Abstract
A thermoanalytical instrument, and especially a differential scanning calorimeter, has first and second measurement positions, a heater and a temperature sensor associated with each of the measurement positions, and a controller. The controller, which has an associated means for setting a predetermined temperature program, controls a heating power of the first heater to cause the temperature measured at the first position to follow the temperature program. The controller also controls both heaters to eliminate any temperature difference between the measured first and second temperatures. The controller also provides a means for determining the lower of the measured first and second measured temperatures and applies additional power to the heater associated with that lower measured temperature.
Claims
1. A method for operating a thermoanalytical instrument as a differential scanning calorimeter in a dynamic power compensation mode, the thermoanalytical instrument comprising a first and a second measurement position, each of the measurement positions having a primary heater, a compensation heater and a temperature sensor associated therewith, and a controller, provided with a predetermined temperature program of temperature target values versus time and operatively arranged to control heating power provided to the primary and compensation heater at each measurement position, the method comprising the steps of: applying, from the controller, a heating power signal to the respective primary heaters to cause the temperatures at the respective measurement positions to follow the predetermined temperature program; and while the predetermined temperature program is being applied, using the controller to iteratively perform the steps of: receiving a series of time-based temperature signals from the respective temperature sensors; determining the measurement position having the lower measured temperature, based on the time-based temperature signals; and providing, as needed, an additional heating power signal to only the compensation heater associated with the measurement position having the lower measured temperature.
2. The method of claim 1, further comprising the step of: applying an individual offset voltage to each compensation heater.
3. The method of claim 1, wherein: a first control loop of the controller operates in the step of applying the heating power signal; and a second control loop of the controller operates in the step of applying the additional power.
4. The method of claim 3, wherein: a means for restricting the additional power applied to only one of the heaters or compensation heaters at any moment in time, operates in the step of applying additional power, the means for restricting being located in the second control loop.
5. A computer program for performing the control method of claim 1 on a thermoanalytical instrument, wherein: the computer program is stored in a memory device of the controller.
6. A method for controlling a thermoanalytical instrument that comprises a first and a second measurement position, a first and a second heater, a first and a second compensation heater and a first and second temperature sensor, the heaters and sensors associated with the respective measurement positions, and a controller operatively arranged to control heating power to the first heater to cause the measured first temperature to essentially follow a predetermined temperature program of temperature target values versus time, and to control the first and second heaters and the first and second compensation heaters to minimize any difference between the measured temperatures, the method comprising the steps of: applying, from the controller, the predetermined temperature program of temperature target values versus time directly to the first and second measurement positions through the respective first and second heaters; determining, in the controller, the lower measured temperature in the first and second measurement positions; and applying, as needed and from the controller, additional power to the compensation heater associated with the measurement position having the lower measured temperature.
7. The method of claim 6, further comprising the step of: applying an individual offset voltage to each compensation heater.
8. The method of claim 6, wherein: a first control loop of the controller operates in the step of applying the temperature program; and a second control loop of the controller operates in the step of applying the additional power.
9. The method of claim 6, wherein: a means for restricting the additional power applied to only one of the heaters or compensation heaters at any moment in time, operates in the step of applying additional power, the means for restricting being located in the second control loop.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] The different embodiments of the invention are discussed in relation to the following figures, wherein similar elements in the figures are referred to with the same reference symbol and wherein:
[0057] FIG. 1 is a schematic electronic circuit for a DSC with power compensation, as known in the prior art;
[0058] FIG. 2 is a schematic electronic circuit for a DSC with dynamic compensation, wherein the DSC comprises a heater and a compensation heater for each measurement position;
[0059] FIG. 3 is schematic electronic circuit for a DSC with dynamic compensation, wherein the DSC comprises a heater for each measurement position;
[0060] FIGS. 4a and 4b are representations of the sample and reference heater power, the reference offset power demand and the compensation power demand of a DSC with power compensation, with FIG. 4a showing melting of polypropylene and FIG. 4b showing crystallization of polypropylene;
[0061] FIGS. 5a and 5b representations of the sample and reference heater power, the reference compensation power demand and the sample compensation power demand of a DSC with dynamic compensation, with FIG. 5a showing melting of polypropylene and FIG. 5b showing crystallization of polypropylene;
[0062] FIG. 6 is a schematic electronic circuit for a DSC with dynamic compensation comprising means for controlling a switching of the compensation heaters and an offset voltage applied to each compensation heater;
[0063] FIG. 7 is a schematic electronic circuit for the data acquisition of a DSC with dynamic compensation comprising means for controlling a switching of the compensation heaters and an offset voltage applied to each compensation heater;
[0064] FIG. 8 an electronic setup for a DSC with dynamic compensation and induced switching of the temperature input into the first control loop in relation to the measurement position controlled by the second control loop;
[0065] FIG. 9 presents comparative measurements of the cold crystallization of polyamide 6 showing the advantage of means for preventing a sensitivity drop of the second control loop;
[0066] FIG. 10 presents DSC measurements on polypropylene showing the advantage in offset temperature, baseline offset, drift and curvature of dynamic compensation in comparison to power compensation; and
[0067] FIG. 11 presents DSC measurements on polypropylene showing the headroom advantage of the dynamic compensation in comparison to power compensation.
