WO2022023751A1

WO2022023751A1 - A temperature control system

Info

Publication number: WO2022023751A1
Application number: PCT/GB2021/051955
Authority: WO
Inventors: Naigin KARIATT; Suchith Chandrakumar MENON; Hashid Valiyakath Ummar FAROOK; Vaishali SHARATH
Original assignee: Micromass UK Ltd
Current assignee: Micromass UK Ltd
Priority date: 2020-07-29
Filing date: 2021-07-29
Publication date: 2022-02-03
Anticipated expiration: 2023-01-29

Abstract

A temperature control system for an ion source, the ion source comprising a heater including a gas source and a heating element, the heater for directing heated gas onto a capillary insertable into the ion source, the control system configured to: measure the temperature of the heating element; compare the measured temperature of the heating element to a predetermined target temperature; and initiate a cooling operation by directing gas over the heating element, to reduce the temperature of the heating element, if the measured temperature is greater than the predetermined target temperature.

Description

Title: A temperature control system Description of Invention

The present invention relates to a temperature control system for an ion source, and to a control system for a heating element. Background of the invention

The invention generally relates to an atmospheric solids analysis probe (ASAP). Such probes and the associated instrument for use with ASAP, are provided by several manufacturers, including Waters Corporation, Milford, MA, U.S.A.

ASAP is a useful and relatively cheap tool for use in the direct analysis of volatile and semi-volatile, solid and liquid samples and may be used in the analysis of speciality chemicals, synthetic polymers, energy sources and food.

A sample is introduced into an ion source housing (e.g. an API source), in which the sample is volatilised using a heated gas, such as nitrogen, and the sample is then ionised using, for example, a corona discharge pin. The ionised sample may subsequently be analysed in a mass spectrometer.

The sample is introduced into the source by loading it onto the tip of a capillary. The capillary may comprise a conventional glass capillary. The capillary may be a solid rod, or a tube, with open ends. To assist in the loading of a capillary into a source, it is known to provide a holder comprising a clamp mechanism which serves to retain the proximal end of the capillary (opposite the tip at the distal end which carries a sample) in the capillary holder. This may provide a user with a more robust method of handling the capillary, and may also assist in the guiding of the capillary into the source. The capillary holder, and/or the source instrument, may comprise a guide mechanism to ensure the correct alignment of the capillary as it is loaded into the source.

When the capillary is arranged in the ion source housing, the distal end of the capillary is arranged adjacent the outlet of a nozzle for directing heated gas onto the capillary.

The nozzle contains a heating element, which is used to heat the gas passing through the nozzle from a gas source, over the heating element.

In use, there may a requirement for the temperature of the heated gas exiting the nozzle to be reduced. In such instances, a user may need to wait until the temperature of the heating element has reduced sufficiently through convection with the surrounding air. This may increase the cycle time of the instrument. Furthermore, in use, a user may wish to open the door to the chamber of the ion source, to access components therein. As they do so, there is a risk that the user may inadvertently contact the hot end of the nozzle, potentially burning their hand. Still further, there is a desire for an improved algorithm to control the temperature of the heating element.

The present invention seeks to address at least one of the aforementioned problems. Accordingly, the present invention provides a temperature control system for an ion source, the ion source comprising a heater including a gas source and a heating element, the heater for directing heated gas onto a capillary insertable into the ion source, the control system configured to: measure the temperature of the heating element; compare the measured temperature of the heating element to a predetermined target temperature; and initiate a cooling operation by directing gas over the heating element, to reduce the temperature of the heating element, if the measured temperature is greater than the predetermined target temperature.

In at least one embodiment, the temperature control system is configured to detect whether power is being supplied to the heating element, and only initiate the cooling operation if no power is detected being supplied to the heating element.

In at least one embodiment, the temperature control system is configured to initiate the cooling operation only if the measured temperature is greater than the predetermined target temperature by a predetermined minimum difference.

In at least one embodiment, the temperature control system is further configured to: determine if a door to a chamber of the ion source is closed; and only initiate the cooling operation if the door is determined to be closed.

In at least one embodiment, the temperature control system is further configured to: determine if a door to a chamber of the ion source is open; and terminate said cooling operation after the door has been detected to be open for at least a predetermined period of time. In at least one embodiment, the temperature control system is configured to terminate said cooling operation after a predetermined amount of time has elapsed.

