Thermometer Lag

Liquid-in-glass thermometers require a finite amount of time to achieve a final, equilibrium temperature. The time required can vary for individual thermometer types depending on the diameter of the thermometer, the size and volume of the bulb, the heat conductivity of the material into which the thermometer is placed, and the circulation rate of that material. 

For taking the temperature of a material that maintains a constant-temperature, thermometer lag is only a nuisance. Most laboratory thermometers placed in a 75°C bath will come within 0.01 °C of the final temperature within 19 to 35 seconds of first contact. The time range is dependent on the type and design of the thermometer. 

---

Finally, every contact thermometer has a thermometer lag due to the time for heat conduction. It takes a certain time for the heat to flow into or out of the thermometer. For a typical laboratory thermometer with a bulb of size 4.9 x 25 mm, filled with mercury, this effect has a time constant of 1 - 2 s to reach half of the initial temperature difference. Thus, if one wants an accuracy of 0.001 times the initial temperature difference, one must wait for a period of 10 such half-times. 

In an ideal first-order system, only one capacity causes a time lag between the measured quantity and the measurement result. Typically, an unshielded thermometer sensor behaves as a first-order system. If this sensor is rapidly moved from one place having temperature Tj to another place of temperature T2, the change in the measured quantity is close to an ideal step. In such cases, the sensor temperature indicated by the instrument has a time histoty as shown in Fig. 12.13. 

And it will be rough indeed, since the temperature of the thermometer usually lags that of the sample. 

If you don t know the melting point of the sample, heat the oil fairly quickly, but no more than KTC per minute to get a rough melting point. And it will be rough indeed, since the temperature ofthe thermometer usually lags that of the sample. 

Accuracy. Mercury-in-glass thermometers are relatively inexpensive and can be obtained in a wide variety of accuracy and temperature ranges. For example, between 0 and 100°C, thermometers with a 0.1°C graduation interval are readily available. Factors that affect the accuracy of the thermometer reading include changes in volume of the glass bulb under thermal stress, pressure effects, and response lag. With proper calibration by NIST [9,10] or traceable to NIST, an accuracy of from 0.01 to 0.03°C can be achieved. Table 16.5 summarizes... 

A thermowell is required to protect the thermal element of a thermometer or temperature transmitter thermobulb from corrosion and erosion, to give it adequate support, and to permit its removal without interrupting the process. The use of the thermowell will unavoidably introduce a temperature time lag to the changes in the process temperature and response to the temperature relayed to the instrument. This is caused by the transmission of heat through the thickness of the metal well and the inevitable dead air space between the well and bulb. 

In batch distillation the column temperature increases during the course of the operation, frequently by sudden increments the adjustment of the jacket temperatures by hand is consequently a matter of some difficulty. As the result of temperature lag, differences up to 30 deg. C between the jacket and column may occur, but these differences may be reduced considerably by automatic control. For this purpose air thermometers, thermocouples or resistance thermometers can be employed as sensing devices. Thermocouples are placed at the top and bottom of the column these together act on-the coil of a galvanometer, the position of which is sensed at short intervals by an electric switching device. The latter opens or closes the circuit... 

---