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in Electrochemical Applications
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FAQs
What influence have connection cords between cell and impedance analyzer to the measurement data?
One of the most critical elements of an impedance measurement setup are the cell and the connection cords between cell and measurement system. By reason of its impedance (capacitance, inductance and resistance) the cords work as a low-pass filter that will damp high frequencies and cause a small phase shift. Beside this the cables are antennas that can pick up every electromagnetic field such as the 50/60 Hz power line frequency.
So what can you do? You should use relatively thick cables (more than 3 mm in diameter) as short as possible if you examine low impedance objects. The connectors should be high quality and of BNC- or Lemosa-type. You should not lay the cables in parallel with a line or another high voltage or high frequency cord.
back
Can you export a graphic created with the IM6 to PC programs?
Yes, you easily can export IM6 graphics to any MS-DOS or Windows software which is able to import HPG -files. HPGL ("Hewlett Packard Graphic Language") is a standard file format today that is sup-ported by the IM6 software and nearly every good graphic and word processing program on the market. Examples are Corel Draw, Designer, Word for Windows and many others.
The handling is easy: In every page of the Thales software you can create a graphic that you can print out. In the printer menu you select the option HPGL file that will store the complete graphics as in the HPGL file format on disk. In the PC program you only have to select "Import" in the File menu and chose the HPGL extension hgl. The file selector will show you all HPGL files and you can select one to be loaded.
back
When I'm examining objects dominated by double layer capacities, my cyclic voltammograms (CV) sometimes show strange effects. What is the rea-son for these effects?
Though it seems simple, capacities are not at all easy objects for an electrochemical measuring system. One reason for problems may be the instability of the feedback loop of the potentiostat used. The capacity adds phase shift to the loop. So undetected parasitic oscillations of the potentiostat may produce irregular results such as "starry skies" or severe current offsets.
Another problem arises with the measuring method itself used by certain equipment. In the era of computers it seems easy to change from the traditional analog scan technique to its digital approximation by a small steps stairs technique. This task can be done by a software controlled D/A converter. The advantage of such a technique is the possibility to easily produce arbitrary wave forms, for instance stable slow signals (slew rate down to zero). The measurement itself too can be performed with the computer using A/D converters. This is the base for flexible and user-friendly CV equipment - but the stair-sweep-approximation idea is too simple to be good! Signals, created by a DA-converter remain discrete. Even if one tries to increase the resolution dramatically, it will stay finite. On the other hand the theories of CV techniques claim steady sweep signals.
Regarding capacitive objects will help to focus on this important difference. If one applies a voltage signal U(t) to a capacitor, the current I(t)=C·dU/dt will flow. A constant slew rate results in a constant current. Discrete steps will cause current pulses with d-pulse shape - that means in theory infinite height for an infinite short time interval but with a well defined integral charge of Q=C·DU. In practice the d-pulse will be distorted to a short pulse of high amplitude. Its shape is determined by the pulse response of the potentiostat and parasitic effects. If the measurement technique samples the response signal after a short time delay relative to the step, the result must be wrong! The only way to get a correct current result is to measure the charge by integrating the total step interval and calculate the mean current. This calculated current value is identical with the measured one with the continuous method.
How can you find out, if your equipment uses this "clean" integrating CV technique? Take an aluminum electrolyte capacitor of 1 mF and perform a test measurement. Choose, for instance, a triangle scan of ±1 V at ±100 mV/s slew rate. The capacitance of 1 mF should cause a square wave of ±0.1 mA current to flow. Consider the capacity tolerance of -5 % / +25 %. Check the correct capacity using an impedance measurement at low frequency (e.g. 1 Hz): Now the measured current should fit the calculated current exactly.
If not, call the service-hotline of your equipment's manufacturer!
back
How can I reduce the measuring time for my impedance spectra?
A reduction of the measuring time will often be demanded in case of examining non steady state systems or in case of using very low frequency ranges down to mHz or μHz. At low frequencies, polychromatic stimulation (PS) at first glance promises advantages in comparison to monochromatic (MS) experiments. By a rough estimation the measuring time of PS methods is fixed by the sum of all periods ne at the lowest frequency which is investigated, namely the ω0 fundamental.