The present invention provides a temperature control system for an ion source, the ion source comprising a heater including a gas source and a heating element, the heater for directing heated gas onto a capillary insertable into the ion source, the control system configured to: measure the temperature of the heating element; compare the measured temperature of the heating element to a predetermined target temperature; and initiate a cooling operation by directing gas over the heating element, to reduce the temperature of the heating element, if: the measured temperature is greater than the predetermined target temperature; the measured temperature is greater than a predetermined minimum temperature; no power is being supplied to the heating element; and a door to the chamber of the ion source is determined to be closed.

In at least one embodiment, the temperature control system is configured to initiate said cooling operation only if a signal is received to indicate that the ion source is operational.

The present invention provides a temperature control system for an ion source, the ion source comprising a heater including a gas source and a heating element, the heater for directing heated gas onto a capillary insertable into the ion source, the control system configured to: measure the temperature of the heating element; and initiate a cooling operation by directing gas over the heating element, to reduce the temperature of the heating element, if: the control system receives a request to initiate the cooling operation; the measured temperature is greater than a predetermined temperature

(Thys); and a door to the chamber of the ion source is determined to be closed. The present invention provides a temperature control system for an ion source, the ion source comprising a heater including a gas source and a heating element, the heater for directing heated gas onto a capillary insertable into the ion source, the control system configured to: measure the temperature of the heating element; and initiate a cooling operation by directing gas over the heating element, to reduce the temperature of the heating element, if: the measured temperature is greater than a predetermined temperature (Thys); and a door to the chamber of the ion source is determined to be open.

In at least one embodiment, the temperature control system is configured to terminate said cooling operation after the door has been detected to be open for at least a predetermined period of time.

The present invention provides a control system for a heating element, configured to: measure the temperature of the heating element; compare the measured temperature of the heating element to a predetermined target temperature; and selectively operate in: a first mode in which power is provided to the heating element to increase the temperature at a predetermined rate; and a second mode in which power is provided to the heating element until the measured temperature is substantially equal to the predetermined target temperature. In at least one embodiment, the control system comprises: a first controller configured to receive said predetermined rate and compare said predetermined rate to a rate of change of the measured temperature of the heating element, and send a first control signal to the heating element based on the comparison; a second controller configured to receive said predetermined target temperature and compare said predetermined target temperature to the measured temperature of the heating element, and send a second control signal to the heating element based on the comparison. In at least one embodiment the control signal from each controller is a value based on the extent of the respective comparison.

In at least one embodiment, the control system further comprises a heater controller to receive the first and second control signals, the heater controller configured to control the heating element based on the control signal with the lowest value. The present invention provides a control system for a heating element, configured to: a) measure the temperature of the heating element; b) receive a first target temperature from a user; c) set a second target temperature which is less than the first target temperature; d) compare the measured temperature of the heating element to the second target temperature; and e) provide power to the heating element until the measured temperature is substantially equal to the second target temperature; f) increase the second target temperature and repeat steps d) and f) until the measured temperature is substantially equal to the first target temperature. In at least one embodiment, the control system is configured to provide power to the heating element so as to increase the temperature of the heating element at a predetermined rate.

Embodiments of the present invention will now be described, by way of non- limiting example only, with reference to the figures, in which:

Figure 1 shows an ion source for use with a control system embodying the present invention. Figure 1A shows an enlarged view of part of Figure 1 ; Figure 2 schematically illustrates a temperature control system embodying the present invention;

Figure 3 schematically illustrates a control system for a heating element embodying the present invention; and

Figure 4 schematically illustrates another control system for a heating element embodying the present invention. Figure 1 illustrates an ion source 50 having an ion source housing 51 which, in use, is mounted to the housing of a mass spectrometer (not shown) so as to define a chamber 52 in the ion source housing 51. The ion source 50 further comprises a heater 53 having a nozzle 54, shown in Figure 1 A. The heater 53 comprises a heating element (not shown) and a gas source (not shown). A power supply is connected to the heating element, and the supply of power to the heating element causes the heating element to produce heat. As gas from the gas source is passed over the heating element, the gas is caused to heat up and exit the nozzle 54. The ion source 50 may further comprise a corona discharge pin 55, which serves to ionise the heated sample on the distal end of a capillary 56 which is receivable in the chamber 52.

