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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to cutaneous gas sensors and is directed more
particularly to an improved temperature control system which protects a
patient from exposure to cutaneous gas sensors which are operating at
excessive temperatures, and which significantly reduces the time that
elapses between the application of the sensor to the patient and the
generation of useful data.
Among the non-invasive patient monitoring instruments which have been
developed recently is an instrument known as a cutaneous gas sensor. Gas
sensors of this type make use of known gas detection techniques to measure
the partial pressure of a gas, such as oxygen or carbon dioxide, which
diffuses outwardly through the pores of a patient's skin. Cutaneous gas
sensors have also been developed which simultaneously measure the partial
pressure of both oxygen and carbon dioxide. One cutaneous gas sensor of
the latter type is described in "Cutaneous Blood Flow and its Relationship
to Transcutaneous O.sub.2 /CO.sub.2 Measurements", by A. V. Beran, et al.,
"Critical Care Medicine", Vol. 9, No. 10, pp. 736-741 (1981).
Because the rate at which blood gases diffuse through human skin is related
to the temperature of the skin, cutaneous gas sensors include heating
elements whereby the temperature of the skin at the measurement site may
be maintained at a temperature that is higher than normal body
temperature. A typical gas sensor will, for example, be maintained at a
temperature, such as 42.degree. to 43.degree. C., which is several degrees
higher than the normal human body temperature of 37.degree. C. This
elevated temperature is maintained by a closed loop temperature control
circuit which continuously compares the actual sensor temperature with a
desired setpoint temperature and increases or decreases the current flow
through the heating element as necessary to maintain the desired
temperature.
Existing temperature control circuits for cutaneous gas sensors have two
important deficiencies which limit their usefulness. One of these is that
temperature control circuits lack adequate provision for shutting off the
flow of current through the heating element in the event that one or more
parts of the circuit malfunction. Such a shutoff is extremely important
because cutaneous gas sensors are often applied to patients, such as
infants or comatose individuals, who are unable to remove a gas sensor the
control circuitry of which has failed in a way that causes it to apply
excessive current to the heating element. This excessive current can
result in serious injury, particularly in cases in which gas sensors are
allowed to operate unattended for hours at a time.
If, as is often the case, the gas sensor operates under the control of a
programmed microcomputer, the solution to the problem of automatically
shutting off the flow of heater current is made more difficult by the fact
that microcomputers can malfunction as a result of power line transients,
electrical noise and cosmic rays. In some cases, these malfunctions can
cause the microcomputer to become unable to limit the flow of heater
current, or even to initiate the flow of excessive heater current.
Another deficiency of presently available cutaneous gas sensors is that
they take a long time to come into thermal equilibrium with a patient
after first being applied thereto. When, for example, a sensor at a
temperature of 42.degree. C. is first applied to a patient whose body is
at a temperature of 37.degree. C., the temperature of the part of the
patient to which the sensor is applied (the measurement site) will
initially remain below the temperature at which useful data can be taken.
This condition will continue until enough additional power can be applied
to the heater to raise the temperature of the measurement site to
42.degree. C. Since the amount of power supplied to the sensor depends
upon the difference between the actual and desired temperature of the
measurement site, the power supplied to the heater is gradually reduced as
the temperature of the measurement site approaches 42.degree. C. As a
result, the temperature of the measurement site tends to approach the
desired value in an asymptotic manner. During this asymptotic approach, 30
minutes or more may pass between the time that the sensor is first applied
and the time that useful data can be produced.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided improved
temperature control circuitry which does not exhibit either of the
above-described deficiencies.
In accordance with one important feature of the present invention there is
provided a closed loop temperature control system which includes a
plurality of independently operating circuits for protecting a patient
from being burned by gas sensors which have begun to operate at excessive
temperatures. In the preferred embodiment, the temperature control system
includes both hard-wired and programmable temperature monitoring circuits,
either of which can shut off the flow of power to the sensor when the
temperature of the sensor takes on an unacceptable value. In addition, the
temperature control system includes circuitry for monitoring the operation
of the programmable temperature monitoring circuit and for shutting off
the flow of heating power to the sensor when it determines that the latter
circuit has or may have malfunctioned. Together, these circuits provide a
patient with in-depth protection against burns resulting from circuit
malfunctions or other out of limit conditions.
