The D175 Electrode Impedance Meter from Digitimer is a compact battery powered device designed to allow checking of electrode impedances prior to recording or stimulating through surface electrodes attached to the skin.
In the case of electrical stimulation, high electrode impedances can reduce the amount of current that can pass through the target tissue, resulting in lower than expected stimulation and a poor evoked response. Likewise, high impedances will reduce the signal to noise ratio and therefore the quality of recordings, such as ECG, EEG and EMG that rely on low impedance surface electrodes.
The D175 Electrode Impedance Meter, is a tool that allows suitably trained operators to rapidly assess surface electrode impedances in order to decide if they are low enough for the procedure being undertaken. It features green/red bi-colour LEDs allowing the operator to set a threshold impedance at which an LED will light as red rather than green. This facilitates electrode impedance checking, by providing the operator with an easily visible indication of “good” (green) or “bad” (red) electrode impedances. The impedance values that the D175 can display are 0, 0.5, 2, 3, 5, 7, 12, 20, 30, 50 kohm.
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The D175 measures the total impedance through a pair of electrodes placed on the skin, by passing a very small biphasic (charge neutral) current between them. Once electrodes have been positioned on the recording/stimulation sites, a pair of electrode lead wires should be connected to the RED and BLACK electrode sockets (1.5mm DIN 42802) on the top of the D175. The sockets are colour coded for no reason other than to help the user identify which electrode is being used as the reference. With the electrodes connected and with the D175 switched ON (and in METER MODE), the D175 should display the measured impedance. If the 7kΩ LED is lit, the actual total impedance lies somewhere between 5 and 7 kΩ.
Having identified one or more electrodes that have a “good” impedance level, these can then be used as reference electrodes for the remainder. If bad/high impedances are found, it may be necessary to replace one or both of the electrodes following additional skin preparation, according to the standard recommended practice.”
When stimulating, a lower electrode impedance reduces the voltage that a constant current stimulator needs to use to pass the requested current. A low impedance also allows the maximum current to be higher before the stimulator goes “out of compliance”. For recording electrodes, lower impedances can drastically improve the signal to noise ratio of the recordings and good skin preparation and cleaning is always recommended in order to obtain the best quality recordings.
The two electrode sockets on the D175 are a male 1.5mm DIN42802 connector which is a type commonly used on medical electrodes which for safety reasons need to have touch-proof connectors.