Figure 2 schematically illustrates a temperature control system 1 for an ion source 50 such as that shown in Figure 1.

The control system 1 is configured to measure the temperature 2 of the heater 53, or the heating element of the heater 53. The control system 1 is further configured to compare the measured temperature 2 of the heating element with a predetermined target temperature 3 (which may be set by a user or software). The comparison between the measured temperature 2 and the predetermined target temperature 3 may be performed by a comparator 4, which outputs the difference between the measured temperature 2 and the predetermined target temperature 3. The temperature control system 1 is configured to initiate a cooling operation 12 by directing gas from the gas source over the heating element, to reduce the temperature of the heating element, if the measured temperature 2 is greater than the predetermined target temperature 3. The cooling operation 12 may comprise passing gas over the heating element at a predetermined flow rate.

The temperature control system 1 may be configured to initiate the cooling operation 12 only if the measured temperature 2 is greater than the predetermined target temperature 3 by a predetermined minimum difference (Ton), indicated as step 5 in figure 2. If the difference between the measured temperature 2 and the predetermined target 3 is greater than said predetermined minimum difference (Ton), then module 6 may be configured to a “set” condition, indicating that the predetermined minimum difference (Ton) between the measured temperature 2 and the predetermined target temperature 3 has been met.

Conversely, if the difference between the measured temperature 2 and the predetermined target temperature 3 is less than a predetermined trigger value (Toff), then the module 6 may be configured to a “clear” condition, indicating that the difference between the measured temperature 2 and the predetermined target temperature 3 is not sufficiently great enough to be of concern to the temperature control system 1.

After a cooling operation has been initiated, the measured temperature 2 should start to reduce. When the difference between the measured temperature 2 and the predetermined target temperature 3 reduces to less than the trigger value (Toff), the cooling operation will be terminated.

In at least one embodiment, Ton is greater than Toff, so as to avoid hysteresis.

The temperature control system 1 is further configured to initiate the cooling operation if the measured temperature 2 is determined 8 to be greater than a predetermined minimum temperature TL. TL may be set to be substantially equal to the minimum temperature that the heater could likely reach through cooling.

A temperature control system 1 embodying the present invention may be further configured to assess whether the predetermined target temperature 3 is within a predetermined acceptable range (T_L to T_H). This is to ensure that the predetermined temperature 3, which may be entered by a user, is within an acceptable range of values. Such an arrangement may then prevent a user inadvertently entering a target temperature which may be unobtainable and/or dangerous. In at least one embodiment, a temperature control system 1 embodying the present invention only initiates the cooling operation if the predetermined target temperature 3 is within said predetermined acceptable range. (TL - TH)

Further, a temperature control system 1 embodying the present invention may be configured to detect 9 whether power is being supplied to the heating element, and only to initiate the cooling operation if no power is detected being supplied to the heating element. A cooling operation must not be performed if power is still being provided to the heating element, since a cooling operation may not serve to sufficiently reduce the temperature of the heating element. Still further, a temperature control system 1 embodying the present invention may be operable to determine 10 if a door to the chamber 52 of the ion source 50 is closed. The cooling operation may only be initiated if the door is determined 10 to be closed. The door of the ion source 50 may not be an access port in the usual sense. The door opening operation may simply comprise the removal of the ion source housing 51 from the mass spectrometer so as to provide access to the components in the chamber 52. Accordingly, the ‘door’ as referred to herein may be the ion source housing 51 itself.

The temperature control system 1 may be further configured to initiate the cooling operation only if a signal is received 16 to indicate that the ion source is operational.

In the schematic illustration in figure 2, an AND gate 11 is illustrated as having five inputs:

• Whether 4, 5 the measured temperature 2 is greater than the predetermined target temperature 3.

• Whether 8 the measured temperature is greater than a predetermined minimum temperature.

• Whether 9 power is being supplied to the heating element.

• Whether 10 the source door is closed.

• Whether 16 a signal is received to indicate that the ion source is operational.