In accordance with another important feature of the present invention, the
temperature control system includes circuitry for raising the setpoint
temperature of the system above its normal value, for a predetermined
time, each time that the sensor is first applied to a patient. By
increasing the rate at which power is supplied to the sensor, this
circuitry causes the temperature of the measurement site to rise to the
desired value in a non-asymptotic manner. As a result, the sensor is able
to provide useful data within a fraction of the time required by
previously used temperature control systems.
In accordance with still another important feature of the present
invention, the temperature control system of the invention is so designed
that, without jeopardizing the safety of the patient, the temperature
monitoring circuits are prevented or inhibited from shutting down the
system during normal transients of the type which occur as a result of the
start up of the system or as a result of changes in the temperature
setpoint. During the time that this inhibited condition exists, the system
retains the ability to protect the patient against unsafe sensor
temperatures. Thus, the system is able to combine flexible temperature
control with a high level of patient protection.
DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present invention will be apparent from
the following description and drawings in which:
FIG. 1 is a combined block-schematic diagram of the preferred embodiment of
the temperature control system of the present invention,
FIG. 1a shows, in simplified form, the internal arrangement of the
temperature sensing elements of a cutaneous gas sensor of a type which is
known in the art,
FIG. 1b shows, in simplified form, the internal arrangement of the
temperature sensing elements of a cutaneous gas sensor of a type which is
suitable for use with the present invention, and
FIGS. 2a, 2b and 2c together comprise a flow chart which depicts the
operation of the computer-controlled portion of the temperature control
system shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1a, there is shown a simplified representation of a
cutaneous gas sensor 10 of a type which is known in the art. In this
sensor, the gas sensing electrodes and their associated components have
been omitted for the sake of clarity. Sensor 10 has an inner surface 12
which is applied to the patient at the measurement site and an outer
surface 14 which is exposed to the ambient air. Included within sensor 10
is a heating element 16 and first and second temperature sensing elements
18 and 20 which may comprise thermistors.
In operation, first sensing element 18 is used to sense the temperature of
inner surface 12 (and therefore the temperature of the patient) and cause
an external control circuit to vary the current through heating element 16
as necessary to maintain inner surface 12 (and the patient) at the desired
setpoint temperature. Simultaneously, second sensing element 20 is used to
sense the temperature of outer surface 14. By sensing both the
sensor-patient temperature and the ambient temperature, some known
temperature control circuits attempt to determine the relative amounts of
heat that are lost to the patient and to the ambient air. Since the amount
of heat that is lost to the patient is dependent upon the rate of blood
flow at the measurement site, these circuits are thought to provide an
indirect measure of the rate of blood flow in the patient's skin.
Referring to FIG. 1b, there is shown a simplified representation of a
cutaneous gas sensor 10' of a type which is suitable for use with the
present invention. Like sensor 10 of FIG. 1a, sensor 10' of FIG. 1b
includes a heating element 16 and first and second temperature sensing
thermistors 18 and 22. Unlike sensor 10, however, sensor 10' includes
temperature sensing thermistors which are positioned so that they both
measure the temperature at inner surface 12 of sensor 10'. Because
thermistors 18 and 22 both sense the same temperature, one of them (18)
can be used as a control thermistor to control the amount of current flow
through heating element 16, while the other (22) is used as a monitoring
thermistor to detect the presence of unacceptable sensor temperatures. It
will therefore be seen that, while the gas sensors shown in FIGS. 1a and
1b both include two temperature sensing elements, these sensing elements
have different positions and functions.
Referring to FIG. 1, there is shown a block-schematic diagram of the
preferred embodiment of the temperature control system of the present
invention. In this diagram, heating element 16 and temperature sensing
thermistors 18 and 22 of FIG. 1b have been drawn in schematic form to show
their connections to the circuitry with which they operate. Heating
element 16 is connected between a suitable d.c. voltage source V1 and
system ground G through a switching transistor 26, a variable conducting
transistor 28 and a resistor 30. Thermistors 18 and 22 are connected
between a second d.c. voltage source V2 and system ground G via the
virtual grounds at the inputs of operational amplifiers 32 and 34,
respectively.
