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MODEL 396/396VP/397/398/398VP SECTION 4.0
MAINTENANCE
4.0 MAINTENANCE. The Model 396, 396VP, 397,
398, and 398VP Sensors are disposable type sensors
and therefore require minimum maintenance. Every
sensor should be kept clean and free of debris and
sediment at all times. The frequency of cleaning by
wiping or brushing with a soft cloth or brush is deter-
mined by the nature of the solution being measured.
The sensor should be removed from the process peri-
odically and checked in buffer solutions.
SAFETY WARNING
SENSOR IS IN PRESSURIZED SYSTEM!
May cause spray and bodily injury. Before
removing sensor, be absolutely certain that
the process pressure is reduced to 0 psig
and the process temperature is lowered to a
safe level.
If the sensor will not calibrate, refer to your analyzer/
transmitter instruction manual for proper test proce-
dures. If it is determined that the sensor has failed, it
should be discarded and replaced.
4.1 ELECTRODE CLEANING. If the electrode is coat-
ed or dirty, clean as follows:
1. Remove the sensor from process.
SAFETY WARNING
SENSOR IS IN PRESSURIZED SYSTEM
may cause spray and bodily injury.
Before removing sensor, be absolutely cer-
tain that the process pressure is reduced to
0 psig and the process temperature is low-
ered to a safe level.
2. Wipe the glass bulb with a soft, clean, lint free cloth
or tissue. If this does not remove the dirt or coating,
go to Step 3. (Detergents clean oil and grease;
acids remove scale.)
3. Wash the glass bulb in a strong detergent solution
and rinse it in clean water. If this does not clean the
glass bulb, go to Step 4.
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. Do not let the solution come in contact with
skin or clothing. If contact with skin is made, immedi-
ately rinse with clean water.
SECTION 4.0
MAINTENANCE
4. Wash the glass bulb in a dilute 5% hydrochloric acid
solution and rinse with clean water. Soaking the
sensor overnight in the acid solution can improve
cleaning action.
NOTE
Erroneous pH results may occur immedi-
ately after acid soak, due to reference
junction potential build-up.
Replace the sensor if cleaning does not
restore sensor operation.
4.2 AUTOMATIC TEMPERATURE COMPENSATOR.
The temperature compensator element is temperature
sensitive and can be checked with an ohmmeter.
Resistance increases with temperature.
The 3K element will read 3000 ohms ±1% at 25°C (77°F)
and a Pt-100 will read 110 ohms. Resistance varies with
temperature for a 3K and Pt-100 element and can be
determined according to Table 4-2 or the following formula:
R
T
=R
o
[l+R
1
(T-20)]
Where R
T
= Resistance
T = Temperature in °C
Refer to Table 4-1 for R
o
and R
1
values
TABLE 4-1. R
o
and R
1
Values for
Temperature Compensation Elements
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
Temperature
Compensation Element
R
o
R
1
3K 2934 .0045
PT-100 107.7 .00385
TABLE 4-2.Temperature vs Resistance
of Auto T.C. Elements