TM 10-6630-222-12&P
0004 00
OPERATOR AND UNIT MAINTENANCE MANUAL
(INCLUDING REPAIR PARTS AND SPECIAL TOOLS LIST)
FOR WATER QUALITY ANALYSIS SET: PURIFICATION (WQAS-P)
DESCRIPTION AND USE OF CONTROLS AND INDICATORS FOR ULTRAMETER 6P
a.
Reference Junction
The most common sensor problem will be clogged junction because a cell was allowed to dry out. The symptom is a drift
in the "zero" setting at 7 pH. This is why the Ultrameter does not allow more than 1 pH unit of offset during calibration.
At that point the junction is unreliable.
b.
Sensitivity Problems
Sensitivity is the receptiveness of the glass surface, which can be diminished by a film on the surface, or a crack in the
glass. These problems also cause long response time.
c.
Temperature Compensation
pH sensor glass changes its sensitivity slightly with temperature, so the further from pH 7 one is, the more effect will be
seen. A pH of 11 at 40C would be off by 0.2 units. The Ultrameter senses the cell temperature and compensates the
reading.
B.
ORP/Oxidation-Reduction Potential/REDOX
1.
ORP as an Indicator
ORP is the measurement of the ratio of oxidizing activity to reducing activity in a solution. It is the potential of a solution
to give up electrons (oxidize other things) or gain electrons (reduce).
Like acidity and alkalinity, the increase of one is at the expense of the other, so a single voltage is called the Oxidation-
Reduction Potential, with a positive voltage showing a solution wants to steal electrons, (oxidizing agent). Chlorinated
water will show a positive ORP value, for instance.
2.
ORP Units
ORP is measured in milli-volts, with no correction for solution temperature. Like pH, it is not a measurement of
concentration directly, but of activity level. In a solution of only one active component, ORP does indicate concentration.
Also, as with pH, a very dilute solution will take time to accumulate a readable charge.
3.
The ORP Sensor
An ORP sensor uses a small platinum surface to accumulate charge without reacting chemically. That charge is measured
relative to the solution, so the solution "ground" voltage comes from a reference junction same as the pH sensor uses.
4.
The ORP Sensor
The pH/ORP Sensor Top View figure shows the platinum button in a glass sleeve. The same reference is used for both the pH
and the ORP sensors. Both pH and ORP read out 0 for a neutral solution. Calibration at zero compensates for error in the
reference junction.
A zero calibration solution for ORP is not practical, so the Ultrameter uses the offset value determined during calibration to
7 in pH calibration (pH 7 = 0 mV). Sensitivity of the ORP surface is fixed, so there is no gain adjustment either.
5.
Sources of Error
The basics are presented in pH/ORP because sources of error are much the same as for pH. The junction side is the same,
and though the platinum surface will not break like the glass pH surface, its protective glass sleeve can be broken. A
surface film will slow the response time and diminish sensitivity. It can be cleaned off with detergent or acid, as with the
pH glass.
END OF WORK PACKAGE
