INLINE HEATER OVERHEATING SYSTEM AND METHOD

Abstract

An inline heating system including an inline heater including a heater element; a control unit configured to cause one of voltage or current to be applied to power the inline heater; and a current or voltage meter positioned and arranged to measure the other of current or voltage at the heater element due to the applied voltage or current, wherein the control unit is further configured to determine a heater element resistance using the applied voltage or current and the measured current or voltage as part of a no flow or low flow condition detection algorithm implemented by the control unit, wherein the voltage or current applied to power the inline heater is stopped if the no flow or low flow condition is detected.

Claims

1. An inline heating system comprising: an inline heater including a heater element; a control unit configured to cause one of voltage or current to be applied to power the inline heater; and at least one of a current or voltage meter outputting to the control unit and arranged to measure at least one of the current or voltage at the heater element due to the applied voltage or current, wherein the control unit is further configured to determine a heater element resistance using the applied voltage or current and the measured at least one of the current or voltage as part of a no flow or low flow condition detection algorithm implemented by the control unit, wherein the voltage or current applied to power the inline heater is stopped if the no flow or low flow condition is detected.

2. The inline heating system of claim 1, wherein the control unit includes the at least one of the current or voltage meter.

3. The inline heating system of claim 1, wherein the control unit is configured to determine multiple heater element resistances using the applied voltage or current and the measured at least one current or voltage at different times over a sample period to determine if the no flow or low flow condition is detected.

4. The inline heating system of claim 1, wherein the heater element is a first heater element, and which includes at least one additional heater element, wherein the control unit is configured to determine a heater element resistance for each heater element as part of the no flow or low flow condition detection algorithm.

5. The inline heating system of claim 4, wherein the at least one current or voltage meter is a first current or voltage meter, and which includes at least one additional current or voltage meter arranged to measure the at least one current or voltage at the at least one additional heater element.

6. The inline heating system of claim 4, wherein the control unit is configured to stop the voltage or current applied to power the inline heater if the no flow or low flow condition is detected at any of the heater elements.

7. The inline heating system of claim 1, wherein the control unit is configured to compare a rate of change in determined heater element resistance to a set maximum rate of change in determined heater element resistance as part of the low flow condition detection algorithm implemented by the control unit.

8. The inline heating system of claim 7, wherein the rate of change in determined heater element resistance is a positive rate of change.

9. The inline heating system of claim 1, wherein the control unit is configured to compare the determined heater element resistance to a calibrated heater element resistance as part of the low flow condition detection algorithm implemented by the control unit.

10. The inline heating system of claim 9, wherein the calibrated heater element resistance is determined remote from the inline heating system and then stored in the control unit.

11. The inline heating system of claim 9, wherein the calibrated heater element resistance is specific to at least one of (i) fluid flowrate through the inline heater or (ii) input power to the inline heater.

12. The inline heating system of claim 1, wherein the control unit is configured to measure the at least one of the current or voltage at a sample rate that is of at least one of (i) 100 Hz or greater or (ii) at least two times greater than a frequency of a pulsed voltage source powering the inline heater.

13. The inline heating system of claim 1, wherein the control unit includes an analog heater protection circuit configured to filter the measured at least one of the current or voltage.

14. An inline heating system comprising: an inline heater including a heater element; a voltage source or a current source positioned to apply a voltage or current to power the inline heater; and an analog heater protection circuit configured to (i) measure at least one of the current or voltage at the heater element due to the applied voltage or current, (ii) determine a heater element resistance using the applied voltage or current and the measured at least one of the current or voltage, and (iii) provide a stop power signal or disable an enable signal if the inline heater element resistance indicates a no flow or low flow condition.

15. The inline heating system of claim 14, wherein the analog heater protection circuit includes at least one of: measurement circuitry, divider circuitry, a differential circuit, a comparator, or a lowpass/timer circuit.

16. An inline heating system comprising: an inline heater including a heater element; and a control unit configured to compare a rate of change in heater element resistance to a set maximum rate of change in heater element resistance as part of a no flow or low flow condition detection algorithm implemented by the control unit, wherein power applied to the inline heater is stopped if the no flow or low flow condition is detected.

17. The inline heating system of claim 16, wherein the control unit is further configured to compare at least one determined heater element resistance to at least one calibrated heater element resistance as part of the low flow condition detection algorithm implemented by the control unit.

18. The inline heating system of claim 17, wherein the at least one calibrated heater element resistance is determined remote from the inline heating system and then stored in the control unit.

