THERMOCOUPLE CALIBRATION PROCEDURE:
The following information is intended
to give the reader a review, in some detail, of the equipment
requirements and proper techniques needed to accurately calibrate
thermocouples and thermocouple materials.
Branom Instrument calibrates thermocouple
and thermocouple wire in accordance with one of the following
American Society for Testing and Material (ASTM) Standards:
E207-88, standard method of Thermal EMF Test of single thermoelement
materials by comparison with a secondary standard of similar
EMF temperature properties. E220-86, standard method for calibration
of thermocouples by comparison techniques.
In general these standards describe the
type of temperature source, measuring equipment, standards,
and procedures needed to accurately perform a calibration.
Each of these elements will now be looked at more closely.
CONTROLLED TEMPERATURE SOURCE:
The temperature source used in the process
of calibrating should as a minimum be stable enough to provide
a constant temperature (approx.+/-.2 degrees F) for a short
length of time (approx.20 mm.) at any temperature at which
the temperature bath or other source is to be used. The temperature
source should have a zone of uniform temperature into which
the thermocouple measuring junction may be inserted. The length
of the temperature source must be adequate to permit a depth
of immersion sufficient to assure that the measuring junction
temperature is not affected by a temperature gradient along
the thermocouple wires.
CONTROLLED TEMPERATURE SOURCES:
- Fixed Point: When highly accurate measurements
must be made, fixed point cells are used. A fixed point
cell consists of a metal sample inside a graphite crucible
with a graphite thermometer well submerged into the metal
sample. When the metal sample is heated to the freezing
point, it will produce a very stable and constant temperature.
In order to better understand the operation of fixed point
cells, the following definitions are useful.
- Fixed Point: A reproducible temperature
of equilibrium between different phases of a material.
- Freezing Point: The fixed point between
the solid and liquid phases of a material.
REFERENCE JUNCTIONS:
A thermocouple's output is based on the
difference in temperature between the measuring junction (hot
junction) and the reference junction (cold junction). See
Figure A.
REFERENCE JUNCTION TEMPERATURE:
A controlled temperature must be provided
in which the reference junction is maintained at a constant
chosen temperature. The reference junction temperature should
be controlled to a better accuracy than that expected from
the thermocouple calibration. The most commonly used reference
temperature is 32 degrees F., but other temperatures may be
used if desired.
ICE BATH:
One of the most common reference junctions
is the ice bath. The ice bath is made up of a mixture of melting
shaved ice and water. The ice bath is a convenient and inexpensive
way to achieve an ice point, it can be reproduced with ease
and with exceptional accuracy. Junctions formed between the
thermocouple materials and instrument leads can be simply
immersed into the slush mixture, or alternatively glass "U"
tubes containing a quantity of mercury approximately 3/4"
to 1" depth can be placed into the slush mixture. Quick electrical
connection can then be made between thermocouple and instrument
leads through the mercury. (Figure B).
Note: An improperly used ice bath can
result in serious errors. The largest error which is likely
to occur arises due to melting of the ice at the bottom of
the bath until the reference junctions are below the ice level
and surrounded by water alone. This water may be as much as
7 degrees F above the ice point.
AUTOMATIC ICE POINT:
The automatic ice point is an electrical
refrigerated device in which an equilibrium between ice and
water is constantly maintained. The change of volume of water
in freezing is used to control heat transfer. Some commercially
available devices provide wells into which the user may insert
reference junctions formed from his own calibrated wire. Others
are provided with many reference junction pairs brought out
to terminals which the user may connect into his system.
ELECTRONIC COMPENSATION:
This method employs a compensation circuit
containing a source of current and a combination of fixed
resistors and a temperature sensitive resistor (TSR). This
device can be designed to produce similar EMF to that of the
thermocouple being calibrated. The Electronic Compensator
will make EMF compensations to the thermocouple circuit based
in the difference in EMF from 32 to ambient temperature.
MEASURING INSTRUMENTS:
The choice of a specific instrument to
use for measuring the thermocouple output will depend on the
accuracy required of the calibration being performed. In general,
an instrument such as the Fluke 702 calibrator or Altek 422
is sufficient for most thermocouple calibrations.
