IMPROVEMENTS IN OR RELATING TO THERMOMETERS

Abstract: A thermometer comprises a temperature sensing device (1) and a separate temperature reading device (2). The sensing device comprises a tuned circuit (3, 4, 5) whose resonant frequency varies with temperature. The reading device measures the resonant frequency of the tuned circuit and converts this to the corresponding temperature for display (12). The thermometer may be used for clinical purposes and allows the sensing device, which does not contain any components which would be damaged by heating, to be sterilised by heat. The separate reading device is not subject to contamination and so does not require sterilising.

Description: Improvements in or relating to thermometers The present invention relates to thermometers, and is particularly suitable for clinical use. The most commonly used means for measuring body ■5 temperature in a clincial environment is the glass-encapsulated mercury thermometer. Such a thermometer has the advantage that it is relatively inexpensive and no power source is required. However, such a thermometer tends to be difficult to read and is io subject to breakage resulting in the release -of toxic mercury. Electronic thermometers using a variety of temperature sensors have also been used in the clinical environment to provide digital or analogue readout of temperature. •3 " Such thermometers are fast acting, easy to read, and simple to operate. However, they tend to be relatively expensive and cannot be sterilised at elevated temperature. Pyroelectric non-contact thermometers have also been Z.O used and have the advantages of being fast acting and accurate and not being subject to contamination because of their non-contact nature. However, such thermometers are expensive and require the use of a skilled operator. According to the present invention, there is provided a thermometer comprising a temperature sensing device and ^" a separate temperature reading device, the temperature sensing device comprising a tuned circuit whose resonant frequency varies with temperature, the temperature reading device being arranged to measure the resonant frequency of the tuned circuit and to convert the \ Ό measured resonant frequency to temperature. Preferably the tuned circuit is a parallel resonant circuit. Preferably the tuned circuit includes a temperature dependent capacitor. \ 3 Preferably, the tuned circuit includes an inductor and the reading device includes a sensing inductor for non-contact inductive coupling to the inductor of the tuned circuit. Preferably, the reading device includes variable frequency source arranged to supply a swept SO frequency signal to the sensing coil and means for indicating the frequency at which absorption of power in the tuned circuit takes place. The variable frequency source may be arranged to provide a continuous sweep of frequency or to sweep the frequency in discrete steps. Preferably, the reading device includes a look-up table for-converting the measured resonant frequency to temperature. S The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a thermometer constituting a preferred embodiment of the invention; Figure 2 illustrates continuous sweep of frequency; Figure 3 illustrates sweeping of frequency by discrete steps; Figure 4 shows an external view of the apparatus of figure 1; and Figure 5 is a block diagram of a thermometer constituting another embodiment of the invention. The thermometer shown in figure 1 comprises a temperature sensing device 1 and a separate temperature reading device 2. The temperature sensing device 1 σ comprises a parallel tuned circuit comprising an inductor 3. a temperature-sensitive capacitor 4. and a trimming capacitor 5. The temperature-sensitive capaci.tor 4 has a known temperature co-efficient so that its capacitance varies in a known manner with 5 temperature. The resonant frequency of the tuned circuit therefore also varies in a known manner with temperature and the resonant frequency provides a measure of the temperature of the temperature sensing device. iσ The inductor 3 is arranged to be inductively coupled to a sensing coil 6 forming part of the temperature reading device 2. The inductor 6 is connected to an output and sensing stage 7 which . drives the inductor and measures "" the energy loss in the inductor. The output and sensing Ϊ5 stage 7 is driven by a variable frequency source 8 whose frequency is controlled by a frequency control circuit 9. The output and sensing stage 7 is also connected to a threshold detector 10 which has an output for controlling the frequency control circuit 9 to stop O varying the frequency of the variable frequency source 8. The frequency control circuit 9 has an output which is connected to a look-up table 11 for converting the measured resonant frequency to the corresponding temperature. The look-up table 11 has outputs connected 2.S to a visual display 12 and a voice synthesiser 13. A control circuit 14. for instance in the form of a micro¬ processor arrangement, is provided to control operation of the- temperature reading device. A power source 15. for instance battery powered or solar powered or £ recharged, is provided to supply power to the electronics of the temperature reading device 2. The thermometer of figure 1 operates in the following way. The temperature sensing device 1 is used as a probe in contact with an element whose temperature is to iO be measured, for instance in the same way as a conventional clinical mercury thermometer. The device 1 is physically separate from the reading device 2 and is preferably moulded into a waterproof, heat-resistant, and shock-resistant package using any suitable encapsu- lation technology. The device 1 does not contain any components which can be destroyed by high temperatures and may therefore be heat-sterilised in the usual way. After waiting for a period sufficient for