Temperature Vs. Resistance - Emerson ROSEMOUNT 320HP Instruction Manual

MODEL 320HP
4.4.3. Glass Electrode Check.
The glass electrode may be checked as follows: Refer to
Figure 4-3.
Failure to connect the glass electrode shield to the reference
lead as shown will make the reading very noise sensitive, but
this will probably not interfere with the basic test. Bad glass
electrodes show limited response, or no response at all; they
seldom become noisy. A steady reading of 3 to 6 pH regard-
less of the buffer solution used indicates a cracked glass
electrode bulb. A steady reading near 7.0 pH can be caused
by a coated or internally shorted pH electrode, connector or
cable.
1. Glass electrodes can sometimes be rejuvenated by
washing in strong detergent and then a 10 to 20% HCI
solution. If the sample stream contains large percentages
of non-aqueous liquids, such as alcohols, glass dehydra-
tion may occur, but recovery will be rapid in a buffer solu-
tion.
2. If the electrode is worn out, response will be unalterably
slow and short of span. It must be replaced in this case.
4.4.4 Liquid Junction.
This plug must remain porous. To check it out, remove the
plug and top off the reference cell to the brim with deionized
water. Dry the plug and screw it back into the cell while
watching the tip. DO NOT use thread sealants other than
TEFLON tape on the threads of the plug. Water beads should
be forced through the foam, not just around the edges or
threads. If it looks clogged, replace it.
4.4.5 Automatic Temperature Compensator.
The temperature compensator element is temperature sensi-
tive 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 tem-
perature for a 3K and Pt-100 element and can be determined
according to Table 4-3 or the following formula:
R T =R o [l+R 1 (T-20)]
=
Where R T Resistance
T = Temperature in °C
o a
Refer to Table 4-4 for R
18
nd R 1 values.
MAINTENANCE AND TROUBLESHOOTING
TABLE 4-3
TEMPERATURE vs RESISTANCE OF AUTO
T.C. ELEMENT
Temperature °C
0
10
20
25
30
40
50
60
70
80
90
100
TABLE 4-4
R o and R 1 VALUES FOR TEMPERATURE
COMPENSATION ELEMENTS
Temperature
Compensation Element
3K
PT-100
SECTION 4.0
Resistance
(Ohms) ± 1%
3K PT-100
2670 100.0
2802 103.8
2934 107.7
3000 109.6
3066 111.5
3198 115.4
3330 119.2
3462 123.1
3594 126.9
3726 130.8
3858 134.6
3990 138.5
R o R 1
2934 .0045
107.7 .00385