REFERENCE THERMOMETERS:
The reference thermometer to be used for
the comparison calibration of a thermocouple will depend upon
the temperature range covered, the accuracy desired, the capabilities,
or the preference of the calibration laboratory. The following
are different examples of reference thermometers.
PLATINUM RESISTANCE THERMOMETERS:
A standard platinum resistance thermometer
(SPRT) is the most accurate standard available, however, it
is the most expensive standard, and other standards are acceptable
alternatives depending upon the temperature range covered,
the accuracy desired, the capabilities, or the preference
of the calibration laboratory. The following are different
examples of reference thermometers.
LIQUID-IN-GLASS THERMOMETERS:
Liquid-in-glass thermometers are available
to cover the range from -300 to 950 degrees Fahrenheit. with
an accuracy of from .1 to 3 Fahrenheit depending on the type
of thermometer and the width of the range covered. They are
relatively inexpensive but they are fragile, and if the highest
degree of accuracy of which they are capable is to be achieved,
an individual thermometer must cover a very narrow temperature
range so that the graduation intervals can be as large as
possible. A further disadvantage of the liquid-in-glass thermometer
is that because of their fine graduations reading errors are
a distinct possibility. Taylor Instruments offers Superior
Grade Certified Secondary Reference Thermometers individually
or in matched Celsius or Fahrenheit sets, which Branom stocks.
TEST ASSEMBLY PLACEMENT IN THE FURNACE:
Depth of immersion is the most important
consideration if accurate calibration results are to be obtained.
The depth of immersion must be sufficient to eliminate the
effects of heat transfer away from the junction. It is impossible
to establish a minimum depth of immersion that would be useable
under all circumstances since heat transfer characteristics
are dependent on the mass of material being put into the temperature
source.
WIRING CONNECTION FROM TEST ASSEMBLY
TO READOUT INSTRUMENT.
The actual wiring necessary to connect
the test assembly, reference junction and readout instrument
will depend on the quantity of thermoelements in the test
assembly, the type of reference junction used and whether
or not a switching device is used, but the basic requirements
are the same. Thermocouple extension wire is used to connect
the thermoelements to the reference junction. Copper wires
are used between the reference junction and readout instrument.
THERMOCOUPLE WIRE, WIRING PROCEDURE:
Ideally, the samples of the thermocouple
material to be calibrated and the standard thermocouple element
should be cut long enough so that they reach directly from
the temperature source to the reference junction without the
need for extension wires. If this is not possible extension
wires may be used, but they must be securely connected directly
to the test assembly conductors. If extension wires must be
used, remove any oxide layer that may be on the surface of
the test assembly conductors and attach an extension wire
of the same material to each conductor by laying the extension
wire alongside the conductors and joining them securely by
means of an alligator clip.
THERMOCOUPLE CALIBRATION WIRING PROCEDURE:
When calibrating thermocouples, it is
faster and more convenient to use a thermocouple switching
box. The extension wires from the thermocouples are placed
into one side of the reference junction. Multiple pairs of
copper leadwire will exit the reference junction and will
be connected to the switch box. One pair of copper leadwires
will run from the readout instrument to the thermocouple switch
box.
For a more in-depth look at thermocouples
and thermocouple calibration the reader is encouraged to read
ASTM STP 470, manual on the use of thermocouple in temperature
measurement.
JUNCTION LOCATION:
One of the primary advantages of calibrating
thermocouple materials against a base-metal standard of similar
EMF output is that the sample(s) to be calibrated are welded
to the base-metal standard forming a common junction thus
achieving good isothermal conditions between the test thermoelement
and the standard. Furthermore, because the test thermoelement
and the standard produce nominally the same EMF vs. platinum
(pt-67) the EMF output changes little over a fairly broad
temperature range, thereby reducing the need for precise temperature
source control. See Figure C.
MEASUREMENT: Set your controlled
temperature source to the specified temperature and allow
it to adequately stabilize. Immerse the test assembly into
the test temperature medium and provide sufficient time for
the test assembly to stabilize. Once the test assembly is
stable the EMF generated between the test specimen and the
reference standard can be recorded. Avoid soaking the test
assembly at temperature for a prolonged period of time, as
it can cause permanent changes to occur in the thermoelements.