In contrast the minimum measuring time of MS (sine wave) experiments is given by the sum of periods ne,i of all frequencies wi to be measured.
A comparison of typical measuring times for both methods is quite difficult as there are some unknown scaling factors. Under the assumption of an ideal measuring object where no ranging processes will occur, the scaling factor fres in first order depends on the number of measuring points of the lowest decade.
The ratio of e.g.
This shows a slight advantage for PS. If, however, at least one ranging process is necessary during measurement, this factor will be reduced to about 1.3. In PS methods ranging processes dominate the measuring time (T, 2T, 3T, etc), while MS can use dynamic ranging. Then the measuring time is extended only by a factor of 1/n, depending on the measuring frequency nω0. You see clearly, that the simple speculation to save time will not work very good, even neglecting all aspects of accuracy. But in addition, using PS, the total energy of the signal will be spread over all examined frequencies. Only the average fraction Ef ≈ E/n, compared with the monochromatic case will be available for one frequency line. To avoid nonlinear response, the AC amplitude has to be selected sufficiently low. Thus polychromatic stimulation has to be repeated n times to obtain a sufficient signal to noise ratio. This leads to significant longer measuring times than in the case of single sine methods which contradicts the expectations of the user. This fact has been proved by Miloco in 1994 [1]
Nevertheless, regarding non steady state system research, there is a gap, where PS shows clear advantage. In the higher frequency range (>100 Hz) the pure measuring time will no longer dominate the overall time. Most of it is spent by the instrument's microprocessor system analyzing the incoming data after each frequency sample. In the PS case, the sum of all this analysis time appears at the end of a measurement, the measurement itself is much faster. This is an advantage when studying non steady state systems. For this application the PS method was developed further by Schindler and Popkirov [2]. A version of PS can be performed with the IM6, extended by the TR8M transient recorder module.
t must be mentioned, that regarding non steady state behavior, there are several possibilities to increase the reliability of the AC impedance results, if measurement time reduction is impossible:
a) IM6 on-line drift correction: Periodic stimulation offers the detection of non steady state behavior of the investigated system. The IM6 performs an automatic drift correction in the lower frequency mode.
b) LKKT (Logarithmic Kramers Kronig Transform): By means of the LKKT [3] not only violations of the EIS fundamentals of the examined system may be detected. The LKKT version in the THALES analysis software allows the reconstruction of non-affected spectra, if the violations are not too strong.
c) Interpolation of impedance spectra series versus time: Series measurements vs. time offer a further possibility to correct impedance spectra of drifting systems. Under the assumption of slow rates of changes the data may be corrected. The spectra can be transformed by an interpolation routine generating new ones, belonging to unique times. Finally each of those spectra will represent a certain state of the electrode. [4,5] This method is also available in the THALES software package.
[1] Ruben H. Milocco: Minimal measurement time in electrochemical impedance identification. Electrochimica acta, Vol.39, No.10, pp 1433-1439, 1994
[2] G.S. Popkirov and R.N. Schindler: A new impedance spectrometer for the investigation of electrochemical systems. Rev. Sci. Instrum. 63 (11), 1992
[3] H. Gφhr, B. Rφseler and C.A. Schiller: Advantages of Hilbert Transform compared with Kramers-Kronig Rule when examining the causality of electrochemical impedance spectra. Presented at the 46th ISE Meeting, Xiamen, China, 1995
[4] Z. Stoynov: Nonstationary impedance spectroscopy. Electrochem. Acta 38, no. 14, pp. 1919-1922 (1993)
[5] H. Gφhr, H. Bode, A. Burghart and C.A. Schiller: Impedance measurements at electrodes of continuously changing state. Presented at the 2nd EIS Symposium, Santa Barbara, USA, 1992
back
One of the most critical elements of an impedance measurement setup are the cell and the connection cords between cell and measurement system. By reason of its impedance (capacitance, inductance and resistance) the cords work as a low-pass filter that will damp high frequencies and cause a small phase shift. Beside this the cables are antennas that can pick up every electromagnetic field such as the 50/60 Hz power line frequency.
