Impedance testing

C. Kleesattel, "The Ultrasonic Contact Impedance Testing Method," The ECHO, Vol. 27, Krautkramer GmbH. 

Simplicity and reliability of operation make AC impedance measurements attractive as a technique in the evaluation of coating integrity. As opposed to classical salt spray test, analysis times are shorter with the AC impedance technique and quantitative data are obtained permitting relevant mechanistic Information to be derived. Impedance test methods are likely to find many applications in the resolution of unsolved practical problems ( .) ... 

Parameters such as ageing. Influence of pigment concentration and Influence of coalescence of emulsion paints can also be studied using AC Impedance test methods. 

Results of Impedance teste conducted on specimens with the polyurethane paint always showed capacitance In the pF cm range. In spite of no rinsing oTCrati n being carried out. The resistances are In the 10-10 0 cm range. 

With thermal surface impedance testing, heat is injected into the test objects surface from a hot gas pulse. The resulting surface temperature transient is analyzed to determine the bond quality in nearly real time. The surface temperature transients are sensed using an innovative noncontacting, emissivity-independent infrared sensor.27 This method is not adversely affected by surface blemishes or roughness. 

Preliminary conductivity measurements indicate that the polymers based on the anionic system are ionically conductive, whereas the nonionic based polymers are non-conductive. AC impedance tests were done on a thick film ( limn thick) using sodium sulfate as the electrolyte in a specially designed closed cell. The resistivity of polystyrene obtained from middle phase microemulsions was found to be in the rjange of lOMO ohm-cm, compared to lO o -10 2 ohm-cm for bulk polystyrene. A thin film of the polymer was also obtained on graphite electrodes by UV irradiation. Electrochemicd measurements using such polymer coated electrodes also suggest that the film is conductive. SEM micrographs before and after the electrochemical measurements indicate that the polymeric film is stable and porous. 

Figure 2. Bio-electronic measurement system (a) Electrical model [6], [7] of an electrode, (b) Electrical model of an electrode partially covered by cells (ff). (c) Circuit for cell-electrode impedance testing. Values for a 50 X 50 om electrode size are R =5 A kfl, Z(co)=Cj R, with = 0.37 nE and R = 25 Mf2.

Frequency Response Analysis is the most widely used technique for impedance testing. Similar to the lock-in technique, it can extract a small signal from a very high background of noise automatically... 

A 230V ring main circuit of socket outlets is wired in 2.5 mm single PVC copper cables in a plastic conduit with a separate 1.5 mm CPC. An earth fault loop impedance test identifies Zg as 1.15Verify that the 1.5 mm CPC meets the requirements of lET Regulation 543.1.3 when the protective device is a 30A semi-enclosed fuse. 

If the inspecting engineer has grounds to question the soundness and quality of these joints then the phase earth loop impedance test described later in this chapter should be carried out. 

The protective conductor resistance values calculated by this method can only be an approximation since the length of the CPC can only be estimated. Therefore, in this case, a satisfactory test result would be obtained if the resistance of the protective conductor was about 0.6 f2. A more precise result is indicated by the earth fault-loop impedance test which is carried out later in the sequence of tests. 

Ultrasonic Pulse Echo Contact Impedance Testing... 

In order that an overcurrent protective device can operate successfully, meeting the required disconnection times, of Regulations 411.3.2.2, that is, final circuits not exceeding 32 A shall have a disconnection time not exceeding 0.4 s. To achieve this, the earth fault loop impedance value measured in ohms must be less than those values given in Appendix 2 of the On Site Guide and Tables 41.2 and 41.3 of the lEE Regulations. The value of the earth fault loop impedance may be verified by means of an earth fault loop impedance test as described in Chapter 14 of this book. The formula is ... 

Electrochemical impedance tests usually investigate the interface between an electrode material and a solution (for example corrosion tests may investigate different coated metals in a salt solution, while battery/fuel cell tests may investigate different electrode materials in an electrolyte). Electrochemical impedance tests provide complementary information to that obtained from dc electrochemical techniques such as cyclic voltammetry, pulse voltammetry, ohmic drop analysis, and chronoamperometry. 

The potentiostat is a very important part of an electrochemical impedance test system. The potentiostat consists of the following elements ... 

One important capability of the potentiostat is to maintain the required dc conditions on the cell while the frequency response analyzer is performing the impedance (ac) analysis. The actual dc conditions required depend on the application. For tests on fuel cells it may be necessary to set up a particular dc steady state current to investigate the impedance of the cell under load conditions. In other cases it may be necessary to run the impedance test at the open circuit potential of the electrochemical cell, in which case the open circuit voltage is measured (Figure 3.2.1)... 

Figure 3.2.1. Setting up the dc conditions on the cell for an impedance test.

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