How do I perform the one-bucket test to identify faulty electrodes? Or, why does the blue CM in Range flicker and the signals pulsate once every half second?
The blue CM in Range light reflects two things: 1) whether the CMS and DRL electrodes are adequately connected to the participant’s body, and 2) whether all of the other active electrodes, cables and connectors are intact. If any of these are NOT true, then the blue CM in Range light will flicker and the signals and offsets will pulsate once every half second until the problem is rectified. We call this state CM Out of Range (CMOR). The blue LED labeled CM in Range on the front panel of the AD box is reflected on the top right of the ActiView software display. This makes it easy to recognize there is a problem. To identify which electrodes are involved, use the one-bucket test.
- Fill a plastic or glass container with about 1/2 gallon of tap water with roughly a teaspoon of Na/Cl (non-iodized table salt): this represents a virtual patient
- Connect CMS/DRL to the system, turn power on and submerge only those two electrodes in water. If the CM in range light (blue LED) does not come on, then CMS/DRL is faulty or something inside the box is broken. Stop the test. If the blue LED light does come on, continue to the next step.
- Connect the questionable electrode set to the system and submerge all electrode contacts in water and observe the electrode offsets in ActiView>Electrode Offsets tab. If the CM in range light goes out, then remove half of the electrodes in that set from the water. If the CM in range light comes on, then there is a problem in one of the electrodes that was removed from the water. In that case, switch the two sets of electrodes (take out those that are in the water and replace them with those that were out of the water).
- Repeat this step of removing half the electrodes until one or more faulty electrodes have been identified.
- Connect only CMS/DRL to the AD box and submerge those electrodes first in a bucket of water.
- If the blue CM in Range light comes on, then connect the other electrodes you want to test into the AD box, and submerge them in the same bucket of water.
- If the blue light remains on, then click to the Electrode Offset tab and make sure the offsets for the channels to which the suspect electrodes are connected are at a stable offset level less than +-40 mV.
- If you see any connected channels with offset greater than +-40 mV but less than +0-262 mV, they may have contaminated electrode pellets.
- If you see any connected channels with offset at -262 mV, most likely the electrode on that channel has two broken wires or connector pins.
- If all offsets are less than +-40 mV, then there are no catastrophic problems with the electrode electronics, electrode cables, connectors or AD box.
- To diagnose further, check the signal in the monopolar display page with a scale of 100 uV/div while showing between 8 and 64 channels (use Channels selector at left). At this scale, it is perfectly normal for the signals to be drifting slightly across the screen in the first few minutes following placement in water.
- Normally functioning electrodes will yield a time-varying voltage that is less than a few mV in amplitude.
- Abnormal / malfunctioning electrodes may show a flat line, a high-frequency / broadband interference, or a low-frequency (1/f) interference due to a variety of possible causes.
- A truly flat-line would only appear if the electrode also has an offset at -262 mV.
- A high frequency / broadband signal may result from ionic contamination of the electrode pellet or loss of Chloride from the electrode pellet.
- A low-frequency (wandering) signal can result from more than one cause having to do with a compromised electrode pellet.
- For the last two situations, brushing lightly with a soft toothbrush, rinsing in water, and soaking in non-Iodized NaCl solution for a few minutes may correct the problem.
- Otherwise, the electrodes may need to be replaced.
Once you identify the faulty electrodes, contact us at via the Request Info form and let us know the serial number of the electrode, and if it is a set of electrodes on a ribbon cable let us know which electrode is (or electrodes are) causing the problem. On occasion, this type of problem may be caused by a loose or damaged connector on the ribbon cable, but this is rare. Most often, the problem is caused by a single broken wire on an electrode. If both wires on an electrode are broken, then the CM in range light will not go off, but the offset on that channel will read -262 mV (the negative extent of the input range).