Rejuvenating Reference Electrodes; Temperature Element - Emerson Rosemount 398R Reference Manual

Reference Manual
00809-0100-3098
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.
b.
c.
d.
Table 6-1: Resistance as a Function of Temperature for Selected RTDs.
Temperature °C
0
10
20
25
30
40
50
60
70
80
90
100
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
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.
Figure 6-4: Three-wire RTD circuit
Maintenance
Clean the sensor as thoroughly as possible.
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.
Soak the sensor in pH 4 buffer at room temperature overnight.
Calibrate the sensor in buffers and retest it in the process liquid.
configuration (Figure 6-4) is used. To check the RTD, disconnect the leads and measure the
Resistance (Ohms) ±1
3K
2670
2802
2934
3000
3066
3198
3330
3462
3594
3726
3858
3990
Maintenance
February 2019
PT-100
100.0
103.8
107.7
109.6
111.5
115.4
119.2
123.1
126.9
130.8
134.6
138.5
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