19. The inline heating system of claim 17, wherein the at least one calibrated heater element resistance is specific to at least one of (i) fluid flowrate through the inline heater or (ii) input power to the inline heater.

20. The inline heating system of claim 16, which is provided as part of a peritoneal dialysis machine, hemodialysis machine, hemofiltration machine, hemodiafiltration machine, continuous renal replacement therapy machine, water purification unit, dialysis fluid preparation unit, or a blood warmer.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0048] FIG. 1 is a schematic flow diagram illustrating an inline heater in combination with a pump, pressure sensor and temperature sensor, which may be used in many different types of dialysis modalities, such as peritoneal dialysis (“PD”), hemodialysis (“HD”) machine, hemofiltration (“HF”), hemodiafiltration (“HDF”), continuous renal replacement therapy (“CRRT”) and blood warming.

[0049] FIG. 2 is a sectioned view of the inline heater of FIG. 1 illustrating multiple heater elements and sensing components used for the low or no flow detection of the present disclosure.

[0050] FIG. 3 is a schematic block diagram illustrating an alternative embodiment for the system of the present disclosure, which uses an analog heater protection circuit.

[0051] FIG. 4 is a collection of graphs correlating inline heater power input, dialysis fluid flowrate and heater element resistance over a common time period.

DETAILED DESCRIPTION

[0052] Referring now to the drawings and in particular to FIG. 1, an inline heating system 10 is illustrated. Inline heating system 10 may be used in many medical fluid applications including but not limited to peritoneal dialysis (“PD”), hemodialysis (“HD”) machine, hemofiltration (“HF”), hemodiafiltration (“HDF”), continuous renal replacement therapy (“CRRT”) and blood warming. While dialysis is a primary focus for inline heating system 10, the system is not limited to same. For example inline heating system 10 may be employed in a water purification unit that prepares purified water for the creation of dialysis fluid, and which heats the water either for downstream use or for disinfection. Inline heating system 10 may be employed alternatively in a dialysis fluid preparation unit that prepares dialysis fluid online, and which heats the dialysis fluid for downstream use or for disinfection.

[0053] Inline heating system 10 includes a fluid source 12s, which may be a PD fluid source, HD fluid source, replacement fluid source (HF, HDF, CRRT), or water source for example. Inline heating system 10 includes a fluid destination 12d, which may be the patient's peritoneal cavity (PD), a dialyzer (HD, HDF), a blood line (HF, HDF, CRRT), a dialysis fluid preparation unit if inline heating system 10 is employed in or provided by a water purification unit, or a dialysis machine (e.g., PD cycler) if dialysis inline heating system 10 is employed in or provided by a dialysis fluid preparation unit. Fluid source 12s and fluid destination 12d may alternatively both be the patient when inline heating system 10 is provided as part of a blood warmer.

[0054] Fluid from source 12s (which will hereafter be termed dialysis fluid even though the fluid is not limited to same as discussed above) is pumped along a fluid line 14 via a pump 16. Pump 16 may be any type of fluid pump, e.g., a durable (reusable) pump that contacts the dialysis fluid directly, such as a piston, gear, centrifugal or rotary vane pump. Pump 16 may alternatively have a disposable component, such as a pneumatic pump operating with a disposable cassette, an electromechanical pump operating with a disposable cassette, or a peristaltic pump operating with a peristaltic tubing segment.

[0055] It is contemplated that there be many different components located along fluid line 14 between fluid source 12s and fluid destination 12d, such as one or more valve, air trap and various sensors. For ease of illustration inline heating system 10 is shown having a pressure sensor 18p and a temperature sensor 18t. The output of pressure sensor 18p may be used as feedback for controlling pump 16 so as not to exceed a positive or negative pressure limit. The output of the temperature sensor 18t may be used as feedback for controlling the input power to inline heater 20. Temperature sensor 18t is located downstream from inline heater 20 so as to sense the temperature of the dialysis fluid exiting the heater. If desired, an additional temperature sensor may be located upstream of inline heater 20 to provide feedforward information concerning the temperature of dialysis fluid entering inline heater 20.

[0056] FIG. 1 further illustrates that inline heating system 10 of the present disclosure includes a control unit 50, which may be the control unit of a dialysis machine (e.g., PD cycler), water purification unit or a dialysis fluid preparation unit. Control unit 50 in the illustrated embodiment includes one or more processor 52, one or more memory 54 and a video controller 58. Control unit 50 receives, stores and processes signals or outputs from pressure sensor 18p, temperature sensor 18t and other sensors provided by the machine or unit, such as a conductivity sensor (not illustrated). Control unit 50 uses pressure feedback from pressure sensor 18p to control pump 16 to pump dialysis fluid at a desired pressure or within a safe pressure limit (e.g., within 0.21 bar (three psig) of positive pressure to a patient's peritoneal cavity). Control unit 50 uses temperature feedback from temperature sensor 18t to control inline heater 20 to heat the dialysis fluid to a desired temperature, e.g., body temperature or 37° C.

[0057] Video controller 58 of control unit 50 interfaces with a user interface 60 of the machine or unit, which may include a display screen operating with a touchscreen and/or one or more electromechanical button, such as a membrane switch. User interface 60 may also include one or more speaker for outputting alarms, alerts and/or voice guidance commands. User interface 60 may be provided with the machine or unit as illustrated in FIG. 1 and/or be a remote user interface operating with control unit 50. Control unit 50 may also include a transceiver (not illustrated) and a wired or wireless connection to a network, e.g., the internet, for sending treatment data to and receiving prescription instructions from a doctor's or clinician's server interfacing with a doctor's or clinician's computer.

[0058] Referring now to FIG. 2, inline heater 20 is illustrated in more detail. In the illustrated embodiment, inline heater 20 includes a housing 22 that surrounds a fluid heating pathway 24. Pathway 24 is illustrated as being straight for ease of illustration but may have alternative shapes, such as a coiled or serpentine shape. Also, while inline heater 20 is illustrated as a durable component that does not operate with a disposable unit, inline heater 20 may alternatively operate with a fluid carrying disposable unit, such as a disposable unit defining a coiled or serpentine heating pathway.

[0059] Inline heater 20 in the illustrated embodiment includes a plurality of heater elements 26a, 26b, such as two heater elements. Inline heater 20 may alternatively include only a single heater element or more than two heater elements 26a, 26b. Heater elements 26a, 26b in the illustrated embodiment are located between housing 22 and fluid heating pathway 24, on the inside of housing 22 and on the outside of fluid heating pathway 24. Inline heater 20 may be configured differently however. For example, heater elements 26a, 26b may instead be located along the center of housing 22. Fluid heating pathway 24 here extends around the outside of the centrally located heater elements 26a, 26b, between the heater elements and housing 22, which in any heater embodiment may be thermally insulated. Control unit 50, for any configuration of inline heater 20, controls the power provided to heater elements 26a, 26b. Control unit 50 may do so by controlling either the voltage or current to heater elements 26a, 26b. The more voltage or current supplied, the more power is provided to inline heater 20, and thus more heating of dialysis fluid flowing through inline heater 20 takes place, resulting in a higher dialysis fluid temperature.

[0060] In the illustrated embodiment, control unit 50 controls the voltage from a voltage source 62 to each heater element 26a, 26b. Voltage source 62 may be a 90 to 130 VAC, a 200 to 240 VAC voltage source, or a voltage source supplying direct current voltage to heater element 26a, 26b. A positive voltage source line 64a extends to one end of each heater element 26a, 26b, while a negative voltage source line 64b extends to the other end of each heater element 26a, 26b.

[0061] Control unit 50 in the illustrated embodiment also includes current meters 66a, 66b for heater elements 26a, 26b, respectively. Current meters 66a, 66b are connected in series with heater elements 26a, 26b to be able to measure the current applied to each heater element. In the illustrated embodiment, current meters 66a, 66b are located along positive voltage source lines 64a extending to one end of each heater element 26a, 26b. In an alternative embodiment shown in dashed line, a single current meter 66 may be used to read the current flowing through multiple heater elements 26a and 26b, wherein control unit 50 may index or switch through readings from each heater element in a cyclical manner. Control unit 50 may alternatively operate with single current meter 66 so as to read both currents (through lines 64a and 64b) along the common line having single sensor 66. Here, it is not required to cycle sensor 66 to read the different lines 64a, 64b individually.

[0062] It is contemplated for control unit 50 to instead measure both voltage and current simultaneously to obtain good accuracy for the resistance value R.sub.e of each heater element 26a, 26b. As discussed below in connection with FIG. 3, an analog heater protection circuit may be provided with control unit 50 to detect the rate of change or derivative, discussed herein, in either one or both of current and/or voltage.

[0063] Inline heater 20 of inline heating system 10 is disadvantageous from one standpoint in that if it is attempted to heat dialysis fluid while no dialysis fluid is flowing, inline heater 20 can overheat. A flow switch may be placed ahead of inline heater 20 to make sure that flow is present as a condition for control unit 50 to energize the heater. Flow switches add cost however and can become stuck or otherwise not function properly.