To the end that the conduction of transistor 28 may be controlled so that
the actual temperature of sensor 10' remains substantially equal to the
desired setpoint temperature, the circuit of FIG. 1 is provided with a
closed loop temperature control circuit 35. This control circuit
preferably includes control thermistor 18, amplifier 34, an error
amplifier 36, heating element 16, transistor 28 and resistors 30 and 31.
Circuit 35 may also include a current limiting circuit, here shown as the
circuit including resistor 30 and a transistor 37, for limiting the
magnitude of the current flow through heating element 16.
As shown in FIG. 1, amplifier 36 has a first input 36a which is connected
to amplifier 34 to receive the actual temperature signal produced by
control thermistor 18, an input 36b which is connected to receive a signal
indicative of the desired setpoint temperature, and an output 36c which is
connected to transistor 28. In operation, amplifier 36 generates an output
signal which varies the flow of current through transistor 28 in
accordance with the difference between the actual temperature signal at
input 36a and the the desired temperature signal at input 36b, and thereby
maintains the difference between those temperatures approximately equal to
zero. The latter difference may be made even more nearly equal to zero by
utilizing an error amplifier which includes integrating circuitry that
causes amplifier 36 to control transistor 28 in accordance with the
integral of the difference between the actual and desired temperature of
sensor 10'. Because the operation of error amplifiers in closed loop
temperature control circuits is well known, the operation of amplifier 36
and circuit 35 will not be described in detail herein.
The temperature at which circuit 35 operates is determined by a temperature
setpoint signal that is generated by a digital microcomputer 40, and
applied to amplifier input 36b through a storage device, which here takes
the form of a suitable multi-bit latch 42, and a digital to analog
converter or DAC 44. Because the signal is applied in this way, computer
40 is able to change the temperature setpoint of temperature control 35 by
merely outputting a new digital signal to latch 42. Once this signal is
stored in latch 42, however, computer 40 plays no continuing role in the
operation of circuit 35. In other words, computer 40 sets the operating
temperature for circuit 35, but is not otherwise necessary to maintain the
operation thereof. The use of computer 40 as a temperature setting device
for temperature control circuit 35 allows circuit 35 to continue to
operate in the intended manner to maintain sensor 10' at the desired
temperature, even if computer 40 itself should cease functioning.
Computer 40 may comprise a conventional single board microcomputer which
includes a central processing unit or CPU 46, a program memory 48, part or
all of which may be embodied in read-only memory (ROM), a random access
read/write memory (RAM) 50 and a suitable data/address bus 52. Computer 40
also includes a plurality of input/output (I/O) ports through which it may
communicate with an operator or with its associated circuitry. These I/O
ports include: (a) a port 54 which is connected to a set of manually
operable switches, such as a keyboard 56, through which the operator may
enter data or commands into the computer, (b) a port 58 which is connected
to a suitable human readable display 60, such as a CRT or printer, through
which the computer may communicate with the operator, and (c) ports 62-70
through which the computer may communicate with the remaining circuitry of
FIG. 1. Additional ports may be provided for the reception of gas
concentration data from sensor 10' or for the handling of signals from
supporting circuits such as alarm circuits. Because these additional ports
and circuits do not relate to the present invention, they are omitted for
the sake of clarity.
As explained earlier, a patient may become severely burned if a failure in
the temperature control circuitry causes the sensor to operate at a
temperature which is not within safe operating limits. In order to prevent
this from occurring, the circuit of FIG. 1 is provided with a switching
device, here shown as a transistor 26, and with a plurality of protective
circuits which are arranged to control the conductive state thereof. When
the temperature control system is operating in the intended manner, these
protective circuits allow transistor 26 to conduct. Under this condition
the magnitude of the current through heater 16 is controlled by
temperature control circuit 35. When, however, the system malfunctions or
otherwise establishes an out of limits condition, one or more of these
protective circuits will cause transistor 26 to turn off, thereby shutting
off the flow of current through heater 16 without regard to the condition
of temperature control circuit 35.
In the embodiment of FIG. 1, the temperature control system includes three
different protective circuits. These include a first temperature
monitoring circuit 72 which preferably utilizes hardwired circuitry, a
second temperature monitoring circuit which preferably utilizes
programmable circuitry such as computer 40, and a failure detecting
circuit 73 which preferably utilizes hardwired circuitry. Of these, the
first two protective circuits 72 and 40 carry out one or more tests to
determine whether sensor 10' has malfunctioned or otherwise produced a
temperature which is not within safe limits. The third protective circuit
73 carries out a test that identifies malfunctions in computer 40. These
protective circuits are afforded joint control over the state of switching
transistor 26 by connecting the same to transistor 26 via a NAND gate 74.
