Thermometer, resistance bulb

Figure 12 Resistance Thermometer Circuit with Precision Resistor in Place of Resistance Bulb...

The thermometer and stirrer should be inserted with the thermometer carefully mounted in such a way that the sensing element (bulb of a mercury thermometer, resistance coil of a platinum resistance thermometer, or calibrated thermistor) is about halfway between the bottom of the test tube and the upper surface of the liquid and concentric with the tube so that the stirrer can easily pass around it. If the thermometer is too close to the bottom, a bridge of frozen solvent can easily form, which will conduct heat away from the thermometer and result in low readings. 

The most common are thermocouples, resistance bulb thermometers, and thermistors. All provide measurement in terms of electrical signals. Independently of their constructional differences, their basic dynamic behavior can be examined in terms of the temperature profiles in Figure 13.6a and b. The temperature-sensing element is always inside a thermowell (Figure 13.7). In the first case (Figure 13.6a) we assume... 

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... 

A3.1.1 Aniline-Point Tube, of heat-resistant glass, of the shape and dimensions shown in Fig. A3.1, and fitted internally with a thin-walled glass thermometer tube, sealed at the lower end. The latter tube accommodates a tight-fitting cork stopper carrying the thermometer, the bulb of which rests on a cork ring or disk placed at the bottom of the... 

As normally used in the process industries, the sensitivity and percentage of span accuracy of these thermometers are generally the equal of those of other temperature-measuring instruments. Sensitivity and absolute accuracy are not the equal of those of short-span electrical instruments used in connection with resistance-thermometer bulbs. Also, the maximum temperature is somewhat limited. 

In this method, the temperattire is determined by thermometers applied to the accessible surfaces of the windings of the motor (see also Figure 10.10). The term thermometer may include mercury or alcohol bulb thermometers as well as embedded thermocouples and resistance thermometers, provided the latter are applied to points accessible to the tistial btilb thermometer also. 

Many large air-conditioning plants incorporate automatic control of the humidity and temperature of the issuing air. Temperature control is effected with the aid of a thermocouple or resistance thermometer, and humidity control by means of a thermocouple recording the difference between the wet- and dry-bulb temperatures. 

Melting point instruments consist of a resistance heater to increase temperature, a sample holder and a temperature-measuring device. In its simplest form, an oil bath can be heated with a Bunsen burner, while a capillary tube with a few milligrams of sample is attached to a thermometer with the sample next to the mercury bulb. 

Filled-bulb temperature sensors are also widely used. An inert gas is enclosed in a constant-volume system. Changes in process temperature cause the pressure exerted by the gas to change. Resistance thermometers arc used where accurate temperature or diflcrcntial-temperature measurement is required. They use the principle that the electrical resistance of wire changes with temperature. 

Dewar flask (short, wide mouth, 1 pt) cork stopper with three holes coil-type stirrer stopwatch two 10-mL weighing bottles 50-mL burette burette clamp and stand two 100-mL volumetric flasks one 10-, one 25-, and one 50-mL pipette pipetting bulb battery jar wash bottle precision ciyoscopic thermometer and magnifying thermometer reader, or a digital resistance thermometer with 0.01°C resolution, or a calibrated thermistor. 

If only two lead wires are carried from the thermometer coil to the indicator, the resistance measured is the sum of the resistances of the coil, the platinum or gold lead wires to the head of the thermometer, and the copper lead wires from this point to the indicator. The resistance of the platinum or gold lead wires will depend upon the form of temperature gradient along the thermometer from the bulb to the head and upon the depth of immersion. Hence, this variable resistance is introduced into the temperature measurement, and changes in the resistance of the lead wires will be interpreted as changes in the temperature of the thermometer coil. 

Since biological systems are extremely sensitive to changes in temperature and cells typically have sharp optima for growth and product formation (often different values), temperature sensors are the most common probes in biotechnology. Thermistors, platinum resistance sensors, and thermocouples are all used to provide an on-line signal thermometer bulbs are also employed but cannot be used for automated monitoring. 

Probably the most common method of measuring moderate temperatures of liquids and gases in the chemical and petroleum industries involves a primary element (detector) and a thermowell. The primary element is usually a thermocouple, resistance thermometer, or gas-filled expansion bulb. If a thermocouple is used, it is most apt to be in the form of a pencil or sheathed assembly, which provides rigidity and mechanical protection. 

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