the temperature of the temperature-sensitive capacitor 4 to 5.0 reach that of the element whose temperature is to be measured, the reading device 2 is brought near to the temperature sensing device 1 so that the sensing coil 6 is in close proximity to the inductor 3 so as to provide inductive coupling therebetween. Under control of the 25 control circuit 14. the frequency control circuit 9 begins varying the frequency of the variable source 8. The frequency of the source 9 may be swept continuously from minimum to maximum, or vice versa, as illustrated- in figure 2. Alternatively, the frequency may be swept through a plurality of consecutive discrete steps, from minimum to maximum frequency or vice Versa, as illustrated in figure 3. The output and sensing stage amplifies the output signal of the variable frequency source and supplies this to the sensing coil 6. which ιθ couples energy to the inductor 3 of the tuned circuit. The output and sensing stage 7 simultaneously provides an output signal to the detector 10 representative of the amount of energy absorbed by the tuned circuit from the sensing coil 6. As the frequency of the variable frequency source 8 approaches the resonant frequency of the tuned circuit, the amount of energy absorbed by the tuned circuit increases until it reaches a maximum at the resonant frequency. The threshold detector 10 is arranged to p detect the amount of absorption at or close to the resonant frequency, and supplies a signal to the frequency control circuit 9 to stop varying the frequency of the variable frequency source 8. A signal representative of the frequency is supplied by the 5- frequency control circuit 9 to the look-up table 11 which is preferably stored in a read only memory. The look-up table 11 converts the measured resonant frequency to the corresponding temperature and supplies this to -the display 12 and to the voice synthesiser 13. The voice synthesiser 13 may be any suitable circuit for 5 providing an audible indication of temperature and is preferably of the type generally available in which the human voice is simulated by electronic means. The presence of the voice synthesiser allows the thermometer to be used by the blind or in situations where it would i be impossible or inconvenient to read the visual display 12. The control circuit 14 performs various housekeeping functions of the reading device 2 such as controlling the display time, providing automatic reset, and l5 providing automatic power-down in order to conserve the power source 15. Figure 4 illustrates the appearance of two possible embodiments of temperature sensing devices la and lb and of the temperature reading device 2. The temperature 0 sensing devices have shafts 20a and 20b with tips 21a and 21b in which the temperature-sensitive capacitor 4 is provided. The tips are arranged to contact the element whose temperature is to be measured, and the embodiments shown in figure 4 are particularly suitable 2-5 " for clinical purposes to replace conventional mercury-in- glass thermometers. Thus, the tips 21a and 21b may be placed under the tongue of a patient with the shafts 20a and 20b extending to the exterior of the mouth. At the end of the shafts opposite the tips, there are provided „5" sensing heads 22a and 22b. These heads contain the inductor 3 and are disposed outside the mouth of the patient so as to be readily'coupleable to the reading device 2. Figure 4 shows the visual display 12 of the reading id device 2 and a protective grill 23 covering an output electro-acoustic transducer of the voice synthesiser 13. The upper end 24 of the reading device 2 is bought into proximity with the heads 22a and 22b of the devices la and lb in order to detect the temperature, the ( 5 " sensing inductor 6 being disposed immediately adjacent the upper end 24. Figure 5 shows another embodiment of temperature reading device which may be used with the temperature sensing device 1 of figure 1. The reading device of figure 5 2.0 includes a sensing inductor 26 similar to the inductor 6 of figure 1. The inductor 26 is connected to the output of a driver 27 whose input is connected to a frequency synthesiser 28 having a plurality of frequency control input lines connected to outputs of a microprocessor ^5 arrangement 29. The synthesiser is preferably of the phase-locked loop type including a programmable frequency divider for determining the output frequency. The ou'tput of the microprocessor arrangement 29 is connected to a read only memory 30 containing a look-up 5 table for converting resonant frequency to temperature. The output of the memory 30 is connected to a visual display 31 which is controlled by a display enable circuit 32. The driver 27 is connected to a current sensor 33 which to measures the driver current, and hence the amount of energy absorbed by the temperature sensing device. The driver 27 and the current sensor 33 are equivalent to the output and sensing stage 7 of figure 1. The output of the current sensor 33 is connected to the input of a t5 threshold circuit 34 which is equivalent to the detector 10 of figure 1. The threshold circuit 34 has an output connected to the display enable circuit 32. The microprocessor arrangement 29 is provided with a reset switch 35 for resetting the temperature reading device so as to give a reading of temperature. The switch 35 is connected between a positive supply line a reset input of the microprocessor arrangement 29. The reset input is also connected to a 1 millisecond mono- stable multivibrator 36 which is arranged to set a /S S bi-stable 37. The bi-stable 37 thus enables, via a diode 38, a clock oscillator connected to a clock input 10 of the microprocessor arrangement 29 and comprising an amplifier 39. a resistor 40. and a capacitor 41. In use. the reading device of figure 5 is brought into proximity with the temperature sensing device so that ό" the