TN 10 Data Logger and RMU Errors

Special precautions should be taken when using data loggers and remote monitoring units (RMUs) with reference electrodes.  The amount of current flowing through a reference electrode is inversely proportional to the circuit impedance.  This current causes the reference to polarize, i.e. shift potential, and the amount of shift is proportional to the amount of current flowing.  If the shift is small, or of short duration, the reference will usually recover.  When the shift is large and/or of long duration, the reference may be permanently damaged.

Most modern potential measuring devices have an input impedance of at least 10 MΩ in order to minimize IR drop error due to current flowing through the circuit.  These high input impedances are used during the measurement cycle to minimize errors from voltage drops in the measuring circuit while the measurement is actually being made.  When the unit is in stand-by or off mode, the impedance may be significantly lower (sometimes only a couple thousand ohms) depending upon the individual components used and the overall circuitry.  A drop in impedance during stand-by or off mode will not affected many transducers but a reference electrode will polarize if this occurs.  If a data logger or RMU is to be used with reference electrodes, the input impedance must be a minimum of 10 MΩ at all times including during active measurements, standing by between measurements, shut down with power connected or shut down with power disconnected.

Testing for changes in input impedance of a data logger or RMU as it goes through its cycles is relatively simple.  A 1½ volt battery is connected to the input terminals through a 10 MΩ resistor (see figure).  The voltage across this resistor is monitored with a portable voltmeter as the unit is put through its cycles.  This voltage should be about 0.15 volts for units claiming 100 MΩ input impedance during measurements.  If the measured voltage increases to greater than ¾ volt during standby or off cycles, then the unit is capable of damaging a reference electrode connected to it and should not be used.  Many RMUs have multiple independent input channels.  Each channel must be separately tested through all operating modes to be assured that all of the channels are suitable for reference electrodes.