Once the reading is taken, raise the test
temperature to the next higher temperature, first removing
the test assembly from the temperature source, or advance
the test assembly to the next temperature source. Allow the
temperature source and the test assembly to stabilize as before,
and take a second set of readings at the new temperature.
In all cases take the reading in sequence from the lowest
to the highest temperature. A base metal reference standard
shall be used for one series of temperature changes only.
ASTM E 220 THERMOCOUPLE CALIBRATION:
The Test thermocouple junction should
be located so that it is in intimate contact with the junction
of the standard. Without making a radiograph of the thermocouple
it is impossible to know exactly where the junction is located.
A few generalizations can be made which enables junctions
to be located quite closely. First, the cap weld on a metal
sheathed thermocouple is normally about as thick as one-half
the sheathed diameter. Second, a "U" junction is normally
about one-half the sheathed diameter. Using these generalizations,
a thermocouple .125" diameter, will have a grounded junction
approximately .063" below the tip of the cap. The thermocouple
standard should be tied to the thermocouple (s) with a fine
gauge wire. The junction of the standard should be bent so
that it is in contact or at least very close to the point
where it has been calculated that the junction is located.
See Figure D.
PLATINUM VERSUS PLATINUM RHODIUM THERMOCOUPLE:
Platinum vs. platinum 10% Rhodium standard
thermocouples (ANSI Type S) are exceptionally accurate and
stable devices. NIST offers calibration uncertainty of.9 F.
from 0 to 1112 F, and 1.26 F. from 1112 to 2012 F. When used
as a working standard, understandably due to the high cost
of these materials, they cannot be discarded after each use.
Consequently, care must be taken to avoid contamination, work
hardening and other sources of de-calibration.
BASE METAL THERMOCOUPLE STANDARDS:
An alternate approach to calibrating thermocouples
and thermocouple materials, and one which gives a high degree
of accuracy, is calibrating with a secondary standard that
has similar FMF properties to those of the test element. That
is, calibrating a KP element against a KP standard.
All thermocouple materials in the USA
are referenced to a pure platinum element (PT-67) which is
retained at the National Institute of Standards and Technology.
It is important that regardless of the type of standard used
that traceability to this NIST standard be accomplished.
CALIBRATION PROCEDURE:
The instruments mentioned previously as
standards, fixed point cells, platinum resistance thermometers
and liquid-in-glass thermometers can all be used to accurately
calibrate thermocouples if the proper calibration procedures
are followed. As previously mentioned, the two most common
procedures are ASTM E207 & P220. We will now examine both
more closely.
ASTM E207 WIRE TO WIRE CALIBRATION:
In order to achieve the maximum amount
of accuracy when using base-metal standards, it is desirable
whenever possible to make wire to wire readings, that is,
reading the EMF developed between the thermocouple material
to be calibrated and the base-metal standard of similar material.
Thermocouple material should be free of
contaminants, insulated wire should be stripped of insulation.
Insulation should be removed carefully in order to avoid cold
working by nicking or stretching. Any one of these conditions
could cause erroneous calibration results.
FLUIDIZED BEDS:
The fluidized bed is a unique method of
providing closely controlled temperatures. The bath consists
of a very fine mesh aluminum oxide, a heated chamber into
which the medium is placed, and a means for slowly agitating
the bath by introducing a flow of air. By careful control
of heat input and air flow, temperatures of the bath can be
controlled within close limits thereby producing isothermal
conditions between the calibration standard and the test setup.
Fluidized beds are useful for calibrating over the temperature
range from ambient to 1600 degrees F.
STIRRED LIQUID BATHS:
Stirred liquid baths operate on the same
principal as fluidized beds and are an excellent means of
establishing closely controlled temperatures. Although stirred
liquid baths using molten salts or liquid tin are available
with a temperature range as high as 932 degrees F., the most
common application is in the range of ambient to 500 degrees
F. utilizing silicone oil as the bath material.
TUBE-TYPE HEATING ELEMENT FURNACE:
For temperatures above 500 degrees F.
an electrically heated tube furnace is recommended. Tube furnaces
operate in the range of 500 to 3100 Degrees. F.
Figure A:
Figure B:
Figure C:
Figure D:
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