[0064] Inline heating system 10 solves the overheating problem by configuring control unit 50 to determine the resistance R.sub.e of each heater element 26a, 26b of inline heater 26. The resistance R.sub.e of each heater element 26a, 26b increases in a typical manner upon an increase in temperature. The resistance R.sub.e of each heater element 26a, 26b is in one embodiment calculated by knowing an amount of voltage or current applied to each heater element 26a, 26b (in FIG. 2 voltage V.sub.e to the heater elements is controlled by control unit 50) and measuring the other of the current or voltage in each heater element 26a, 26b (in FIG. 2 current I.sub.e along heater elements 26a, 26b is measured by control unit 50) while inline heater 20 is being powered. Heater element resistance R.sub.e is then equal to V.sub.e/I.sub.e. The calculated resistance is applied to an algorithm stored in memory 54 of control unit 50, which is interrogated by processor 52 to determine if a low flow or no flow condition within inline heater 20 exists.

[0065] Referring now to FIG. 3, in an alternative embodiment, system 10 of the present disclosure may employ an analog heater protection circuit 70, which performs some or all of the functionality described above as being performed via software using one or more processor 52 and one or more memory 54. It should be appreciated that control unit 50 as discussed herein also includes analog heater protection circuit 70 and any other heater protection circuitry that may be employed by system 10. Control unit 50 includes all supervisory control side and protective side hardware and software and all lower level hardware (including heater protection circuit 70) and software.

[0066] As illustrated in FIG. 3, analog heater protection circuit 70 may include measurement circuitry 72, which takes measurements V.sub.e and I.sub.e. Circuitry 72 may include a current probe, e.g., an ACS723 current probe, which measures the current I.sub.e. Measurement circuitry 72 may include a suitably sized resistor grid to measure the voltage V.sub.e. Measurement circuitry 72 may also include a low pass filter that filters two voltage signals (one representing current I.sub.e and the other representing voltage V.sub.e). A standard RC lowpass filter, for example, may be used to suppress the most prevalent noise created by the pulsed controller. The two voltage signals (possibly filtered) are fed from measurement circuitry 72 to divider circuitry 74, which includes a divider, e.g., an AD538 divider, creating a new voltage corresponding to heater element resistance R.sub.e.

[0067] The voltage corresponding to heater element resistance R.sub.e is fed from divider circuitry 74 to a differential circuit 76 that creates a second new voltage representing a rate of change of heater element resistance R.sub.e. Differential circuit 76 may be (i) a passive capacitive differential circuit that includes a capacitor and resistor, (ii) a passive inductive differential circuit that includes an inductor and a resistor, or (iii) an active differential circuit, which may also include a capacitor and a resistor as well as an operation amplifier. The second new voltage representing the rate of change of heater element resistance R.sub.e is then delivered from differential circuit 76 to a comparator 78, the output of which may be used to disable or to turn off inline heater 20 if the rate of change of heater element resistance R.sub.e meets or exceeds a threshold rate of change of heater element resistance R.sub.e set at comparator 78. A lowpass/timer circuit 80 may be provided after comparator 78 to remove potential glitches/disturbances. Lowpass/timer circuit 80 ensures that the no/low flow signal does not go high based on noise that may have gotten through to the lowpass filter/timer circuit. Lowpass/timer circuit 80 instead ensures that the signal is stable/triggered for a certain amount of time and that inline heater 20 is not depowered or shut off for small signal spikes or noises that may occur.

[0068] Referring now to FIG. 4, a collection of graphs over a common time period correlating inline heater power input applied to heater elements 26a, 26b, dialysis fluid flowrate through inline heater 20 and heater element resistance R.sub.e is illustrated. The graphs highlight for example that as power to heater elements 26a, 26b increases, e.g., from 100 Watts, to 200 Watts, to 300 Watts, heater element resistance R.sub.e increases in turn from about 9.52 Ohms, to about 9.6 Ohms, to about 9.66 Ohms, respectively, which coincides with an increase in heater element temperature. The graphs also highlight that whenever dialysis fluid or other fluid flowrate is stopped, while input power continues to be applied, heater element resistance R.sub.e increases sharply, which also coincides with a sharp increase in heater element temperature.