If all of the five inputs are true/positive, the AND gate 11 will output a signal which, in turn, initiates the cooling operation 12. A further AND gate 14 may be provided downstream of the AND gate 11 , which also requires a positive indication from a fault monitoring module 13 before the cooling operation 12 is initiated. If the fault monitoring module 13 indicates that there are no faults, then two positive inputs to the AND gate 14 will serve to initiate the cooling operation 12.

The fault monitoring module 13 may perform various fault checks including:

• Reading discontinuity check on the heater temperature readbacks

• Check the cold-junction compensation thermistor is within range · Check whether the readback temperature is within a range of, for example, 0 to 650 °C.

The temperature control system 1 illustrated in figure 2 may further comprise a timer module 15, which may serve to limit the maximum cooldown period to a predetermined maximum time. Once the timeout is reached the cool down operation 12 is stopped. This may ensure that the cool down operation 12 is not indefinite due to a fault condition. It may also ensure that gas (e.g. nitrogen) from the gas source is not indefinitely pumped into the lab environment.

The above set of five conditions which may initiate the cooling operation 12 may be grouped together as “condition group A”.

Another set of conditions (condition group B) which may initiate the cooling operation 12 will now be described.

The temperature control system 1 measures the temperature 2 of the heating element. A module 18 then assesses whether the measured temperature 2 is greater than a predetermined temperature (Thys), or lower than a predetermined set minimum required heater temperature (Ts) which is not user settable. The measured heater temperature 2 is monitored and once it drops below the predetermined set minimum required heater temperature (Ts) the cooling operation is stopped. Once the cooling operation is terminated, the temperature of the heater may increase slightly due to the ambient temperature inside the ion source. This may increase the measured temperature above the predetermined temperature Ts very slightly and trigger the cooling operation 12 again. In order to avoid such a condition, temperature hysteresis functionality may be provided. Setting the predetermined temperature (Thys) may help in creating this temperature hysteresis by providing an upper threshold.

A temperature control system 1 embodying the present invention may further be configured to receive a request 19 from a user and/or software to initiate a cooling operation 12. For example, the user/software may set the instrument to standby so that the temperature control system then initiates a cooling operation (assuming the other conditions are met), such that the heating element is cooled ready for the next use of the instrument. A temperature control system 1 embodying the present invention may further be configured to assess whether the door to the chamber of the ion source is closed.

Figure 2 illustrates a further AND gate 20, which requires the control system 1 to receive a request 19 to initiate the cooling operation, the measured temperature 2 to be greater than the predetermined temperature (Thys) and for the door to be closed,. Assuming that these conditions are met, the AND gate 20 outputs a positive signal which, in turn, initiates the cooling operation 12. Figure 2 illustrates an OR gate 21 which receives an input from the AND gates 11 and 20. A positive signal from one or both of the AND gates 11 , 20 is required in order to trigger the OR gate 21. A temperature control system 1 embodying the present invention may further be configured to initiate a cooling operation 12 if a third set of conditions (condition group C) is met.

As with condition group B, module 18 assesses whether the measured temperature 2 is greater than a predetermined temperature (Thys), or lower than a predetermined set minimum required heater temperature (Ts) which is not user settable.

In this embodiment, the temperature control system 1 is configured to initiate the cooling operation 12 if the measured temperature 2 is greater than a predetermined temperature (Thys) and if the door to the chamber of the ion source has been detected to be open.

In the schematic illustration of figure 2, these conditions are shown to feed in to an AND gate 24. If the measured temperature 2 is greater than a predetermined temperature (Thys) and the door of the chamber of the ion source has been detected to be open 22, then the AND gate 24 will send a positive signal towards the OR gate 25.

The temperature control system may be further configured to terminate the cooling operation after the door has been detected to be open for at least a predetermined period of time (set by timer block 23).

The skilled person will appreciate that the two OR gates 21 , 25 are configured such that the cooling operation 12 is initiated if one or more of the condition groups A to C is satisfied. The AND gate 11 determines whether the condition group A is satisfied; the AND gate 20 indicates whether the condition group B is satisfied; and the AND gate 24 determines whether the condition group C has been satisfied. The logic circuit in Figure 2 demonstrates just one embodiment of how the functionality of the claimed control system 1 may be provided. It will be appreciated that other logic circuits are possible. It will be appreciated that the functionality may also be provided by a computer processor. With reference to figure 3, the present invention further provides a control system 100 for a heating element 120, configured to: measure the temperature 101 of the heating element 120 and compare the measured temperature of the heating element 120 to a predetermined target temperature 102. The control system 100 is configured to operate in: a first mode in which power is provided to the heating element 120 to increase the temperature at a predetermined rate 103; and a second mode in which power is provided to the heating element 120 until the measured temperature 101 is substantially equal to the predetermined target temperature 102.

A user may enter a rate 103 at which the temperature of the heating element should increase over time (the slope). This may be compared, by a first comparator 104, with a measured rate 105 of temperature increase. The output of the first comparator 104 may feed into a first controller 106 which controls the heating element 120. A thermocouple 121 associated with the heating element may measure the temperature 101 of the heating element 120.

A user may also enter a predetermined target temperature 102. This may be compared, by a second comparator 107, with the measured temperature 101 of the heating element/thermocouple. The output of the second comparator 107 may feed into a second controller 108 which controls the heating element 120.

A control selector 109 is provided, to select which of the first controller 106 and second controller 108 is used to control the heating element 120. The selection may be made based on predetermined criteria. The control system 100 is configured to operate in a first mode, in which the first controller 106 is selected by the control selector 109 to control the heating element. The control system is also configured to operate in a second mode, in which the second controller 108 is selected by the control selector 109 to control the heating element.

The control system 100 may be configured initially to operate in said first mode, before transitioning to said second mode when predetermined criteria is/are met.

The first 106 and second 108 controllers may comprise PID controllers.

The output of PID controller 106, 108 may be a value of between, for example, 0 and 100. The PID controllers 106, 108 may be configured such that, when the measured value (slope 105/temperature 101) moves closer to the setpoint 103, 102, the output value of the PID controller 106, 108 will move closer towards a stable value. This will be the value required to maintain the heater 120 at that particular setpoint in a stable condition. If the difference between the setpoint 103, 102 and the measured value 105, 101 is increasing, the output of the PID controller 106, 108 will increase and finally saturate at 100. The stable value for maintaining the heater system may increase as the temperature setpoint 102 is higher. This is because more energy is required to maintain the heater 120 at higher temperature. The slope PID 106 may always have this stable value moving up along with the heater temperature as the temperature of the heater is increasing to maintain the slope. At the initiation of the control system 100, having entered a predetermined slope 103 and predetermined target temperature 102, the output of PID controller 108 may be high due to the setpoint temperature 102 being much higher that the initially measured temperature 101. However as the temperature increases the measured temperature 101 comes closer to the heater setpoint temperature 102 and so the output of PID controller 108 will start to decrease. On the other hand the output of PID controller 106 may increase as the temperature goes up and the power required to maintain that particular slope will increase.

The control selector 109 may choose the lower of the two values from the PID controllers 106 & 108. As initially the output of PID controller 106 is lower, the control selector 109 passes that value to the heater controller 110, thus meaning that the system focusses on maintaining the slope of the heater. In time, as the output of PID controller 106 increases and the output of PID controller 108 decreases, there will be a point where the output of PID controller 108 becomes smaller than the output of PID controller 106. At this point, the PID controller 108 may effectively take control of the heater 120 and the system will now start focusing on maintaining the final setpoint temperature rather than the slope.

Control selector 109 may select the output from the PID controller 106, 108 with the lowest value, which may ensure a substantially smooth transition from slope control to the temperature control of the heater.

As illustrated in Figure 4, there may also be provided a control system 200 for a heating element 120, configured to measure the temperature 101 of the heating element and receive a first target temperature 202 from a user. Rather than provide the first target temperature 202 to a PID controller 208, the control system 200 sets a second target temperature which is less than the first target temperature. The second target temperature is then fed to the controller 208 and compared with the measured temperature 101 to control the heating element 120. The control system 200 then increases the second target temperature at a predetermined rate. For example, the second target temperature may be increased at 1 °c per second. Accordingly, at any one time, the controller 208 is controlling the heating element 120 based on the difference between the second target temperature and measured temperature, without any regard to the first target temperature. This causes the controller 208 to increase the temperature of the heater 120 at substantially a uniform rate.

The second target temperature may be incrementally increased until the measured temperature is substantially equal to the first target temperature.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and/or to encompass equivalents.

