Electrical circuits.

0001This invention relates to electrical circuits and to temperature monitoring apparatus.

0002Use of disposable temperature probes for single patient application is common today because of the high cost of cleaning and resterilizing medical products. The well-known 700 series disposable temperature probe is considerably more costly than single thermistor disposable temperature probes. Most of the additional cost of the 700 series probe is attributable to the cost of the sensor element of the probe, which employs two thermistors.

0003Unfortunately, manufacturers of patient monitors continue to make monitors which use only the 700 series, dual thermistor probes. Users of such patient monitors are forced to purchase 700 series probes or an electronic adaptor. The typical electronic adaptor accepts signals from a low cost thermocouple-type probe and simulates the resistances of the 700 series thermistor elements. While such an adaptor allows use of low cost thermocouple probes, it also adds the cost of maintenance, battery replacement, and calibration, as well as introducing its own errors to the temperature monitoring procedure.

0004It is therefore an object of the invention to improve or reduce the cost of disposable temperature probes and to provide a circuit employing a single thermistor and having a resistance versus temperature characteristic comparable to that of a two-thermistor or 700 series disposable probe.

0005According to one aspect of the present invention there is provided an electrical circuit characterised in that the circuit includes a single thermistor and electrical circuit components connected with the thermistor such that the resistance versus temperature characteristic of the circuit simulates the output of a dual thermistor circuit.

0006The electrical circuit components are preferably connected to simulate a dual thermistor circuit of the kind having first and second thermistors each having a first terminal connected together and a second terminal connected together via a passive resistor. The electrical circuit components may be a shunt resistance that alters the slope of the characteristic and a series resistance that adds a constant resistance to the characteristic. The electrical circuit components may be contained in a cable and are preferably housed within an end connector of the cable. The cable preferably has an end connector at each end, the electrical circuit components being housed with an end connector that is adapted to mate with a connector at the end of a cable connected with a single thermistor, and the end connector at the other end of the cable being adapted to mate with a temperature monitor configured for use with a dual thermistor. The end connectors may have mating male and female contacts.

0007According to another aspect of the present invention there is provided temperature monitoring apparatus having a temperature monitor configured to monitor temperature at a dual thermistor temperature probe, characterised in that the apparatus includes a single thermistor temperature probe and electrical circuit components connected with the probe to simulate the resistance versus temperature characteristic of a dual thermistor probe.

0008The circuit components are preferably included in a cable having a connector at one end which is connected to the single thermistor and a connector at the other end which is connected to the temperature monitor. The circuit components preferably include a shunt resistance connected across the single thermistor and a series resistance in series between the single thermistor and the temperature monitor such that the shunt resistance alters the slope of the resistance versus temperature characteristic of the single thermistor, and the series resistance adds a constant resistance to the characteristic.

0009A circuit according to the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a circuit schematic of a prior art dual thermistor circuit;</li><li>Figure 2 is a circuit schematic of a prior application of the circuit of Figure 1;</li><li>Figure 3 is a circuit schematic of an application of the circuit of Figure 2 to a patient monitor;</li><li>Figure 4 is a circuit schematic of a prior art dual thermistor circuit;</li><li>Figure 5 is a circuit schematic of the preferred embodiment of the invention;</li><li>Figure 6 is a graph of error versus temperature between the resistance versus temperature characteristics of circuits according to Figure 4 and Figure 5;</li><li>Figure 7 is a circuit schematic showing application of the preferred embodiment in a patient monitor;</li><li>Figure 8 is a top view of a male-female connector;</li><li>Figure 9 shows an end view of each of the connector elements of Figure 8; and</li><li>Figure 10 is a sectional view taken at 10 - 10 of Figure 8.</li></ul>

0010The Yellow Springs Instrument (YSI) 700 series thermistor is a composite structure consisting of two thermistors T1, T2 with one lead connected in common at a junction 11, as shown in Figure 1. Both thermistors T1, T2 are assumed equilibrated to the same temperature. The resistance in Kohms versus temperature characteristic of the YSI thermistors T1 T2 is illustrated in the following table: <tables id="tabl0001" num="0001"><img file="EP0448414A2_D0001.tif" /></tables>

0011The thermistor assembly of Figure 1 is typically connected to a half bridge circuit (Figure 2) consisting of two fixed resistors, R1 and R2, and a regulated voltage source V_reg . The values of the fixed resistors R1, R2 are chosen to minimize nonlinearity of the voltage output (V_out) over the range of the desired measurement temperatures.

