The ESA400™ Electrochemical Signal Analyzer is designed to address both the past problems and the future needs of electrochemical noise measurements. Its primary goal is to assist in the evaluation of noise as a technique for the routine study of chemical processes by providing a convenient package for versatile data acquisition and sophisticated data analysis.

For acquisition, the ESA400™ partners with any of four Gamry Potentiostats to generate reliable data in either potentiostatic, galvanostatic, or zero resistance ammeter (ZRA) mode. Careful attention is paid to sample continuity, acquisition rate, filtering, and autoranging to provide the most accurate signal representation.

To transform the noise into information, the ESA400™ provides an impressive package of signal analysis tools: blockwise statistics, Fourier and MEM frequency domain analysis, correlation analysis, histograms, and the powerful JFTA (Joint Time- Frequency Analysis). These algorithms can be used to calculate quantitative results from the data. When there is information buried in electrochemical noise, the ESA400™ gives you the power to find it.

Electrochemical Noise Overview

Various physical and chemical processes can give rise to seemingly random low-frequency signals. These phenomena include pitting and crevice corrosion, uniform corrosion, coating failure, inhibitor activity, cracking, passive film stability, adsorption, and gas generation. The potential and/or current fluctuations from these stochastic processes, taken as a group, are referred to as electrochemical noise.

Three Electrode Biased ZRA/Electrometer. ---->

Noise signals may be acquired in several ways. Performing the experiment with two identical electrodes under open-circuit conditions with a Zero Resistance Ammeter/ Electrometer allows a measurement to be made with no external perturbation, closely simulating ambient real-world conditions. Both potential and current can be measured simultaneously.

The ESA400™ can also be used in our unique Biased ZRA mode. Imposing a potential between two identical electrodes tends to move the anodic corrosion processes to the positively polarized electrode. This insures that more of the relevant current is measured. It also provides a useful way to electrochemically stress a material to measure its resistance to localized corrosion.

Some researchers find it advantageous to study the system under either potentiostatic or galvanostatic control to accelerate a particular process such as passivation. In this case, the current and potential, respectively, are monitored versus time. The ESA400™ allows both potentiostatic and galvanostatic control in addition to ZRA mode.

The ESA400™ can also apply a computer-generated white noise signal to the system under test. While this may seem an anomaly for an electrochemical noise instrument, it is useful for improving the performance of the instrument when used for impedance analysis.

Data Acquisition Done Right

In all cases, it is extremely important to differentiate the signals generated by the chemical process from the electronic noise of the instrument. We pay special attention to maintaining data integrity by using filters tuned to the sampling rate according to the Nyquist anti-aliasing criterion, which states that signals at frequencies greater than ½ the sampling frequency will appear as lower frequencies. High frequency computer noise is effectively eliminated in the ESA400™ by a series of analog and digital filters. The filters are designed to be linear in phase, insuring that peak shapes and higher order statistical moments are not distorted.  The screen capture above depicts White Noise sampled at 100 Hz filtered (Red Dots) vs Unfiltered (Blue).

The Right Hardware for Signal Analysis

Electrochemical noise signals are often very small. The ESA400™ utilizes the offset and gain capability of the Gamry Potentiostat to achieve 1 µV and 100 fA resolution to observe even the most subtle interactions. Think of this as using a magnifying glass to zoom in on a relatively stationary portion of your signal. A unique DC offset circuit subtracts out the background level so you can subsequently apply a final amplification of x1, x10 or x100. Offset and gain are calculated and adjusted continuously in the background during the data acquisition.

To eliminate switching transients, a single potentiostat is dedicated to gathering data for one sample. To increase the productivity of your laboratory, the ESA400™ can control up to four potentiostats simultaneously , each running an independent noise experiment with different potentiostat ranges and settings.

The time-scale of an electrochemical noise measurement is difficult to predict. Accordingly, the ESA400™ has been designed to monitor potential and current signals continuously in an uninterrupted fashion. If you wish, you can collect data for months! This way, you won’t miss any long-term events that may require an initiation period. You are literally limited only by the free space on your hard drive! To conserve space on your hard drive, we save the data as a binary file of voltage and current records.

The data can be collected at a selectable sampling rate from 1000 Hz to 0.1 Hz. The choice of data acquisition rate is dependent on the time scale of the phenomena being studied. Electrochemical Impedance Spectroscopy performed using the Gamry EIS300™ is an excellent method to identify an appropriate sampling frequency.

Tools for Signal Analysis

Researchers are still studying signal analysis techniques to determine which are the most useful. To help you decide, we’ve supplied the ESA400™ with a versatile, inclusive collection of mathematical tools for signal analysis.

Data from noise experiments is acquired in the time domain, i.e., current and/or potential is measured versus time. For analysis, it is often convenient to convert this data into the frequency domain in which a function of the signal amplitude is plotted versus frequency. This spectral representation can be accomplished with the Fast Fourier Transform (FFT) or the Maximum Entropy

To help you recognize when a significant event has occurred, the ESA400™ continuously calculates several statistics on the I and V data streams. In addition a real-time FFT can be calculated. The statistical functions are calculated on user-specified blocks of raw data ranging from 32 to 16,384 data points per block. Up to four of the following calculations can be displayed in real time, one per screen quadrant.

The same statistics may also be calculated on previously acquired data stored in files.

V & I vs. Time:

RMS:

Variance^*:

Kurtosis:

FFT:

Block Average:

Standard Deviation^*:

Skewness:

Resistance:;

* Population-based definition

The Fourier Transform in the Analysis section provides the same analysis as in the Real-Time FFT. You may choose the width of the region being analyzed, the resolution, and the appropriate smoothing window to eliminate edge effects.

The ESA400™ introduces the Joint Time-Frequency Analysis, a powerful visual technique, for viewing and summarizing electrochemical noise data.

Traditionally, signals have been analyzed in either the time or the frequency domain. JTFA analyzes signals in both time and frequency domain at the same time. The amplitude of a signal is plotted as an intensity plot with the X-axis corresponding to time and the Y-axis corresponding to frequency.

In addition, the JFTA shows the original time series data and either instantaneous or overall spectra of the current and voltage signals for a powerful one chart summary of your signals.

This method gives the energy of the voltage and current signals occurring at a given frequency. It also gives the ratio of the two power spectra. The power of the voltage signal is defined by:
                                        
where is the Fourier transform of the signal.

Power Spectral Density, Maximum Entropy Method

The Power Spectral Density (PSD) when calculated via a Fourier Transform tends to be noisy at high frequencies. The MEM algorithm smoothes the PSD by fitting a time-domain signal of the form:
                                            
               .
The order, M, of the MEM can be adjusted.

This analysis shows the correlation between the current and voltage signals. It is defined as the Fourier Transform of the cross correlation function:
               
where m   measures the time interval, e.g., m sample periods between the two signals . When two signals are highly correlated, R_I,V increases as m decreases. Correlated V and I signals indicate that they are probably coupled through the electrochemical interface and, therefore, associated with electrochemical processes.

The impedance spectrum is by now well known to electrochemists and corrosion scientists. The ESA400™ calculates an impedance by ratioing the frequency domain representation of the voltage signal to that of the current signal. Both MEM and FFT spectra are calculated. Quite often, however, there is not enough noise present in laboratory environments to generate meaningful spectra. To obtain better accuracy in impedance spectrum, the ESA400™ can apply a computer-generated white noise signal to the system under test.

The histogram is a plot of the number of points at a given voltage or current for the entire data record. It is the distribution function on which the various statistical calculations are based.

The ESA400™ can count the number of excursions above or below a limit. This analysis is often used to characterize localized corrosion processes. The minimum width of peaks or valleys can be specified.

Often it is handy to remove a background drift in voltage or current. This is done by fitting a straight line to the data set and subtracting it from each data point. Linear Detrending is typically performed prior to calculation of the RMS value of the data.

Used to compare two non-identically scaled data sets, Data Normalization transforms each set to an equivalent set with a mean of 0 and standard deviation of 1. In this way, multiple data sets can all be plotted on the same "normalized" axes for ease of comparison.

The Analysis features of the ESA400™ can even be applied to data acquired with non-Gamry potentiostats. Data files in a tab-delimited format can be conveniently imported into the ESA400™ and processed normally. Data can also be exported from the ESA400™ in a tab delimited format to spreadsheets, databases, or other software packages.

The ESA400™ system, along with any Gamry Potentiostat, can be pre-configured and sold as a turnkey solution, integrated into a lunchbox type portable or desktop computer.

Gamry Instruments can supply complete systems including Potentiostat and Software installed in a desktopk, notebook, or portable computer. Custom computer configurations, software, training and installation are available by special order. Contact us for further details on these systems.

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Last revised on Monday, August 21, 2006