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Goldy
Locks Advice
In betting circles, sure
bets are called “locks.” As we all know,
there are no such things as sure bets. However, I would
like to think that the advice that I, “GOLDY LOCKS,”
provides is as close to a sure bet as you can get. If
you find my advice helpful drop me a note. (It’s
always easier at review time if you have notes from
customers.) Also, feel free to drop me a note on future
topics or on mistakes I may have made. (I CAN’T
PROMISE THE LATTER WILL FIND THEIR WAY TO THE BOSS.)
Thanks,
Goldy Locks
goldylocks@smartsensors.com
Replacing Thermocouples:
Room for Improvement?
When replacing a failed thermocouple, one has a tremendous
opportunity to improve the life, accuracy and effectiveness
of the replacement, make one’s life easier, and
possibly get a note from a process person on what a Smart
decision he has made. In a recent survey we conducted,
we found that 90% of replacement sensors are replaced
with like sensors, (ie thermocouples replaced with thermocouples
and RTDs with RTDs). So, you are going to be a maverick
if you try to change the sensor type. Consequently, if
the failed sensor is a thermocouple, you need to stick
with it. Don’t replace it with an RTD, even though
RTDs outnumber thermocouples by a wide margin in new installations
or new temperature loops. This article deals with how
you can improve on the sensor that you are replacing given
that you will stay with the same sensor technology.
Lets start with identifying the type/calibration of the
thermocouple you have. If the vendor was Smart,
he color coded the leads. This will enable you to determine
the calibration or thermocouple type. The chart below
provides the color codes for all standard and noble metal
thermocouples.
OK, so let’s review the options you might have in
changing the thermocouple. Let’s keep in mind that
we are looking for an “ata boy” note, so we
don’t want to do anything drastic. Basically, here
are the modification options:
1. Sheath material
2. OD
3. Length
4. Junction
5. Single to dual or dual to single
6. Calibration
You may have noticed that I don’t like to take chances.
That’s why I have such a good track record on locks.
Along those lines, let’s assume that the thermocouple
being replaced was in a well. This essentially eliminates
three of your options. It would not be a good idea to
change the sheath material, OD or length of the thermocouple.
Chances are that the sheath was not the cause of failure.
So if it ain’t broke; don’t fix it. You might
have heard that, “the bigger the thermocouple the
longer the life,” this might tempt you to opt for
a larger OD. There is a good chance that your well has
a .260 bore. If it does, and you increase your OD to 3/8”,
you will get longer life primarily because the larger
OD won’t fit into the existing bore. The replacement
thermocouple will never be put into service giving it
infinite life.
You could save a few bucks by shortening the length and
you might get a note from one of the bean counters. Of
course, you would be violating one of the principle rules
of good thermocouple practice: The tip of the thermocouple
must have good metal-to-metal contact with the bottom
of the well. If it doesn’t, you won’t be happy
with the accuracy or the response time. This is why all
Smart thermocouples are spring-loaded.
Oh, by the way, a simple way to check whether the length
is correct is to use a Smart
formula. OL(overall length of well) + A (nipple union
nipple length) + 1 1/4” = CSL(correct sensor length)
Now for the exciting part. What can you
do to improve the life and performance of the thermocouple?
Single to dual or dual to single
The bigger the thermocouple the longer the life? True
or False? If you said true you get an atta boy. So if
you have a dual thermocouple, you might want to consider
changing to a single. This will increase the gage size
of the thermocouple wire to 18 AWG. Now the non-Smart
followers might say, “what happens to my back-up
thermocouple?” Dual thermocouples are a gimmick
that seldom provide long or meaningful back up when one
of the thermocouples fail. Lets examine the failure possibilities:
Catastrophic Failure - Oh yeah, we might get
2 or 3 seconds more of readings.
Drift - Drift or aging is symptomatic of a material
composition change caused by elements external to the
thermocouple wire. Carbide precipitation or impurities
released from the inner wall of the sheath can be especially
damaging to Type K /Chromel Alumel thermocouples. It is
asking too much to expect that chemical composition changes
won’t occur in all four wires given the confinement
of the four wires and their shared environment.
Calibration
Once again Goldy will employ the, “I don’t
take chances” rule. Let’s assume that the
thermocouple is connected to a field or head-mounted transmitter.
If it is, and the transmitter is programmable, then you
will be able to change the calibration without incurring
any added costs. If there is not a head-mounted transmitter,
then stay with the existing calibration. It would be cost
prohibitive to replace the thermocouple wire, and adding
a transmitter will go beyond the scope of this article.
The areas to consider when changing calibration are:
1. Does the existing calibration match the suggested operating
range of the process?
2. Do I need stronger signal strength?
3. Can the accuracy be improved by changing calibrations?
For example, Type T (Copper Constantan)
is more accurate at lower temperatures than Types J or
K.
4. Do I need the faster response that the thermocouple
provides?
5. Do I need the ruggedness that the thermocouple provides?
Junction:
Here is where you can make a subtle change that could
get you an atta boy note. There are three junctions that
you can choose from: grounded, exposed and ungrounded.
The photo below gives the cut away look of each of these
junctions:
From left to right: ungrounded, exposed and grounded
junctions.
Since exposed junctions are rarely found on thermocouples
that are housed in a thermowell, this leaves you with
two choices--grounded and ungrounded If you have experienced
the joys and pleasures of trouble shooting a ground loop
fault, you have probably already replaced any grounded
thermocouples with ungrounded thermocouples. So, if you
have not guessed it yet, Goldy’s lock of the century
is to replace any grounded thermocouple with one that
has an ungrounded junction. Lets start with determining
how to check for junction identity. Using an ohmmeter,
check the resistance from either thermocouple wire to
the sheath. If it is grounded, the meter will show direct
continuity; and if it is ungrounded, there will be no
continuity.
Why ungrounded?
1. Eliminates ground loop problems
2. Instruments like an isolated signal better
3. Reduces signal noise from radio frequency and electro
magnetic sources
4. Extends the life of the thermocouple
Wait one minute Goldy. There has got to be a downside.
Well, actually, there is. The ungrounded junction has
a slower response time. The following chart provides response
time characteristics per ASTM 470A. It measures a 63%
response time change from ambient to boiling water.
|
Measuring
Junction Typical Response Time
|
|
Sheath
OD |
Measuring
Junction |
Response
Time* |
|
.063 (1/16") |
Grounded |
.09 |
|
|
Ungrounded |
.28 |
|
.125 (1/8") |
Grounded |
.34 |
|
|
Ungrounded |
1.6 |
|
.188 (3/16") |
Grounded |
.7 |
|
|
Ungrounded |
2.6 |
|
.250 (1/4") |
Grounded |
1.7 |
|
|
Ungrounded |
4.5 |
|
|
Exposed Loop |
.09 |
* Sensors
not in thermowell or protection tubes.
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If your process can accept a slower response time, then
ungrounded is the junction for you. It will not only make
your life a little easier, it will make your sensors last
longer. This could get you an atta boy from the bean counters.
Hope this helps! And if it does, let me hear from you.
Best regards,
Goldy
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