The latter gate turns transistor 26 off when any of the protective
circuits applies a low state voltage (i.e., a disable signal) to one or
both of its inputs 74a and 74b.
NAND gate 74 may, if desired, be replaced by an equivalent multi-input
analog circuit. Such a replacement may be desirable, in circuits in which
voltage V1 is supplied by a battery, in order to allow voltage V1 to be
made higher than that used with the remaining circuitry of FIG. 1. This
higher voltage, in turn, allows the sensor to be operated for longer times
between battery charges.
In the preferred embodiment, hardwired temperature monitoring circuit 72
turns off transistor 26 by applying a low state disable signal to gate 74
when the amplified output signal of one or both of thermistors 18 or 22
exceeds an overtemperature reference signal V.sub.R. In addition,
hardwired failure detecting circuit 73 turns off transistor 26 by applying
a low state disable signal to gate 74, when computer 40 fails in such a
way that it becomes unable to periodically apply a reset signal thereto.
Since these conditions can occur either as a result of the failure of
individual circuit components, such as amplifier 36, or as a result of the
failure of computer 40 to properly execute its program, it will be seen
that hardwired protective circuits 72 and 73 protect the patient against
both hardware and software related malfunctions.
Programmable temperature monitoring circuit 40 turns off transistor 26, via
gate 74 and a multi-bit addressable latch 78, when it detects the
occurrence of any one or more of a plurality of unacceptable temperature
conditions. In the preferred embodiment, these conditions are identified
on the basis of the outcome of the following software implemented
temperature tests: (i) do the temperatures indicated by the thermistors
differ from one another by more than a predetermined amount?, (ii) does
the average of the temperatures indicated by the thermistors differ from
the setpoint temperature by more than a predetermined amount?, and (iii)
does the temperature indicated by either thermistor exceed the maximum
permissible sensor temperature? Since these temperature tests can be
failed either as a result of the failure of individual circuit components,
or as a result of the failure of computer 40 to properly execute its
program, it will be seen that programmable monitoring circuit 40 also
protects the patient against both hardware and software related
malfunctions.
In order to accomplish its protective function, temperature monitoring
circuit 72 is provided with inputs 72a and 72b which are connected to
receive the amplified output signals of thermistors 18 and 22,
respectively; an input 72c which is connected to receive suitable
overtemperature reference signal V.sub.R ; an input 72d which is connected
to receive a reset signal; and an output 72e which is connected to gate
74. In the embodiment of FIG. 1, circuit 72 includes a comparator network
that comprises first and second comparators 80 and 82, a pull-up resistor
84, and an RC timing circuit that includes a resistor 86, a capacitor 88,
and a diode 90. These devices are supplied with operating power from a
suitable DC voltage source +V.
In operation, comparators 80 and 82 and pull-up resistor 84 cooperate to
establish at a junction 92 a high state voltage which is approximately
equal to voltage +V when the output signals of thermistors 18 and 22 are
both less than reference signal V.sub.R, and to establish a low state
voltage which is approximately equal to that of system ground G when
either or both of the thermistor output signals exceed reference signal
V.sub.R. Thus, the presence of a high state voltage between junction 92
and ground G indicates a normal temperature condition at sensor 10', and
the presence of a low state voltage therebetween indicates an
overtemperature condition at sensor 10'.
The RC timing circuit including elements 86-90 establishes at a junction 94
a voltage which follows that at junction 92, but which undergoes
high-to-low and low-to-high state voltage transitions which proceed at
different rates. More particularly, resistor 86 and capacitor 88 serve to
briefly delay the appearance of high-to-low state voltage transitions at
junction 94. This is because such transitions will occur only after
capacitor 88 discharges through resistor 86 for a long enough time. This
brief delay is beneficial since it assures that transistor 26 is not shut
off as a result of line voltage transients or induced voltage spikes.
Diode 90 effectively short-circuits resistor 86 and allows capacitor 88 to
charge rapidly in the presence of low-to-high state voltage transitions.
