Emerson 396VP, 398VP, 397 instruction manual Section Maintenance, Sensor is in Pressurized System

MODEL 396/396VP/397/398/398VP	SECTION 4.0
	MAINTENANCE

SECTION 4.0

MAINTENANCE

4.0MAINTENANCE. 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.

4.1ELECTRODE 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.

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.

4.2AUTOMATIC 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:

RT=Ro [l+R1 (T-20)]

Where RT = Resistance

T = Temperature in °C

Refer to Table 4-1 for Ro and R1 values

TABLE 4-1. Ro and R1 Values for

Temperature Compensation Elements

Temperature		Ro		R1
Compensation Element
3K		2934		.0045
PT-100		107.7		.00385
TABLE 4-2.Temperature vs Resistance
of Auto T.C. 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

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