DETAILED DESCRIPTION
[0068] FIG. 1 shows an electronic setup for a DSC with power compensation, as known in the prior art. The DSC comprises at least two measurement positions, a first measurement or sample position S and a second measurement or reference position R. A sample or sample material can be placed on the sample position S and a reference material can be placed on the reference position R. Experiments on a sample can be performed with and without a reference material.
[0069] The sample position S is in thermal contact with a sample heater 1 and a first compensation heater 2. The temperature at the sample position S is determined by a sensor comprising at least one thermocouple 3. Likewise the reference position R is in thermal contact with a reference heater 4 and a second compensation heater 5, which supplies an offset power arising from a constant offset voltage U.sub.off. The temperature at the reference position R is determined with a sensor comprising at least one thermocouple 8. The heaters 1, 2, 4, 5 are preferably designed as individual resistance heaters.
[0070] The sample heater 1 and the reference heater 4 apply a temperature program to the respective measurement positions S, R and are part of a first control loop 6. This control loop 6 also comprises a PID controller 7. The temperature program is fed to the first control loop 6 as indicated by the temperature set points T.sub.set.
[0071] The first compensation heater 2 is integrated in a second control loop 9, which also comprises a PID controller 10. The compensation voltage supplied to the sample position S gives rise to a compensation power and its magnitude is chosen in order to control any temperature difference T between the sample position S and the reference position R to remain substantially zero. Therefore, the input to the second control loop 9 is the product of said temperature difference T and the Seebeck constant of the thermopile as. The control loops 6, 9 are connected with a main controller for controlling the DSC, which is not shown here.
[0072] FIG. 2 shows an electronic setup for a DSC with a compensation according to the invention, which will be referred to as dynamic compensation and shares some features with the power compensated DSC of FIG. 1. For the dynamic compensation as shown in FIG. 2 a second control loop 11 comprises besides a first compensation heater 2 and a PID controller 10 also a second compensation heater 12, which is in thermal contact with the reference position R. The second control loop 11 further comprises determination means 13, which allow selecting the first compensation heater 2 or the second compensation heater 12 to receive a compensation voltage, which is applied to the respective measurement position and gives rise to a compensation power. Which of the two compensation heaters 2, 12 receives the compensation voltage depends on the sign of the differential temperature T between the sample position S and the reference position R. If the temperature difference T=T.sub.sT.sub.R is negative, because the temperature T.sub.R at the reference position is higher than the temperature T.sub.S at the sample position, the compensation voltage applied to the first compensation heater 2 is raised and thus compensation power is applied to said first compensation heater 2, which leads to an increase of the sample temperature T.sub.s in order to reduce the differential temperature T to substantially zero again. If the sign of the differential temperature T is positive, the compensation voltage applied to the second compensation heater 12 is raised in order to reduce the differential temperature T to substantially zero.
[0073] FIG. 3 shows another electronic setup for a DSC with dynamic compensation without compensation heaters. Each measurement position S, R is equipped with a main heater 1, 4 and at least one thermocouple 3, 8 as already described in FIG. 2. The main heaters 1, 4 are on one hand controlled by a first control loop 16, which supplies a temperature program T.sub.set to the measurement position S, R, and on the other hand by a second control loop 17, which comprises similar parts as the one described in FIG. 2. Due to a differential temperature T determined from the thermocouples 3, 8 an extra heating power is supplied to either the sample position S or the reference position R through the second control loop 17. This extra heating power is electronically added to the main heating power delivered by the first control loop 16 to the respective main heater 1, 4.
[0074] In FIGS. 4 and 5, the known power compensation principle is compared on a highly abstract level with the dynamic compensation according to the invention. FIG. 4a shows the heating power distribution at the marked point of a melting curve of a polypropylene sample 14 subjected to a power compensated DSC experiment. FIG. 4b shows the heating power distribution at the marked point of a crystallization curve of a polypropylene sample 14 subjected to a power compensated DSC. FIGS. 5a and 5b show the heating power distribution at the same point of a melting or respectively a crystallization curve of a polypropylene sample 14 subjected to a dynamic compensated DSC experiment.
[0075] During the experiments shown in FIGS. 4a and 4b the sample position S as well as the reference position R are subjected to a temperature program through the sample and reference heater. The voltage supplied by the sample heater and the reference heater at the marked point of the curve is indicated with the reference symbol M. Additionally, the second compensation heater supplies a constant offset voltage O to the reference position R throughout the experiment. To compensate any temperature changes of the sample due to a phase transition a compensation voltage C is also supplied to the sample position S, which is controlled in order to keep the differential temperature between the sample position S and the reference position R substantially zero. At the marked point on the melting curve the first compensation heater is supplied with a compensation voltage C being higher than the offset voltage O, while at the marked point of the crystallization curve the compensation voltage C is lower than the offset voltage O. Because both points mark roughly the same temperature, the main heater voltage M is approximately equal in both situations.