Claims

1. A temperature control system for an ion source, the ion source comprising a heater including a gas source and a heating element, the heater for directing heated gas onto a capillary insertable into the ion source, the control system configured to: measure the temperature of the heating element; compare the measured temperature of the heating element to a predetermined target temperature; and initiate a cooling operation by directing gas over the heating element, to reduce the temperature of the heating element, if the measured temperature is greater than the predetermined target temperature.

2. A temperature control system according to claim 1 , configured to detect whether power is being supplied to the heating element, and only initiate the cooling operation if no power is detected being supplied to the heating element.

3. A temperature control system according to any preceding claim, configured to initiate the cooling operation only if the measured temperature is greater than the predetermined target temperature by a predetermined minimum difference.

4. A temperature control system according to any preceding claim, further configured to: determine if a door to a chamber of the ion source is closed; and only initiate the cooling operation if the door is determined to be closed.

5. A temperature control system according to any of claims 1 to 3, further configured to: determine if a door to a chamber of the ion source is open; and terminate said cooling operation after the door has been detected to be open for at least a predetermined period of time.

6. A temperature control system according to any preceding claim, configured to terminate said cooling operation after a predetermined amount of time has elapsed.

7. A temperature control system for an ion source, the ion source comprising a heater including a gas source and a heating element, the heater for directing heated gas onto a capillary insertable into the ion source, the control system configured to: measure the temperature of the heating element; compare the measured temperature of the heating element to a predetermined target temperature; and initiate a cooling operation by directing gas over the heating element, to reduce the temperature of the heating element, if: the measured temperature is greater than the predetermined target temperature; the measured temperature is greater than a predetermined minimum temperature; no power is being supplied to the heating element; and a door to the chamber of the ion source is determined to be closed.

8. A temperature control system according ot claim 7, configured to initiate said cooling operation only if a signal is received to indicate that the ion source is operational.

9. A temperature control system for an ion source, the ion source comprising a heater including a gas source and a heating element, the heater for directing heated gas onto a capillary insertable into the ion source, the control system configured to: measure the temperature of the heating element; and initiate a cooling operation by directing gas over the heating element, to reduce the temperature of the heating element, if: the control system receives a request to initiate the cooling operation; the measured temperature is greater than a predetermined temperature (Thys); and a door to the chamber of the ion source is determined to be closed.

10. A temperature control system for an ion source, the ion source comprising a heater including a gas source and a heating element, the heater for directing heated gas onto a capillary insertable into the ion source, the control system configured to: measure the temperature of the heating element; and initiate a cooling operation by directing gas over the heating element, to reduce the temperature of the heating element, if: the measured temperature is greater than a predetermined temperature

(Thys); and a door to the chamber of the ion source is determined to be open.

11. A temperature control system according to claim 10, further configured to terminate said cooling operation after the door has been detected to be open for at least a predetermined period of time.

12. A control system for a heating element, configured to: measure the temperature of the heating element; compare the measured temperature of the heating element to a predetermined target temperature; and selectively operate in: a first mode in which power is provided to the heating element to increase the temperature at a predetermined rate; and a second mode in which power is provided to the heating element until the measured temperature is substantially equal to the predetermined target temperature.

13. A control system according to claim 12, comprising: a first controller configured to receive said predetermined rate and compare said predetermined rate to a rate of change of the measured temperature of the heating element, and send a first control signal to the heating element based on the comparison; a second controller configured to receive said predetermined target temperature and compare said predetermined target temperature to the measured temperature of the heating element, and send a second control signal to the heating element based on the comparison.

14. A control system according to claim 13, wherein the control signal from each controller is a value based on the extent of the respective comparison.

15. A control system according to claim 13 or 14, further comprising a heater controller to receive the first and second control signals, the heater controller configured to control the heating element based on the control signal with the lowest value.

16. A control system for a heating element, configured to: a) measure the temperature of the heating element; b) receive a first target temperature from a user; c) set a second target temperature which is less than the first target temperature; d) compare the measured temperature of the heating element to the second target temperature; and e) provide power to the heating element until the measured temperature is substantially equal to the second target temperature; f) increase the second target temperature and repeat steps d) and f) until the measured temperature is substantially equal to the first target temperature.

17. A control system according to claim 16, configured to provide power to the heating element so as to increase the temperature of the heating element at a predetermined rate.