YSI 85 - Manual (Page 29)

YSI, Incorporated Model 85 25
SECTION 10 PRINCIPLES OF OPERATION
The dissolved oxygen sensor utilizes an oxygen permeable membrane that covers an electrolytic cell
consisting of a gold cathode and a porous silver anode. This membrane acts as a diffusion barrier
and an isolation barrier preventing fouling of the cathode surface by impurities in the environment.
Upon entering the cell through the membrane, oxygen is reduced at an applied potential of -0.8 V
referenced to the silver electrode. The reduction current at the cathode is directly proportional to the
partial pressure of oxygen in liquid (expressed as %-air saturation) which is proportional to the
concentration of dissolved oxygen (in mg/L) at a particular temperature. Thus the same partial
pressure of oxygen (% air-saturation) in liquid gives different concentrations of dissolved oxygen
(mg/L) at different temperatures because of the different solubility’s of oxygen at different
temperatures.
The conductivity cell utilizes four pure nickel electrodes for the measurement of solution
conductance. Two of the electrodes are current driven, and two are used to measure the voltage drop.
The measured voltage drop is then converted into a conductance value in milli-Siemens (millimhos).
To convert this value to a conductivity (specific conductance) value in milli-Siemens per cm
(mS/cm), the conductance is multiplied by the cell constant that has units of reciprocal cm (cm-1).
The cell constant for the Model 85 conductivity cell is 5.0/cm + 4%. For most applications, the cell
constant is automatically determined (or confirmed) with each deployment of the system when the
calibration procedure is followed. Solutions with conductivity’s of 1.00, 10.0, 50.0, and 100.0
mS/cm, which have been prepared in accordance with recommendation 56-1981 of the Organisation
Internationale de Métrologie Légale (OIML) are available from YSI. The instrument output is in
µS/cm or mS/cm for both conductivity and specific conductance. The multiplication of cell constant
times conductance is carried out automatically by the software.
10.1 TEMPERATURE EFFECT ON CONDUCTIVITY
The conductivity of solutions of ionic species is highly dependent on temperature, varying as much
as 3% for each change of one degree Celsius (temperature coefficient = 3%/C). In addition, the
temperature coefficient itself varies with the nature of the ionic species present.
Because the exact composition of a natural media is usually not known, it is best to report a
conductivity at a particular temperature, e.g. 20.2 mS/cm at 14 C. However, in many cases, it is also
useful to compensate for the temperature dependence in order to determine at a glance if gross
changes are occurring in the ionic content of the medium over time. For this reason, the Model 85
software also allows the user to output conductivity data in either raw or temperature compensated
form. If "Conductivity" is selected, values of conductivity that are NOT compensated for
temperature are output to the display. If "Specific Conductance" is selected, the Model 85 uses the
temperature and raw conductivity values associated with each determination to generate a specific
conductance value compensated to a user selected reference temperature (see Advanced Setup)
between 15 C and 25 C. Additionally the user can select any temperature coefficient from 0% to 4%
(see Advanced Setup). Using the Model 85 default reference temperature and temperature
coefficient (25 C and 1.91%), the calculation is carried out as in equation (1) below: