FAQ - General

There are two major servo applications for the Solartron FRA (the instruments specifically designed for servo applications are the 1250, 1253A and 1255). These are electrical and mechanical. A servo application might be a 'test control loop' as used in aircraft control surfaces (mechanical application) or monitoring loss of phase in hi-fi amplification (electrical application).

The main emerging applications for impedance measurement are predominantly materials-based (ceramics, piezoelectric materials, quartz crystal microbalances and polymers). There also appears to be a growing trend in the monitoring of curing rates of thermoplastics and adhesives (dipole alignment, molecular viscosity and mobility). 
Bioimpedance measurement is still in its infancy but from current predications it appears that this may be a large market in years to come. At present it is being used to measure drug delivery rates, the amount of oxygen present in transplant organs and the influence of drugs on tissue healing rate (skin impedance measurements). 

The use of impedance measurement for materials used in liquid crystal displays (LCDs), is beginning to emerge in countries such as Japan/Korea, where scientists are using the technique to monitor material characteristics in different liquid crystal phases. 

The main techniques used in corrosion measurement are available in CorrWare 3.5d. The use of electrochemical techniques in corrosion analysis provides complementary information to that obtained from coupon testing and salt spray analysis. These techniques only provide average corrosion rate information over long time periods. 
As a short term diagnostic analysis tool the techniques are invalid. Within the chemical industry (gas and oil pipelines) there is a demand for corrosion analysis techniques that can be performed on-line at regular time intervals. Other manufacturers require techniques which can determine the effectiveness of inhibitor systems such as the coatings and paints industries. 

The information obtained indicates the effects of localized corrosion and will also allow detection of the changes due to flow rate and viscosity. A variety of electrochemical analysis techniques are available including: Linear polarization resistance (LPR) measurements, electrochemical noise (ECN) tests, harmonic analysis and electrochemical impedance spectroscopy (EIS). Each of these will provide valuable data which, when used effectively, will minimize corrosion and increase plant efficiency and product lifetime. Signal measurements are often in the microvolts and nanoamps region. The best results are obtained from readings acquired simultaneously (i.e. current and voltage). 

The use of this technique enables the electrochemist to determine information relating to the corrosion rate, passivity and pitting susceptibility. Using the 1287A Electrochemical Interface or the 1285A Potentiostat plots of log[I] vs. E (termed potentiodynamic scans) may be obtained. The instrument is set to a series of pre-defined scan limits. 
The starting point will be set depending on the nature of the information required. It is normal to start the scan at a potential close to that of the open circuit potential which the instrument software provides. The limits are set to scan to provide either anodic or cathodic information. 

Experimental limits determine that once a certain current density has been passed progressing further is self defeating (i.e. anodic scan where pitting has been initiated, or a cathodic scan where the potential is sufficiently negative for hydrogen evolution to occur). The scan rate is typically expressed in mV/sec. 
At higher scan rates the system under evaluation will not have time to stabilize at each potential. Scanning at higher potentials will have the effect of pushing the values obtained (icorr, Ecorr) to a more positive value. The ASTM (American Standard of Testing and Methods) stipulates 0.1667 mV/sec (10 V/min) for the analysis of corroding systems. 

The technique can be used to determine icorr, which in turn can be used to calculate the corrosion rate. Scans are performed close to the open circuit potential (-200mV for an anodic scan and +200mV for a cathodic scan). The corresponding trace must have a point where the current measured is equal to zero. 

The Tafel plot extrapolated to the zero current/potential gives a set of co-ordinates relating to Ecorr (x axis) and icorr (y axis). The icorr value may be calculated using the Tafel constants (βa + βc) and Rp. The value for βa can be determined by taking the slope for the anodic portion of the curve and βc for the corresponding cathodic part. 
Using the Rp value and the Stern-Geary equation the value for icorr can be determined. The corrosion rate can then be calculated from this value in mm per year. 

Stern-Geary Equation 

From the Ecorr value it is possible to obtain a value for icorr which in turn is related directly to the rate constant of the electrode reaction. 

icorr = corrosion current (A cm-2) 
EW = equivalent weight of the sample (g) 
d = density of sample (g cm-3) 

The CorrWare software package used with the 1287A Electrochemical Interface and 1285A Potentiostat allows autotafel fitting. Individual markers are moved to the appropriate positions on the plot and the Ecorr and icorr values calculated. If the material density and surface area are known it is also possible to compute the corrosion rate. The plotting of different comparable metal electrodes using the same physical/chemical environment provides information relating to general corrosion mechanisms and their rate. Techniques such as Tafel plots cannot be used to provide information relating to corrosion types such as pitting and crevice corrosion, although this information is more readily obtainable using electrochemical noise measurement, where the current and voltage are measured simultaneously in a static (unperturbed) system.

The Dielectric Interface 1296A was launched in July 1997. This instrument is designed to extend the range and capabilities of the 1260A and is specifically aimed at materials testing. Many materials have properties of low conductance (high impedance) and low capacitance, these are often referred to as dielectrics, although many materials not considered to be dielectrics also often exhibit these same properties. The dielectric interface extends the operating range of the 1260A in many ways: 

• The 1296A allows conductivity measurements down to 0.1pMho
• Accurate tan delta measurements down to 0.0001
• This measurement facility is available over a frequency range from 100µHz - 100MHz
• Extends the impedance range of the 1260 to 10Tohms (T = 10^12)
• 1000V bias available either AC or DC via external power supply unit
• Sample can be temperature controlled during analysis
• Internal and external reference capacitors provide increased measurement accuracy

The dielectric interface has the option of selecting internal and external reference capacitors, enabling virtually all measurement needs to be met. 

The dielectric interface is set to address a broad range of applications. These include:

• Relaxation processes on the molecular dynamics of liquid crystals, polymers and liquids
• Charge transport in semiconductors, organic crystals, ceramics, etc
• Analysis of chemical reactions during polymerization and curing processes
• Structural material properties such as phase compositions, phase transitions and crystallization processes
• Non-linear electrical and optical effects
• Novel gas and liquid sensors
• Characterization of insulating and semiconductor materials

Dielectric interface measurements at high frequencies (<10MHz) require the use of external references. These references should be connected with short leads that are of a comparable length to those connecting the sample to the interface. This will minimize any phase shift at high frequencies and therefore improve the sample data obtained. 

Further to the recommendation above it is good laboratory practice for dielectric measurements that two sets of readings are taken, one with the sample present and a further one without the sample. This allows comparisons to be drawn so that any experimental errors present can be eliminated. 

The minimum requirements to run CorrWare and ZPlot are as follows: 

IEEE-488.2 Interface (National Instruments GPIB Board) USB-GPIB, PCI-GPIB, PCMCIA-GPIB PC with:

• Microsoft Windows 2000/XP Pro
• 1 GHz Pentium or equal processor
• 512 MB RAM
• CD
• 2 USB Ports

Solartron does sell electrodes. All of the electrodes cells and accessories supplied through our sister company, Princeton Applied Research, are now available through your Sales Representative. Some of the accessories available include:

• Reference electrodes
• Rotators
• Solid Electrodes
• Cells (Corrosion, micro, flat, polarographic)
• Faraday Cages

For more complete details on their accessory lineup please go to this link.

If the instrument appears to be malfunctioning, please contact technical support to determine if the unit does need service. If your instrument does require service or calibration, please contact your local distributor for details.

Driven shields can be found on the following Solartron instruments, 1280 'Lab in a Box', 1287A Electrochemical Interface and the new 1285A Potentiostat. The use of driven shield technology is to minimize stray impedances but input and cable capacitance is a possible source of error at high frequencies. There are two main methods used to minimize stray impedances. Electrometers are high input impedance voltage buffers positioned close to the sample. Driven shields are driven with a copy of the original signal waveform to minimize the cable capacitance while high impedance buffers are kept within the instrument. With driven shields the signal on the cable inner is the same as the cable outer and, as a result, cable capacitance is not measured and therefore no errors are introduced.

The advantages of the driven shields technique is that stray input and cable capacitance are minimized; these being a particular source of error at high frequency.

Driven shields should never be shorted (connected together) as this defeats the object of having the technology available. Different cables will have varying capacitances connected with them and coupling will increase the overall effect of stray capacitances instead of reducing it. If the electrochemical system/cell in use requires earthing this can be achieved by using the earth connector on the front/rear panel of the instruments.

The polarization voltage (working potential) of Solartron potentiostats (1285A, 1287A and 1280) is +/- 14.5V. This range is wider than most commercially available potentiostats, allowing measurements of an individual battery in a 12V accumulator array (the open circuit potential of a 12V array is nominally 13.6V). Although this is generally sufficient, higher voltages are sometimes required (e.g. titanium oxide films have a breakdown potential of 15-20V). It is possible to achieve these higher voltages by altering the nature of the reference electrode. 

If, for example, the highest potential that can be applied between the working electrode and reference electrode is 14.5V when using a standard hydrogen electrode (SHE), the applied potential is said to be 14.5V with respect to the SHE. If the potential of the reference electrode was increased to a potential of 9V (by means of a series battery as shown in the next diagram) for example then the new potential would be 9V higher than the applied potential with respect to SHE (i.e. if 14.5V is applied with a 9V reference then the total potential will be 23.5V). To produce this increased polarization potential the convention is to attach the battery positive terminal to the RE2 connector on the Solartron potentiostat, with the negative terminal attached to the reference electrode as shown. Potentials less than 14.5V can be achieved by attaching the negative terminal to RE2 allowing the positive terminal to be connected to the reference electrode, giving a range of between -23.5 to + 5.5 volts. 

Make sure your connections are tight (use a screw driver) 

Use only even numbered addresses for your Solartron Instruments. 

Make sure you have the latest instrument driver appropriate for your operating system and your GPIB device. If you’ve changed computers recently this is often a problem. You can verify the correct driver at the National Instruments web site. 

Make sure your instruments are powered up. If you have changed your GPIB address while your instrument is powered up you will need to power down and power up your instrument to initialize the new GPIB address. 

Make certain you have the same GPIB address in software as you do on your instrument as well as the GPIB card address. If you have one card the card address should be 0. If you have more than one card remember to set the card address in software for a specific instrument to the card address it is connected to physically. 

Use the National Instruments measurement and automation software to verify your GPIB software and Hardware-Go to HelpàTroubleshootingàNI488.2 Troubleshooting Wizard. Search for instruments (right click on GPIB folder) will find powered up connected instruments on the GPIB. It will typically not find your Solartron Instruments, but it will find active addresses. This can be useful in determining the address assigned to an instrument. Simply turn off all but one of your instruments and try again. The even numbered address is the instrument left on. 

Once you have the GPIB addresses for each instrument. Setup the Solartron (Celltest, SMaRT, FRA) or Scribner software (ZPlot, CorrWare, Multistat) with the correct Instruments and correct GPIB addresses. To test the GPIB there is either a button marked search or Test GPIB found where you set the GPIB address in software. 

Make certain you have the correct instrument type selected in the Solartron or Scribner software. 

Make sure you don’t have more than one software suite trying to control the same instrument(s) (e.g. Celltest and Multistat). Only one software suite at a time can control the instruments. 

If this does not help then consult the National Instruments Troubleshooting DAQ and GPIB Installation Problems web page. 

Contact technical support if you still can’t find the answer.