1 CALIBRATION OF A THERMISTOR THERMOMETER I. Introduction Calibration experiments or procedures are fairly common in laboratory work that involves any type of instrumentation. Calibration procedures always require a standard substance or device against which the instrument to be calibrated is checked. For example pH meters are calibrated with standard buffer solutions and spectrophotometer are calibrated with standard solutions or mechanical devices of known absorbencies. In physical measurement laboratories the purpose of most calibrations is to provide a means to convert from one measurement unit to another unit. The calibration of a pH meter converts a voltage measurement to a measure of hydrogen ion concentration. The calibration of a spectrophotometer converts an electrical current measurement to a measure of light absorption. II. Thermometers Any property which changes with temperature can be used to make a thermometer. You are familiar with liquid in glass devices which use the thermal expansivity of a liquid to indicate temperature change. Other thermometers utilize the expansion of a gas or the resistance change of a wire or other resistance material. In this experiment, your calibration work will allow the measured electrical resistance of a thermistor to be converted to a temperature. Thermistors are semiconductors whose electrical resistance changes with temperature. The standard device will be a platinum resistance thermometer. You are asked to consult the Internet for additional information on thermistors. III. Platinum Thermometers For the temperature range from about 60 K to 500 K, the platinum resistance thermometer is a primary standard thermometer. Platinum thermometers consist of a small coil of platinum wire encased in a protective sheath. Platinum thermometers must themselves be calibrated. Platinum thermometers can be purchased from several manufacturers, some of these companies have the facilities to calibrate them but the most dependable calibrations are done at the National Institute of Standards and Technology. Ours was purchased from and calibrated by the Leeds and Northrop Company. The calibration report (attachment 1 to given at the interview) contains information about the method of calibration and the calibration constants and fitting equations which can be used to calculate the temperature of our platinum thermometer at any point within its calibrated range from its resistance. With this information, the platinum thermometer can be used either as a stand alone temperature measuring device or as the temperature standard against which other thermometers are calibrated. Remember, the platinum thermometer does not read in degrees. Its resistance must be measured from a known current; the corresponding temperature is calculated using the calibration constants and equations provided in the calibration report. If all has gone as planned, you have already written a spreadsheet program which will convert the resistance of the platinum thermometer to the Celsius temperature. Because of the expensive nature of a platinum thermometer, most labs have only one and use it to calibrate other, less expensive, thermometers. This procedure is what we will be doing in this experiment.2IV. Thermistor Thermometers The thermometer you will calibrate is made from a tiny bead of a semiconductor material. The electrical resistance of this material is very sensitive to temperature (thermal resistor = thermistor) and is stable over time. These characteristics make thermistors a good device for the construction of thermometers. For our purposes, the thermistor bead is enclosed in copper tubing to protect the bead and to generally make the thermistor thermometer more durable and easier to use. Again, please find more information about thermistors on the Internet. V. Calibration Information The actual calibration process is really very simple. The platinum thermometer and the thermistor thermometer are placed in the same constant temperature environment (a computer controlled water bath). The electrical resistance of both devices is determined. The resistance of the platinum thermometer is used to calculate the water bath temperature. This temperature is coupled with the electrical resistance of the thermistor thermometer to yield a temperature-resistance data pair for the thermistor. Then the temperature of the water bath is changed and the process is repeated. This simple procedure is continued for a range of temperatures until the entire temperature range you wish to cover has been completed. During this experiment, you will perform two calibrations of your thermistor. The first calibration will cover the temperature range from 0 to 60°C in about 5° intervals. The second calibration will cover the temperature range from 20 to 30°C in about 1.3° intervals. Both sets of calibration data will be plotted to check the quality of the data. To check the quality of data, plot your data. (Which variable is the independent variable [x-axis] and which is the dependent variable [y-axis]?). VI. Generation of the Calibration Equations Based on information (attachment 2 to given at the interview) provided by the Yellow Springs Instrument Company (a major manufacturer of bead thermistors), you will use an empirical equation developed by Steinhardt and Hart1 (eq. 1) to fit your thermistor data. (1) where: T = Kelvin temperature, R = electrical resistance of your thermistor, A, B, C = Fitting constants found by using regression VI. Electrical Measurements During the calibration of your thermistor thermometer you will use a potentiometer, a computer-interfaced digital ohm meter (data acquisition system), and a computer controlled water bath. Potentiometers are used to measure voltage. They are relatively inexpensive, but very accurate, instruments. Ohm meters are used to measure electrical resistance. The computer controlled bath will provide stable calibrating temperatures between 0°C and 60°C. 1 Steinhart, J. S. and S. R. Hart, "Calibration Curves for Thermistors", Deep Sea Res., 15, 497-503 (1968). () ()31T A B ln R C ln R=+× +×3Platinum resistance thermometers have one major disadvantage. Their resistance change per degree is very small. The typical platinum thermometer has a resistance of 25 ohms at 0°C and 30 ohms at 60°C. For this reason, accurate temperature measurements require that small