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38 Maintenance
NOTE: Erroneous pH results may result immedi ately after acid soak, due to reference junction
potential build-up. Replace the sensor if cleaning does not restore sensor operation.
5.4 396P and 396PVP ORP
5.4.1 Platinum Electrode Check
The platinum electrode may be checked as follows: There are two types of standard solutions
which may be used to check the ORP electrode/transmitter system.
Type 1: One type of commonly used ORP standard solution is the saturated quinhydrone solu-
tion. Refer to Section 5.2.
Type 2: A second ORP standard solution is the Ferric-Ferrous Ammonium Sulfate Solution (PN
R508-16OZ), and it can be ordered as a spare part; otherwise, it can be prepared from the
following recipe: Dissolve 39.2 grams of reagent grade ferrous ammonium sulfate, Fe(NH4)2
(SO4)2 • 6H2O and 48.2 grams of reagent grade ferric ammonium sulfate, FeNH4(SO4)2 •
12H2O, in approximately 700 milliliters of water (distilled water is preferred, but tap water is
acceptable). Slowly and carefully add 56.2 milliliters of concentrated sulfuric acid. Add sufficient
water to bring the total solution volume up to 1000 ml. This standard ORP solution, although not
as simple to prepare as the quinhydrone recipe, is much more stable, and will maintain its milli-
volt value for approximately one year when stored in glass containers. This solution (ferric/ferrous
Temperature °C Resistance (Ohms) ±1%
3K Pt100
0 2670 100.0
10 2802 103.8
20 2934 107.7
25 3000 109.6
30 3066 111.5
40 3198 115.4
50 3330 119.2
60 3462 123.1
70 3594 126.9
80 3726 130.8
90 3858 134.6
100 3990 138.5
Temperature
Compensation Element
R
o
R
1
3K 2934 .0045
Pt100 107.7 .00385
TABLE 5-2. Temperature vs Resistance of Auto T.C. Elements
TABLE 5-1. Ro and R1 Values for Temperature Compensation Elements
CAUTION
The solution used during the following check is an acid and should be handled with care. Follow the
directions of the acid manufacturer. Wear the proper protective equipment. If contact with skin of
clothing is made, immediately rinse with plenty of clean water.
Section 5: Maintenance 396P + 396PVP Sensors Instruction Manual
August 2013 LIQ_MAN_396P_396PVP