1. Field of the Invention
The invention relates generally to fluid-filled heating pads and more specifically to control circuitry for maintaining a desired temperature of fluid in such fluid-filled heating pads.
2. Brief Description of the Prior Art
Water-filled heating pads are generally used in hospitals because they are considered to be relatively safe and efficient. One fluid-filled heating pad currently on the market is sold by Baxter Healthcare Corporation. This heating pad includes a control module that is connected to a pad by a pair of tubes. Water is heated in the control module and is circulated through the tubes to the pad. A thermistor located in the control module monitors the temperature of the fluid within the module.
If the distance between the module and the pad is relatively short, the temperature in the pad is approximately equal to the temperature in the module. However, if the distance is significant, or if the tubes connecting the pad to the module are exposed to significant temperature drops, the fluid in the pad may be significantly lower than the temperature of the fluid within the module. Therefore, it is desired to provide a more accurate means of monitoring the temperature of the fluid in the pad.
All fluid-filled heating pads in which the fluid is electronically heated require a means of calibrating whatever temperature monitoring device is used. As discussed above, thermistors that are located in a control module have been used in the past to monitor the temperature of fluid in a remotely located heating pad. Such thermistors generate a nonlinear signal which needs to be calibrated in order to correlate the thermistor signal to the temperature of the fluid.
Earlier versions of electronically heated fluid-filled pads using such thermistors were calibrated using one or more potentiometers. These potentiometers were typically adjusted by a manufacturer during the manufacture of a regulator module to individually modify the output signal from each thermistor in order to correlate that thermistor's output signal to a given temperature range. This meant that each thermistor had to be manually calibrated during the manufacturing process. More recently, self-calibrating electronic circuits have been developed which do not require the manufacturer to calibrate each unit individually. In a self-calibrating unit, two high-precisions reference resistors are used to provide a "two-point calibration."
The two reference resistors are used to measure and calibrate the current generator which, in turn, is used to measure the resistance of the temperature measuring thermistor. The measured and calibrated current is applied to the thermistor. A voltage, caused by the current, is developed across the thermistor. The voltage across the thermistor is directly proportional to the resistance of the thermistor.
The resistance of a thermistor is inversely proportional to its temperature. Thus as the temperature of the thermistor decreases, the resistance of the thermistor increases, and as the temperature of the thermistor increases, the resistance of the thermistor decreases. Accordingly, the resistance of a thermistor (and through additional calculations its temperature) can be calculated by dividing the voltage developed across the thermistor by the amount of current passing through the thermistor.
The resistance of a thermistor is an indication of the temperature of the thermistor, and the resistance of a thermistor (R.sub.thermistor) can be calculated as follows: ##EQU1## where: V.sub.thermistor =the voltage across the thermistor, and
I.sub.thermistor =the current through the thermistor.
Thus, any error in measuring the current through the thermistor produces an error in determining the temperature of the thermistor.
While the use of a "two-point" calibration method is an accurate way of measuring and calibrating the current generator, a need existed to reduce the complexity of the circuits and calculations used in a control module for a fluid-filled heating pad, yet maintain the necessary precision and overall accuracy of measuring the temperature of a fluid over the very limited temperature range of interest using the resistance temperature characteristics of a thermistor. Therefore, it is an object of the invention to eliminate one of the resistors to reduce the complexity of the electronic circuitry. It is also an object of the invention to provide a thermistor in close proximity to the fluid in the heating pad to be able to more accurately monitor the temperature of the fluid in the pad.