The present invention relates to a thermometer which converts changes in the resistance of a thermistor into changes in a frequency, by utilizing the thermistor in a capacitance-resistance (abbreviated in the following to CR) oscillator circuit and which performs temperature measurement by measuring that frequency.
It is an objective of the present invention to use the exponential changes in the resistance of a thermistor with respect to temperature, to produce an inexpensive and highly accurate thermometer which has a fixed sensitivity over a wide temperature range, and to thereby implement a small size thermometer which can be built into an electronic wristwatch.
Hitherto, the most widely used types of thermometer have been formed of a glass tube with mercury or colored alcohol sealed therein, and which employ the expansion of such a fluid. Such glass thermometers are light and comparatively inexpensive, but on the other hand, if they are made very small in size then the displayed temperature becomes difficult to read, and the accuracy becomes poor as a result. Another type of inexpensive and lightweight thermometer is the bimetal type. However this has the disadvantages of low accuracy and of being very weak and easily damaged if it is made small in size.
Another type of thermometer, used for high-grade applications, is the electrical thermometer which utilizes a thermocouple as a temperature sensing element. In recent years, such thermometers have been equipped with digital temperature display means, for use as digital thermometers, and are widely used in scientific and industrial temperature measurement applications. However due to the high cost of such devices, they are not suitable for general use, e.g. as domestic thermometers. Thermometers based on various other principles have been produced, however none of these combines high accuracy, light weight, and low cost, such as to be suited for use as a general purpose thermometer.
The present invention enables a thermometer to be produced which has a high degree of accuracy, comparable to that of a prior art specialized electronic thermometer, an easily legible display, and possesses advantages such as light weight and low manufacturing cost which are comparable to those of glass rod or bimetal type thermometers. A thermometer according to the present invention uses a thermistor as a temperature sensor. The term "thermistor" is applied to resistive elements whose resistance value varies with changes in temperature. However in the present specification, the term thermistor refers to a negative temperature coefficient thermistor comprising a sintered metal oxide body provided with electrodes. The resistance of a thermistor is extremely sensitive to changes in temperature, and can be freely selected. In addition, such a thermistor can be simple in shape and small in size. Thus, the thermal time constant can be small, and so thermistors are highly suited to measurement of rapidly changing temperatures. In addition to these advantages, thermistors are highly suited to mass production manufacture. However, since the resistance of a thermistor changes in an exponential manner with respect to to temperature, it possesses the disadvantage that it is necessary to convert the exponential characteristic to provide a linear variation with temperature, by some form of compensation.
Hitherto, the following two methods have been adopted for this linearity compensation. In one method, the thermistor is connected in parallel with a resistor, so that the temperature/resistance of the combination is approximately linear. However with such a method, large numbers of highly accurate resistors are required, and the temperature range over which compensation can be applied is narrow, while the accuracy of linearity compensation is comparatively poor. Due to this relatively poor accuracy, such thermometers have only been used for applications which cover only a narrow range of temperature, such as for room heating and cooling systems, etc.
With the second method of thermistor linearity compensation, the resistance of the thermistor is accurately measured, and is converted into a digital value by an A-D converter. Linearity compensation is then applied by computer means. Such a method requires a large amount of equipment, and due to the high cost, it is limited to thermometers for scientific applications.
For the above reasons, thermistors have been used primarily as sensors for detection of specific temperature points, rather than in thermometers. With the present invention, however, linearity compensation is applied to the temperature versus resistance characteristic of a thermometer by means which are light in weight, yet which provide a high degree of accuracy, as will be explained in the following.