So what can you do? You should use relatively thick cables (more than 3 mm in diameter) as short as possible if you examine low impedance objects. The connectors should be high quality and of BNC- or Lemosa-type. You should not lay the cables in parallel with a line or another high voltage or high frequency cord.
back
Can you export a graphic created with the IM6 to PC programs?
Yes, you easily can export IM6 graphics to any MS-DOS or Windows software which is able to import HPG -files. HPGL ("Hewlett Packard Graphic Language") is a standard file format today that is sup-ported by the IM6 software and nearly every good graphic and word processing program on the market. Examples are Corel Draw, Designer, Word for Windows and many others.
The handling is easy: In every page of the Thales software you can create a graphic that you can print out. In the printer menu you select the option HPGL file that will store the complete graphics as in the HPGL file format on disk. In the PC program you only have to select "Import" in the File menu and chose the HPGL extension hgl. The file selector will show you all HPGL files and you can select one to be loaded.
back
When I'm examining objects dominated by double layer capacities, my cyclic voltammograms (CV) sometimes show strange effects. What is the rea-son for these effects?
Though it seems simple, capacities are not at all easy objects for an electrochemical measuring system. One reason for problems may be the instability of the feedback loop of the potentiostat used. The capacity adds phase shift to the loop. So undetected parasitic oscillations of the potentiostat may produce irregular results such as "starry skies" or severe current offsets.
Another problem arises with the measuring method itself used by certain equipment. In the era of computers it seems easy to change from the traditional analog scan technique to its digital approximation by a small steps stairs technique. This task can be done by a software controlled D/A converter. The advantage of such a technique is the possibility to easily produce arbitrary wave forms, for instance stable slow signals (slew rate down to zero). The measurement itself too can be performed with the computer using A/D converters. This is the base for flexible and user-friendly CV equipment - but the stair-sweep-approximation idea is too simple to be good! Signals, created by a DA-converter remain discrete. Even if one tries to increase the resolution dramatically, it will stay finite. On the other hand the theories of CV techniques claim steady sweep signals.
Regarding capacitive objects will help to focus on this important difference. If one applies a voltage signal U(t) to a capacitor, the current I(t)=C·dU/dt will flow. A constant slew rate results in a constant current. Discrete steps will cause current pulses with d-pulse shape - that means in theory infinite height for an infinite short time interval but with a well defined integral charge of Q=C·DU. In practice the d-pulse will be distorted to a short pulse of high amplitude. Its shape is determined by the pulse response of the potentiostat and parasitic effects. If the measurement technique samples the response signal after a short time delay relative to the step, the result must be wrong! The only way to get a correct current result is to measure the charge by integrating the total step interval and calculate the mean current. This calculated current value is identical with the measured one with the continuous method.
How can you find out, if your equipment uses this "clean" integrating CV technique? Take an aluminum electrolyte capacitor of 1 mF and perform a test measurement. Choose, for instance, a triangle scan of ±1 V at ±100 mV/s slew rate. The capacitance of 1 mF should cause a square wave of ±0.1 mA current to flow. Consider the capacity tolerance of -5 % / +25 %. Check the correct capacity using an impedance measurement at low frequency (e.g. 1 Hz): Now the measured current should fit the calculated current exactly.
If not, call the service-hotline of your equipment's manufacturer!
back
How can I reduce the measuring time for my impedance spectra?
A reduction of the measuring time will often be demanded in case of examining non steady state systems or in case of using very low frequency ranges down to mHz or μHz. At low frequencies, polychromatic stimulation (PS) at first glance promises advantages in comparison to monochromatic (MS) experiments. By a rough estimation the measuring time of PS methods is fixed by the sum of all periods ne at the lowest frequency which is investigated, namely the ω0 fundamental.
In contrast the minimum measuring time of MS (sine wave) experiments is given by the sum of periods ne,i of all frequencies wi to be measured.