[0076] When comparing the situation shown in FIGS. 4a and 4b for power compensation with the situation shown in FIGS. 5a and 5b for dynamic compensation it is evident, that the overall compensation power is strongly reduced, leading to an improved signal-to-noise-ratio. At the point on the melting curve in FIG. 5a only the first compensation heater applies any compensation power, while the second compensation heater is inactive. The situation at the marked point of the crystallization curve in FIG. 5b is reversed.
[0077] The dynamic compensation has the advantage that the compensation power as compensation voltage is only applied where and when it is needed, leading to the already mentioned advantages of increased headroom and an absence of an offset temperature.
[0078] FIG. 6 shows a further electronic setup for a DSC with dynamic compensation, which is similar to the setup of FIG. 2, but where a constant offset voltage U.sub.off is supplied to the first compensation heater 2 as well as to the second compensation heater 12. The offset voltage U.sup.off in a chip-calorimeter setup according to the invention can for example be about 0.5 V, which corresponds to about 50 W of heating power based on a compensation heater with a resistance of about 5 k. A thermal resistance of the order of 0.01 K/W, a typical magnitude in such a chip-calorimeter setup, will result in a very small offset temperature of only about 0.5 C. The exact amount for the offset voltage U.sub.off depends e.g. on the quality of the PID controller, the setup, etc. The offset voltage U.sub.off should be chosen in such a way that it is high enough to prevent artifacts without adding a significant temperature offset to the baseline.
[0079] Because of the dynamic compensation according to the invention the data acquisition has to be adapted. For power compensation the compensation power is acquired by measuring the voltage across and the current through the compensation heater. As for the dynamic compensation, where the heating power is alternatively delivered to one or the other of the two measurement positions, an extraction of individual signals, such as compensation voltage and compensation current, at one fixed point in the circuit is no longer possible. To overcome this problem the data acquisition for a thermoanalytical instrument with a first and a second compensation heater comprises: Measuring the voltage U.sub.comp differentially across both compensation heaters and measuring the current I.sub.comp additively as the sum of the compensation heaters currents. For dynamic compensation with additional compensation offset power this is schematically shown in the circuit diagram of FIG. 7 showing the second control loop 15 of FIG. 6 in more detail. For this setup the net compensation power is given by the difference of the power P.sub.S of the first compensation heater 2 and the power P.sub.R of the second compensation heater 12 as
P.sub.comp=P.sub.sR.sub.R=U.sub.SI.sub.SU.sub.RI.sub.R,
with the offset contribution of the inactive compensation heater being always small in comparison to the active one, but due to the applied compensation offset voltage not negligible. The net compensation power P.sub.comp is the signal of interest. What is actually measured, however, can be expressed as:
[00004]
with R.sub.s being the heater resistance of the first compensation heater 2 and R.sub.R that of the second compensation heater 12.
[0080] Because the sample position S and the reference position R remain at approximately equal temperatures during a dynamic compensation experiment and should possess an intrinsic symmetry, it can be assumed that the resistance values are well matched. Therefore the error term (U.sub.SU.sub.R(R.sub.R.sup.1R.sub.S.sup.1)) of the actual net compensation power will be quite small compared to the net compensation power signal and can be neglected.
[0081] FIG. 8 shows an electronic setup of a further DSC with dynamic compensation, wherein interference between a first control loop 18 and a second control loop 19 is prevented by introducing switching means 20. The same decision criterion being used to activate the appropriate compensation heater 2, 12 on measurement positions R or S is also fed into the first control loop 18 via the switching means 20, which controls the temperature T.sub.S, T.sub.R to be used for controlling said first loop 18. Through this measure only one control loop 18, 19 is active at one of the measurement positions S, R. When the control of the second control loop 19 switches to the other measurement position S, R, the control of the first control loop 18 is simultaneously switched to the opposite measurement position.
[0082] FIG. 9 shows comparative experiments on the cold crystallization of polyamide 6 subjected to DSC with dynamic compensation and a heating rate of 50 C./s. The solid line graph represents a measurement without an additional compensation voltage offset U.sub.off and the dotted line graph a measurement with an additional compensation voltage U.sup.off applied to both compensation heaters 2, 12. It is evident from FIG. 9 that the artifacts present around a power of 0 mW in the solid line graph do not appear in the dotted line graph, thereby showing the advantages of applying said additional compensation voltage offset U.sub.off.
[0083] In order to prove the advantages of dynamic compensation over power compensation the results of comparative heating/cooling experiments on polypropylene are presented in FIGS. 10 and 11. The solid line graph was measured with power compensation and the dotted line graph with dynamic compensation comprising means to control the switching between the compensation heaters and an additional compensation offset voltage.
[0084] The graphs presented in FIGS. 10 and 11 show that with dynamic compensation there is less to no offset from room temperature, less to no offset and slope in the baseline and the headroom trouble was also eliminated.
[0085] The principle of dynamic compensation is especially useful for chip-type differential scanning instruments but could also be adapted to other thermoanalytical instruments which so far utilize the power compensation principle.