[0069] One possible algorithm stored in memory 54 and interrogated by processor 52 sets a maximum heater element resistance increase or resistance derivate (change in heater element resistance over time). With control unit analysing heater element resistance R.sub.e rate change or increase over time, a no or low flow condition through the inline heater 20 is detectable before the temperature of heater elements 26a, 26b increases to a point that could damage inline heater 20. When there is no fluid flow in an energized inline heater 20, the temperature of heater elements 26a, 26 increases rapidly and hence the heater element resistance R.sub.e increases rapidly. Control unit 50 is accordingly programmed to look to determine if a change in heater element resistance R.sub.e over a period or of time (rate of change), e.g., to determine if a sample period rate of change of resistance R.sub.e meets or exceeds a set maximum rate of change of resistance R.sub.e over the period of time, the algorithm triggers control unit 50 to automatically deenergize inline heater 20, stop treatment at the corresponding machine or cycler (or stop preparing purified water or stop preparing dialysis fluid at the corresponding unit), and alarm the patient, caregiver or nurse, e.g., at user interface 60.

[0070] One possible implementation of the overheating algorithm of inline heating system 10 is to run a characterizing program that characterizes inline heater 20 at the time of manufacture and/or servicing of the dialysis machine (or other unit). The characterizing program characterises how heater element resistance R.sub.e relates to input power and flowrate. Here, the inline heater is energized while receiving a known flowrate of fluid. Input power is varied while the flowrate is held constant. Temperature sensor 18t may be used to measure the test fluid temperature downstream form inline heater 20 to confirm that the inline heater is heating the test fluid properly to the commanded temperature. The output of pump 16 is monitored to confirm that the commanded flowrate is met (a weight scale may be used in the test setting). The characterising program enables measured heater element resistances R.sub.e (correlated with heater element temperature) for the particular inline heater 20 tested to be known for a commanded flowrate and fluid temperature, and over a common or use range of input power settings. A resulting calibration data table is then stored in memory 54 of control unit 50 of the dialysis machine (or other unit). Control unit 50 uses the calibration data during treatment to detect when inline heater 20 is overheating, that is, to detect that for a given or commanded flowrate and input power setting to achieve a commanded temperature, whether a measured and determined heater element resistance R.sub.e is greater than it should be for the commanded dialysis fluid temperature.

[0071] The algorithm stored in memory 54 of control unit 50 of the dialysis machine (or other unit) may accordingly analyze two factors (i) rate of change in heater element resistance (derivative) indicating that a no or low flow condition may be beginning and (ii) a comparison between a determined heater element resistance R.sub.e to the calibration resistance for the same input power and commanded flowrate. That is, the rate of change in heater element resistance R.sub.e may be high, indicating that a no or low flow condition may be underway, but if the determined heater element resistance (and corresponding temperature) is lower than what the calibration data says the heater element resistance should be for a given flowrate and power input setting, then the algorithm allows control unit 50 of dialysis machine (or other unit) to continue treatment.

[0072] In an embodiment, the cycle rate for the current or voltage measurement to determine heater element resistance R.sub.e may be very high, and thus the algorithm may be configured to take many measurements, e.g., over the period of a second or more, before making a no or low flow determination, deenergizing inline heater 20, stopping treatment, and alarming the patient, caregiver or nurse. Taking many measurements over a safe time period that is too short for inline heater 20 to overheat significantly helps to prevent false trips.

[0073] Taking many measurements also provides an adequate number of samples to efficiently supress measurement signal noise. The measurement signal for heater element resistance R.sub.e tends to be very noisy due to its environment including a pulsed power source. In one implementation, the high sampling rate was 5 kHz, and a multi- or four step decimation procedure was applied. Each decimation step was performed by applying an 8th order Chebyshev lowpass filter type I and by removing samples to a 10th of a fraction of the original or prior step, e.g., to 500 samples, then 50 samples, etc. There are numerous ways of performing the filtering leading to the heater element resistance R.sub.e graph at the top of FIG. 4. It is likely, however, that successful filtering requires a large sample rate, such as 5 kHz. The sample rate will depend on the frequency of the pulsed voltage source, e.g., if a typical 50/60 Hz AC voltage source, a sample rate of at least two times as fast, e.g., 100 to 120 Hz is likely required. However, the sample rate may desirably be at least ten times higher, e.g., 500 to 600 Hz.

[0074] It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. It is therefore intended that any or all of such changes and modifications may be covered by the appended claims. For example, while inline heating system 10 is shown interrogating two heater elements 26a, 26b (e.g., for redundancy where a rise in resistance at either element can trip a no or low flow outcome), the inline heating systems of the present disclosure may instead interrogate only a single or less than all of the heater elements of inline heater 20.