Emerson Rosemount 398R, Reference Manual, Page: 41 / 56

Reference Manual Maintenance
00809-0100-3098 February 2019
Maintenance 35
6.6 Rejuvenating Reference Electrodes
Occasionally, a poisoned or plugged reference electrode can be reconditioned. Although the
electrode seldom recovers completely, the procedure might extend the life of the sensor by a few
weeks.
a. Clean the sensor as thoroughly as possible.
b. Soak the sensor for several hours in a hot (NOT BOILING) 3% potassium chloride solution.
Prepare the solution by dissolving 3 g of potassium chloride in 100 mL of water.
c. Soak the sensor in pH 4 buffer at room temperature overnight.
d. Calibrate the sensor in buffers and retest it in the process liquid.
6.7 Temperature Element
Rosemount 398R/398RVP pH sensors produce a voltage which the instrument converts to pH using
a temperature-dependent factor. A Pt 100 or a Balco 3K RTD built into the sensor measures
temperature. To permit cor-rection for changes in lead resistance with temperature, a three-wire
configuration (Figure 6-4) is used. To check the RTD, disconnect the leads and measure the
resistances shown. The measured resistance should agree with the value in the table to within
about ±1%. If the measured resistance is appreciably dif-ferent (between 1 and 5%), the discrepancy can
be calibrated out. See the instrument Reference Manual.
Table 6-1: Resistance as a Function of Temperature for Selected RTDs.
Temperature °C Resistance (Ohms) ±1
3K PT-100
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
Figure 6-4: Three-wire RTD circuit