This, in turn, assures that the latter transitions appear at junction 94
without any appreciable delay, and thereby further reduce the effect of
line voltage transients or induced voltage spikes. Thus, the RC timing
circuit assures that high-to-low state voltage transitions appear at
junction 94 only when a real overtemperature condition occurs at sensor
10'.
In order to assure that transistor 26 is maintained in a non-conducting
condition after the occurrence of a real overtemperature condition,
junction 94 is connected to gate 74 through a suitable latch, here shown
as a flip-flop 96. This flip-flop is set by a high-to-low state voltage
transition at its set input S to produce a low-state voltage at the Q
output thereof. This low state (disable) voltage, in turn, is applied to
NAND gate 74, through a negative OR gate 98, to cause the output of gate
74 to assume its high state and thereby maintain transistor 26 in a
non-conducting condition. Once established, this condition will persist
unless and until a reset signal is applied to reset input R of flip-flop
96. (The condition under which such a reset signal is applied to flip-flop
96 will be discussed later.) Thus, monitoring circuit 72 affords latched
overtemperature protection to the patient.
Temperature monitoring circuit 72 also includes a conductor 99 which
connects junction 94 directly to negative OR gate 98. Low state signals
applied to this conductor serve to turn-off transistor 26, via gates 98
and 74, only so long as they appear at junction 94. As a result, conductor
99 assures that circuit 72 is able to protect the patient against an
overtemperature condition independently of the occurrence of any circuit
conditions (such as start-up transients or circuit malfunctions) which may
cause flip-flop 96 to be reset. Thus, control circuit 72 affords unlatched
overtemperature protection to the patient.
In order to accomplish its protective function, failure detecting circuit
73 is provided with a multi-bit input 73a which is connected to output
port 68 of computer 40, an input 73b which is connected to the reset (RST)
output of CPU 46 through a conductor 102, and an output 73c which is
connected to NAND gate 74 through OR gate 104 and addressable multi-bit
latch 78. In the embodiment of FIG. 1, circuit 73 includes an address
decoder 106, a retriggerable monostable multivibrator 108, a D-type
flip-flop 110 and an inverter 112.
In operation, address decoder 106 serves to apply a low-state voltage to
the input of multivibrator 108 during those times when a predetermined
address appears on data-address bus 52. Provided that these times occur at
intervals that are shorter than the time necessary for multivibrator 108
to reset itself, multivibrator 108 will remain set and thereby apply a
continuous low-state voltage to the clock (CK) input of flip-flop 110. By
programming computer 40 so that it generates the above address frequently
enough, the presence of a continuous low-state voltage at the Q output of
multivibrator 108 may be used as an indication that computer 40 is
executing its program in the intended manner. Conversely, the appearance
of a high state voltage at the Q output of multivibrator 108 may be used
as an indication that computer 40 is not executing its program in the
intended manner.
If computer 40 is operating normally, multivibrator 108 will be unable to
apply to flip-flop 110 the low-to-high state voltage transition that is
necessary to change to the state thereof. As a result, the Q output of
flip-flop 110 remains in the high state which it was forced to assume by a
reset signal applied thereto by CPU 46, via conductor 102, at the time of
start up. As long as this condition exists, inverter 112 will apply a
continuous low-state voltage to OR gate 104, via conductor 114, and to the
non-maskable interrupt (NMI) of CPU 46, via conductor 116. Since this low
state voltage cannot cause latch 78 to apply a low-state voltage to gate
74, and cannot cause an interrupt of CPU 46, the temperature control
system will maintain conduction in transistor 26, unless the latter is
turned off by one of the other protective circuits, such as circuit 72.
If, on the other hand, computer 40 fails to operate normally, it will be
unable to apply to multivibrator 108 the signals necessary to prevent the
latter from changing the state of flip-flop 110. As a result, flip-flop
110 will apply a low-state voltage to inverter 112. The resulting
high-state voltage at the output of inverter 112 will, in turn, cause OR
gate 104 to reset latch 78 and thereby turn off transistor 26 by applying
a disable signal to input 74b of gate 74. It will be understood, however,
that it is not essential that flip-flop 110 turn off transistor 26 by
resetting latch 78. It may, for example, turn off transistor 26 by
applying a disable signal to an additional flip-flop having an output that
is connected to one input of a three-input version of gate 74.
The high-state voltage at the output of inverter 112 will also activate the
NMI input of CPU 46, via conductor 116. In the preferred embodiment, the
activation of this input will direct the computer 40 to a part of its
program that causes it to take action to protect the patient. This action
preferably includes the outputting to DAC 44 of a reduced temperature
setpoint signal, i.e., a setpoint signal which will cause amplifier 36 to
shut off transistor 28, and the outputting of a signal which will notify
the operator of a failure. (As will be explained more fully later, this
reduced setpoint signal is also outputted to DAC 44 in response to the
occurrence of any of a number of other unacceptable temperature
conditions.) The shutoff of transistor 28 provides a further level of
patient protection by assuring that no current will flow through heater 16
even if transistor 26 and/or gate 74 should malfunction.
From the foregoing, it will be seen that protective circuits 72 and 73 are
adapted to shut-off the flow of heater current, in response to the
occurrence of any one or more of a plurality of malfunctions in either the
hardware or software controlled portions of the temperature control system
of FIG. 1, independently of the operability of computer 40.
While protective circuits 72 and 73 are preferably implemented with the
discrete, hardwired circuit elements shown in FIG. 1, they may also be
implemented with program controlled circuitry, i.e., additional programmed
microcomputers. If the latter are used, however, they should be connected
and programmed so that, like their hardwired counterparts, they can
continue to perform their protective function independently of the proper
functioning of computer 40. Their data/address buses should not, for
example, be directly connected to the data/address bus of computer 40. In
such usage, these additional microcomputers comprise program controlled
equivalents of hardwired circuits 72 and 73. Because of this equivalency,
embodiments which utilize such additional microcomputers will not be
described in detail herein.
As explained earlier, computer 40 serves as a second temperature monitoring
circuit by executing a program which performs the three above-mentioned
temperature tests. The program which computer 40 uses to carry out these
tests will be described later in connection with the flow chart of FIG.
2a. The manner in which computer 40 receives the information necessary to
apply these tests, and the manner in which computer implements the results
of these tests will now be described in connection with the circuit of
FIG. 1.
In the embodiment of FIG. 1, computer 40 receives the output signals of
thermistors 18 and 22 through a suitable multiplexer 120 and an analog to
digital converter 122. Multiplexer 120 may also be connected to the output
of DAC 44, through a conductor 124, to enable computer 40 to read the
temperature setpoint signal which is actually being applied to error
amplifier 36. Reading the latter voltage enables the computer to
automatically calibrate DAC 44, as will be explained more fully presently.
In order to assure that the multiplexed temperature information is
provided to computer 40 at the proper times, computer 40 controls the
operation of multiplexer 120 and analog to digital converter 122, via port
62 and a set of conductors 126. Because the operation of multiplexers and
analog to digital converters are well known to those skilled in the art,
the operation thereof will not be described in detail herein.
If, after processing the thermistor output signals, computer 40 determines
that sensor 10' is not functioning within safe limits, it will take action
to protect the patient. A first part of this protective action includes
applying to multi-bit addressable latch 78, over a set of conductors 128,
a digital signal which will cause output Q.sub.2 thereof to apply a
latched low-state voltage to input 74b of gate 74. The effect of this
action is to shut off transistor 26 and hold the same in a non-conducting
state. A second part of this protective action includes applying to latch
42 a digital signal which will cause DAC 44 to apply a predetermined
reduced temperature setpoint signal (such as one that produces a
temperature of 37.degree. C.) to summing amplifier 36. The effect of this
action is to partly or completely shut off the flow of current through
transistor 28. Thus, the computer is able to limit or stop the flow of
current through the devices connected to both ends of heater 16, and
thereby protect the patient even if one of transistors 26 or 28 should
fail.
Because of the number and sensitivity of the tests that are used to
identify unacceptable operating conditions, the circuitry of FIG. 1 may
exhibit a tendency to shut off the flow of current through heater 16
during the course of normal temperature transients. When, for example, the
system is first turned on, temperature transients may cause the circuit of
FIG. 1 to become latched into a condition in which transistor 26 is
maintained in a non-conducting state, even though the patient is not in
any danger of being burned. In order to prevent this, computer 40 is given
the ability, at the time of turn on, to reset latch 96 and to temporarily
disable the above-mentioned software implemented temperature tests. Even
when it does so, however, the patient is protected by other parts of the
circuitry of FIG. 1. Thus, the temperatur | | |