0012In a clinical patient temperature monitor application, the fixed resistors R1, R2 are located in the patient temperature monitor 19 and interconnection of the thermistor elements T1, T2 is achieved through a three-conductor cable 20 as shown in Figure 3.

0013According to the preferred embodiment, the network of the dual-thermistor elements T1, T2 and R2 shown in Figure 4 and used in Figure 2 may be simplified to a single dual-terminal resistance which varies with temperature. A network exhibiting the same resistance versus temperature characteristics of the network in Figure 4 may then be substituted between the external connection of R1 and ground.

0014A network employing a single thermistor and exhibiting a resistance temperature characteristic substantially similar to the circuit of Figure 4 is shown in Figure 5. The circuit of Figure 5 employs a thermistor T_s connected in parallel with a shunt resistor R_b. The shunt resistor R_b is connected in series with a resistor R_a. The values of R_a and R_b may be selected to be 2.67 KΩ and 63.0 KΩ, respectively, in an exemplary embodiment. A suitable, commercially-available thermistor T is the Thermometrics Type Y, 10K at 25 degrees Centigrade.

0015In operation, the shunt resistor R alters the slope of the resistance-temperature curve of the standard valued thermistor T_s, while the series resistor R shifts the curve. The resultant resistance-temperature curve closely fits that of the dual-thermistor circuit of Figure 4. The resistance in ohms of the network of Figure 4 is set forth in the following table: <tables id="tabl0002" num="0002"><img file="EP0448414A2_D0002.tif" /></tables> The temperature error between the resistance temperature characteristic of the circuit of Figure 4 and that of Figure 5 is plotted in Figure 6. The errors depicted in Figure 6 are low enough to permit the use of the circuit of Figure 7, including the perferred substitute thermistor circuit of Figure 5, for clinical temperature monitoring at around 30 degrees Centigrade.

0016In Figure 7, the simple thermistor sensor T_s is installed in a suitable temperature probe assembly for insertion into or attachment to a suitable body site. The series and shunt resistor R_a, R_b are connected between the thermistor element T_s and the input connection to the patient monitor 19, preferably in an extension cable 21. The thermistors R_a and R_b are connected to only two input points 31, 35 of the three available input points or terminals 31, 33, 35 of the typical patient temperature monitor 19 which is configured for use with a dual-thermistor probe. Terminal 33 of the patient monitor is left unconnected.

0017According to the preferred embodiment, the single thermistor and probe housing assembly is a disposable unit to be used once and then discarded. The thermistor T_s is connected to a cable 63, which includes a low cost end connector 51, as illustrated in Figures 7 to 10. The end connector 51 mates with a connector 53 formed as the termination of the extension cable 21. The connector 53 of the extension cable 21 preferably contains and encapsulates the two fixed resistors R_a and R_b , as shown in Figure 10. The cable 21 is terminated on the end opposite the connector 53 with a connector (not shown) suitable for mating with the input points 31, 35 of the input connector of the patient monitor 19.

0018As further shown in Figures 8 to 10, the end connector 51 of the disposable probe assembly includes a solid extended male connector shell 52 encapsulating first and second female contacts 55. The female contacts 55 are disposed in side-by-side relation to mate with respective male contacts 57 positioned in the extension cable connector 53. The male connector shell 52 is shaped to slide into a corresponding female connector shell 60 formed as part of the extension cable connector 53. The male connector shell 52 has bevelled edges 54 and 56 on one side to ensure proper polarity connection.

0019The female connector shell 60 is contained within an outer boot or shroud 58, which is made of a relatively pliable plastics or other material and is ribbed for strain relief. The inner female shell 53 is preferably made of harder material such as nylon to support rigidly the connector pins. Alternatively, the shroud 58 and female connector shell 53 could be fabricated as one solid piece of the same material.

0020Respective leads 59 are conducted through the cable 63 of the disposable probe assembly to connect to the respective opposite terminals of the single thermistor T_s. The cable 63 and its end connector 51 are thus on the "disposable side" of the device. Respective leads 63 attach to the respective male contacts 57 and to the resistors R_a, R_b such that the resistors R_a, R_b are connected in the manner schematically illustrated in Figure 7.

0021Those skilled in the art will appreciate that various adaptions and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practised other than as specifically described herein.