sensing coil 26 is coupled with the coil or inductor of the sensing device. The reset switch 35 is operated so as to cause the microprocessor arrangement 29 to begin sweeping the frequency of the synthesiser in . discrete steps as illustrated in figure 3. The step lO size may be chosen to provide an adequate resolution of measurement, for instance 0.2 Fahrenheit over a range from 89.6 to 107.6 Fahrenheit, or 0.1 Centigrade over a range from 32 to 42 Centigrade. The driver circuit 27 drives the sensing coil 26 with this frequency and ό the current sensor 33 detects the energy absorbed in the tuned circuit of the temperature sensing device. When the energy absorption exceeds a predetermined value as set in the threshold circuit 34, the display enable circuit 32 is enabled so as to permit the display 31 to r _Q read the temperature corresponding to the resonant frequency in accordance with the look-up table in the memory 30. The output signal from the threshold circuit 34 also resets the bi-stable 37 which disables the clock oscillator 39 in order to prevent further processing by _S the microprocessor arrangement 29. The temperature thus remains latched in the display 31 until the reading device is reset by the switch 35 so as to take a further reading. Where the temperature dependent characteristic of the capacitor 4 is accurately known and the inductance of the inductor 3 is also accurately known, the trimming capacitor 5. which is optional, may simply be used for fine calibration. However, where the exact values are not known, the trimming capacitor 5 is used to provide accurate calibration of the temperature sensing device. ιC In these cases, the look-up table 11 may be a stock item which is identical for all temperature reading devices 2. However, where it is not possible to provide sufficiently accurate or consistent temperature sensing devices 1, the sensing device and reading device may be ι<5 calibrated together with reference to a known temperature standard and the look-up table may be created at the same time in order to provide a sufficient degree of accuracy. Each value in the look-up table may be set by such calibration, or O alternatively a plurality of look-up values spread across the working temperature range may be set by such calibration and the intermediate values determined by inter-polation. In order to provide a better measure of the resonant ^ frequency of the tuned circuit within the temperature sensing device 1 so as to take into account varying Q-values of the tuned circuit and differences in the inductive coupling between the inductor 3 and the sensing inductor 6 or 26 of the reading device due to " -variations in use. the reading device may be modified so that the variable frequency source or synthesiser "over scans" the resonant frequency of the tuned circuit so as to determine the frequency at which maximum absorption of energy in the tuned circuit takes place. For (O instance, the reading device of figure 5 may be modified so that the threshold circuit 34 is replaced by an analogue-to-digital converter whose output is connected to the microprocessor arrangement 29. The microprocessor arrangement 29 then creates a table of ■5 " . energy absorption values against frequency for a complete scan of frequencies, or for a sufficient scan to encompass the resonant peak in the absorption. The microprocessor 29 then supplies to the memory 30 the frequency value corresponding to maximum absorption and < 2θ this is latched into the display 31. The display enable circuit 32 is thus controlled by the microprocessor arrangement 29 in this modified embodiment, and the bi-stable 37 is not provided. It is thus possible to provide a thermometer having all 5 ' of the advantages of the prior art without any of the disadvantages. The temperature sensing device can be sterilised by heat in the usual way, as it does not contain any components which would be damaged by such heating. The reading device is not subject to contamination and therefore does not require sterilising.

Claims: Claims 1. -A thermometer comprising a temperature sensing device and a separate temperature reading device, the temperature sensing device comprising a tuned circuit 5 whose resonant frequency varies with temperature, the temperature reading device being arranged to measure the resonant frequency of the tuned circuit and to convert • the measured resonant frequency to temperature. 2. A thermometer as claimed in claim 1. in which the 10 tuned circuit is a parallel resonant circuit. 3. A thermometer as claimed in claim 1 or 2,- in which the tuned circuit includes a temperature dependent capacitor. 4. A thermometer as claimed in anyone of the preceding '5 claims, in which the tuned circuit includes an inductor and the reading device includes a sensing inductor for non-contact inductive coupling to the inductor of the tuned circuit. 5. A thermometer as claimed in claim 4, in which the * 3C reading device includes a variable frequency source arranged to supply a swept frequency signal to the sensing coil and means for indicating the frequency at which maximum absorption of power in the tuned circuit takes place. 6. A thermometer as claimed in claim 5, in which the variable frequency source is arranged to provide a continuous sweep of frequency. 7. A thermometer as claimed in claim 5, in which the variable frequency source is arranged to sweep the frequency in discrete steps. 8. A thermometer as claimed in any one of the preceding •O claims, in which the reading device includes a look-up table for converting the measured resonant frequency to temperature. 9. A thermometer substantially as hereinbefore described with reference to and as illustrated in the ι5 accompanying drawings.

Inventors: KON ISRAEL, KON, ISRAEL
Applicants: KON ISRAEL, KON, ISRAEL
Application year: 1985
Main classification: G01K7
IPC codes: G01K1, G01K7, G01K13