A comparison of typical measuring times for both methods is quite difficult as there are some unknown scaling factors. Under the assumption of an ideal measuring object where no ranging processes will occur, the scaling factor fres in first order depends on the number of measuring points of the lowest decade.
| points | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| fres | 1.32 | 1.68 | 2.06 | 2.44 | 2.82 | 3.21 | 3.60 | 3.99 | 4.38 |
The ratio of e.g.This shows a slight advantage for PS. If, however, at least one ranging process is necessary during measurement, this factor will be reduced to about 1.3. In PS methods ranging processes dominate the measuring time (T, 2T, 3T, etc), while MS can use dynamic ranging. Then the measuring time is extended only by a factor of 1/n, depending on the measuring frequency nω0. You see clearly, that the simple speculation to save time will not work very good, even neglecting all aspects of accuracy. But in addition, using PS, the total energy of the signal will be spread over all examined frequencies. Only the average fraction Ef ≈ E/n, compared with the monochromatic case will be available for one frequency line. To avoid nonlinear response, the AC amplitude has to be selected sufficiently low. Thus polychromatic stimulation has to be repeated n times to obtain a sufficient signal to noise ratio. This leads to significant longer measuring times than in the case of single sine methods which contradicts the expectations of the user. This fact has been proved by Miloco in 1994 [1]
Nevertheless, regarding non steady state system research, there is a gap, where PS shows clear advantage. In the higher frequency range (>100 Hz) the pure measuring time will no longer dominate the overall time. Most of it is spent by the instrument's microprocessor system analyzing the incoming data after each frequency sample. In the PS case, the sum of all this analysis time appears at the end of a measurement, the measurement itself is much faster. This is an advantage when studying non steady state systems. For this application the PS method was developed further by Schindler and Popkirov [2]. A version of PS can be performed with the IM6, extended by the TR8M transient recorder module.
t must be mentioned, that regarding non steady state behavior, there are several possibilities to increase the reliability of the AC impedance results, if measurement time reduction is impossible:
a) IM6 on-line drift correction: Periodic stimulation offers the detection of non steady state behavior of the investigated system. The IM6 performs an automatic drift correction in the lower frequency mode.
b) LKKT (Logarithmic Kramers Kronig Transform): By means of the LKKT [3] not only violations of the EIS fundamentals of the examined system may be detected. The LKKT version in the THALES analysis software allows the reconstruction of non-affected spectra, if the violations are not too strong.
c) Interpolation of impedance spectra series versus time: Series measurements vs. time offer a further possibility to correct impedance spectra of drifting systems. Under the assumption of slow rates of changes the data may be corrected. The spectra can be transformed by an interpolation routine generating new ones, belonging to unique times. Finally each of those spectra will represent a certain state of the electrode. [4,5] This method is also available in the THALES software package.
[1] Ruben H. Milocco: Minimal measurement time in electrochemical impedance identification. Electrochimica acta, Vol.39, No.10, pp 1433-1439, 1994
[2] G.S. Popkirov and R.N. Schindler: A new impedance spectrometer for the investigation of electrochemical systems. Rev. Sci. Instrum. 63 (11), 1992
[3] H. Gφhr, B. Rφseler and C.A. Schiller: Advantages of Hilbert Transform compared with Kramers-Kronig Rule when examining the causality of electrochemical impedance spectra. Presented at the 46th ISE Meeting, Xiamen, China, 1995
[4] Z. Stoynov: Nonstationary impedance spectroscopy. Electrochem. Acta 38, no. 14, pp. 1919-1922 (1993)
[5] H. Gφhr, H. Bode, A. Burghart and C.A. Schiller: Impedance measurements at electrodes of continuously changing state. Presented at the 2nd EIS Symposium, Santa Barbara, USA, 1992
back
FAQs
What influence have connection cords between cell and impedance analyzer to the measurement data?
Answer...
Can you export a graphic created with the IM6 to PC programs?
Answer...
When I'm examining objects dominated by double layer capacities, my cyclic voltammograms (CV) sometimes show strange effects. What is the rea-son for these effects?
Answer...
How can I reduce the measuring time for my impedance spectra?
Answer...
What influence have connection cords between cell and impedance analyzer to the measurement data?
Answer...
Can you export a graphic created with the IM6 to PC programs?
Answer...
When I'm examining objects dominated by double layer capacities, my cyclic voltammograms (CV) sometimes show strange effects. What is the rea-son for these effects?
Answer...
How can I reduce the measuring time for my impedance spectra?
Answer...
