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Introduction to Thermocouples
is a thermocouple?
thermocouple is a sensor for measuring temperature. It consists
of two dissimilar metals, joined together at one end, which produce
a small unique voltage at a given temperature. This voltage is
measured and interpreted by a thermocouple thermometer.
are the different thermocouple types?
are available in different combinations of metals or calibrations.
The four most common calibrations are J, K, T and E. Each calibration
has a different temperature range and environment, although the
maximum temperature varies with the diameter of the wire used
in the thermocouple.
do I choose a thermocouple type?
thermocouples measure in wide temperature ranges and can be relatively
rugged, they are very often used in industry. The following criteria
are used in selecting a thermocouple:
Chemical resistance of the thermocouple or sheath material
Abrasion and vibration resistance
Installation requirements (may need to be compatible
with existing equipment; existing holes may determine probe
do I know which junction type to choose?
thermocouple probes are available with one of three junction types:
grounded, ungrounded or exposed. At the tip of a grounded junction
probe, the thermocouple wires are physically attached to the inside
of the probe wall. This results in good heat transfer from the
outside, through the probe wall to the thermocouple junction.
In an ungrounded probe, the thermocouple junction is detached
from the probe wall. Response time is slowed down from the grounded
style, but the ungrounded offers electrical isolation (see table below)
|Room-Temperature Insulation Resistance
|Nominal Sheath Diameter||Applied dc Voltage Min.||Insulation Resistance Min.
|< 0.88mm(0.03in.)||50V||100M Ω
|0.08 to 1.5mm(0.030 to 0.059in.)||50V||500M Ω
|> 1.5mm(0.059in.)||500V||1000M Ω
The thermocouple in the exposed junction
style protrudes out of the tip of the sheath and is exposed to
the surrounding environment. This type offers the best response
time, but is limited in use to noncorrosive and nonpressurized
applications. See the illustrations at the right for a full discussion
of junction types.
is response time?
time constant has been defined as the time required by a sensor
to reach 63.2% of a step change in temperature under a specified
set of conditions. Five time constants are required for the sensor
to stabilize at 600 of the step change value. Exposed junction
thermocouples are the fastest responding. Also, the smaller the
probe sheath diameter, the faster the response, but the maximum
temperature may be lower. Be aware, however, that sometimes the
probe sheath cannot withstand the full temperature range of the
Standard diameters: 0.010", 0.020", 0.032", 0.040", 1/16", 1/8",
3/16", and 1/4" with two wires.
Standard OMEGA thermocouples have 12 inch immersion lengths. Other
304 stainless steel and Inconel are standard. Other sheath materials
available; call for price and availability.
High purity Magnesium Oxide is standard. Minimum insulation resistance wire
to wire or wire to sheath is 1 Gohm at 500 volts dc in all
diameters above 1.5mm (1/16").
Iron-Constantan (J), ChromegaŽ-AlomegaŽ (K), Copper-Constantan
(T), and Chromega-Constantan (E) are standard calibrations.
Easily bent and formed. Bend radius should be not less than twice
the diameter of the sheath.
Off-the-Shelf, other sheaths available; call for price and delivery.
Dual Elements: Thermocouples with a sheath diameter of 0.040"
(1.0 mm) thru 1/4" (6.3mm) are available in dual element.
The wires used in OMEGA thermocouples are selected and matched
to meet ANSI Limits of Error. Special limits of error thermocouples
can be made from all 1/16" (1.5 mm) O.D. or larger OMEGACLADŽ
In the thermocouple industry, standard practice is to color the
negative lead red. Other standards that OMEGA uses are: the negative
lead of bare wire thermocouple is approximately 1/4" shorter than
the positive lead, and the large pin on a thermocouple connector
is always the negative conductor.
Thermocouple alloy wire must always be used to connect a thermocouple
sensor to the instrumentation to assure accurate measurements.
The grounded junction is recommended for the measurement of static
or flowing corrosive gas and liquid temperatures and for high
pressure applications. The junction of a grounded thermocouple
is welded to the protective sheath giving faster response than
the ungrounded junction type.
ungrounded junction is recommended for measurements in corrosive
environments where it is desirable to have the thermocouple electronically
isolated from and shielded by the sheath. The welded wire thermocouple
is physically insulated from the thermocouple sheath by MgO powder
exposed junction is recommended for the measurement of static
or flowing non-corrosive gas temperatures where fast response
time is required. The junction extends beyond the protective metallic
sheath to give accurate fast response. The sheath insulation is
sealed where the junction extends to prevent penetration of moisture
or gas which could cause errors.