This is a modified part of a little report I wrote on the ECG project. Please
have the slightly formal tone excused, and any mistakes that I might have made
Standard wet electrode, top and bottom side. Source: cardiologysupplies.com
Measuring the electric activity in the heart on the surface of the skin is possible because the body acts as a conductor. An electrode, as shown in the figure above, is used to tap into the activity. The electrode can be either dry or wet. A dry metal electrode is any conductive metal in direct contact with clean skin. A wet electrode is more common. It is a pad with a plate of metal together with the salt of the metal, usually silver and silver chloride, covered by electrolyte in the form of a sticky gel. Wet electrodes provide far better contact with the skin under movement than dry metal. For the ECG to perform as expected, it is important that the impedance from underneath the skin to the input of the instrumentation amplifier is as equal as possible for both leads, otherwise common mode currents or bias currents may produce a large differential voltage at the amplifier inputs.
Is any wet electrode good enough? During testing, I found that cheap electrodes from ebay performed far worse than the ones from known suppliers like 3M and Philips. The signal would be completely out of range for the ADC. This is probably because cheap electrodes have larger variances in the impedance of each pad.
If you use quality electrodes and still have problems with signal offset, try doing the following:
- Shave the area where the pad is attached
- Rub the skin gently with fine sand paper
- Clean the skin with alcohol
- Use sports tape to secure the pad and wires
Some pads also cause an allergic reaction or irritate the skin. Make sure to change the electrode periodically according to the manufacturer if you are going to wear it for a long period of time.