separation or liquefact-ing of natural gas (LNG), then the favorite choice is the platinum resistance thermometer (PRT). Platinum, as with all pure metals, has an electrical resistance that goes to zero as the absolute temperature decreases to zero. Accordingly, the lower useful limit of platinum is about 20 K, or liquid hydrogen temperatures. Below 20 K, semiconductor thermometers (germanium-, carbon-, or silicon-based) are preferred. Semiconductors have just the opposite resistance-temperature dependence of metals—their resistance increases as the temperature is lowered, as fewer valence electrons can be promoted into the conduction band at lower temperatures. Thus, semiconductors are usually chosen for temperatures from about 1 to 20 K. 

If the temperature range of interest is large, say 1 to 400 K, then diode thermometers are recommended. Diodes have other advantages compared to resistance thermometers. By contrast, diode thermometers are veiy much smaller and faster. Bv selection of diodes all from the same melt, they may be made interchangeable. That is, one diode has the same cahbration cui ve as another, which is not always the case with either semiconductor or metallic-resistance thermometers. It is well known, however, that diode thermometers may rectify an ac field, and thus may impose a dc noise on the diode output. Adequate shielding is required. 

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. 

There are two principal classes of this type of sensor, viz. (i) resistance thermometers (resistance temperature detectors)—which are constructed from normal metallic conducting materials, and (ii) thermistors—which are bulk semiconductor sensors. 

The thermisters (TMs) are semiconductor device with a high resistance dependence on temperature. They may be calibrated as a thermometer. The semiconductor sensor exhibits a large change in resistance that is proportional to a small change in temperature. Normally TMs have negative thermal coefficients. Like RTDs, they operate on the principle that the electrical resistance of a conductive metal is driven by changes in temperatures. Variations in the conductor s electrical resistance are thus interpreted and quantified, as changes in temperature occur. 

Thermistors are resistance thermometers, where the temperature-sensible element is the semiconductor, and are made of a mixture of different metal oxides. The large resistance of the thermistor enables us to lower substantially its dimensions in comparison to the resistance thermometer. Thermistors are very sensible and give a fast response, which is very suitable for use in small calorimeters. 

Another type of resistance thermometer uses metal oxides, instead of metals it is frequently referred to as a thermistor. Electrical resistance of these metal oxides changes rapidly with even rather small temperature changes. Hence, thermistors are often emplyed to measure small temperature changes such as 1°C to 5 °C. The thermistor proper tends to have low purchase prices. Metal oxides, which are semiconductors, include mixtures of the following oxides nickel, manganese, copper, cobalt, tin, germanium, etc. [1]. 

A bolometer is a type of resistance thermometer constructed of strips of metals, such as platinum or nickel, or of a semiconductor. Semiconductor bolometers are often called thermistors. These materials exhibit a relatively large change in resistance as a function of temperature, The responsive element is kept small and blackened to absorb radiant heat. Bolometers are not so extensively used as other infrared transducers for the mid-infrared region, Flowever, a germanium bolometer. operated at 1..3 K, is nearly an ideal transducer for radiation in the. 5 to 400 cm (2000 to 2.3 pm) range. 

Metals, as well as semiconductors have been used as resistance thermometers. As given in Fig. 4.5, the conductivity depends upon the number of charge carriers n, their charge e, and their mobility p. The mobility of the electrons is impeded by the... 

Semiconductors thus have over a wide temperature range the opposite dependence of resistance on temperature to that of metals. Typically, one may have as many at 10 charge carriers per cubic centimeter at room temperature, and specific resistances may vary from 10 to 10ncm. Mathematically Eq. (3) expresses the change of resistance with temperature. Three constants, Roo, B, and 0, are characteristic for a particular semiconductor. The temperature coefficient of the resistivity may be ten times that of a typical metal resistance thermometer. 

For the international practical temperature scale the platinum thermometers serve as standard (interpolation) instruments with characteristics values such the reduced resistance, the temperature coefficient of the resistance and the platinum temperature. Interpolation polynomials of the third to fifth degrees, using as the principal reference points 0, 100 and 419.58 °C (fusion of Zn) are used most frequently (as a standard accessory of commercial products). Another form of resistance thermometers are films or otherwise deposited layers (0.01 to 0.1 pm thick). They can be suitably covered to protect against corrosive media and platinum deposited on ceramics can withstand temperatures up to 1850 K. Non-metals and semiconductor elements can be found useful at low temperatures. 

Resistance thermometer detectors (RTDs) are made of either metal or semiconductor materials as resistive elements and may be classed as follows [3] ... 

For most purposes, the platinum resistance thermometer is still the first choice among metallic resistance thermometers. For measurements extending either over a broad temperature range from 1 to 300 K or for a narrower temperature range below 30 K, semiconductor and diode thermometers are the principal competitors to PRTs and are often to be preferred. 

What are the principal differences between metallic and semiconductor resistance thermometers Under what conditions would you choose one over the other ... 

The basis for resistance thermometry is the feet that most metals and some semiconductors change in resistivity with temperature in a known, reproducible manner. Several materials are commonly employed for resistance thermometers. 

Two other types of variable resistors use external conditions and not human input to control the electron flow of a circuit. The thermistor is a variable resistor that is regulated by temperature (Figure 15-13). Sometimes we call the thermistor a temperature resistor. When the temperature rises, electrons are able to flow with less opposition, so the resistance of the thermistor goes down. When the temperature gets colder, the resistance of the thermistor increases. Thermistors are used in electric thermometers and also in new electronic home thermostats. Thermistors are typically manufactured from semiconductors and metals such as manganese, copper, or nickel. 

---