Temperature Measurements With Thermocouples

Temperature sensors:
Temperature is the measure of average molecular kinetic energy within a substance. This follows that as the kinetic energy of the substance increases so does the temperature. Temperature measurement relies on the transfer of heat energy from the process material to the measuring device. The measuring device therefore needs to be temperature dependent.

There are two main types of industrial temperature sensors namely:

1) Contact

2) Non-contact

Contact Temperature Sensors are the most common and widely used form of temperature measurement. The three main types are:

1) Thermocouples

2) Resistance Temperature Detectors (RTDs)

3) Thermistors

These types of temperature devices all vary in electrical resistance for temperature change. The rate and proportion of change is different between the three types, and also different within the type classes.

Non-Contact Temperature Sensors:

Temperature measurement by non-contact means is more specialized and can be performed with the following technologies:

1) Infrared

2) Acoustics

Thermocouples:
A Thermocouple consists of two wires of dissimilar metals, such as iron and constantan, electrically connected at one end. Applying heat to the junction of the two metals produces a voltage between the two wires. This voltage is called an E.M.F. (electromotive force) and is proportional to temperature.

Most thermocouple metals produce a relationship between the two temperatures and the E.M.F. as follows:

e = a(T1 - T2) + b(T12 - T22)

e is the e.m.f, a and b are constants for the thermocouple, T1 and T2 are the temperatures. The relationship is nearly linear over the operating range.

A thermocouple requires a reference junction, placed in series with the sensing junction. As the two junctions are at different temperatures a thermal E.M.F is generated. The reference junction is used to correct the sensing junction measurement. A schematic for a thermocouple/instrument connection is shown in the diagram below:

Thermocouples are fusion-welded to form a pure joint, which maintains the integrity of the circuit and also provides high accuracy. Grounded junctions provide good thermal contact with protection from the environment. Ungrounded and isolated junctions provide electrical isolation from the sensor sheath.

Thermocouples are usually encased in a protective metal sheath. The sheath material can be stainless steel, which is good for temperatures up to 870 oC. For temperatures up to 1150 oC Inconel is used. A metallic oxide can be compacted into the sheath. This provides mechanical support and also electrically insulates the thermocouple junction. The metal sheathed mineral insulated thermocouple has become the accepted norm in most industries. They use a variety of temperature and corrosion resistant sheaths and have an extremely high purity (99.4%) of Magnesium oxide insulation.

Most thermocouples are manufactured with different tip configurations. For maximum sensitivity and fastest response, the dissimilar-metal junction may be unsheathed (bare). This design, however, makes the thermocouple more fragile. Sheathed tips are typical for industrial applications, available in either grounded or ungrounded forms:

Grounded-tip thermocouples exhibit faster response times and greater sensitivity than ungrounded-tip
thermocouples, but they are vulnerable to ground loops: circuitous paths for electric current between the conductive sheath of the thermocouple and some other point in the thermocouple circuit. In order to avoid this potentially troublesome effect, most industrial thermocouples are often the ungrounded design.

Advantages of Thermocouple Sensors:

• Low cost
• Small size
• Robust
• Wide range of operation
• Reasonably stable
• Accurate for large temperature changes
• Provide fast response

Disadvantages of Thermocouple Sensors:

• Very weak output, milivolts
• Limited accuracy for small variations in temperature
• Sensitive to electrical noise
• Nonlinear
• Complicated conversion from emf to temperature

Thermocouple types:
Thermocouples exist in many different types, each with its own color codes for the dissimilar-metal wires. Here is a table showing the ANSI design letter designation for thermocouple types and their standardized colors along with some distinguishing characteristics of the metal types to aid in polarity identification when the wire colors are not clearly visible:

ANSI  Letter Design	Leg (Terminals)	Metallic Composition	Melting Point		Temperature Range
			°C	°F
B	P	Platinum – 30% Rhodium	1825	3320	0 - 1820 °C  32 - 3308 °F
	N	Platinum – 6% Rhodium
E	P	Chromel	1220	2230	-270 - 1000 °C -454 - 1832 °F
	N	Constantan
J	P	Iron	1220	2230	-200 - 1200 °C -328 - 2192 °F
	N	Constantan
K	P	Chromel	1400	2550	-270 - 1372 °C -454 - 2501 °F
	N	Alumel
N	P	Nicrosil	1340	2440	-270 - 1300 °C -454 - 2372 °F
	N	Nisil
R	P	Platinum – 13% Rhodium	1770	3215	-50 - 1768 °C -58 - 3214 °F
	N	Pure Platinum
S	P	Platinum – 13% Rhodium	1770	3215	-50 - 1768 °C -58 - 3214 °F
	N	Pure Platinum
T	P	Copper	1080	1980	-270 - 400 °C -454 - 752 °F
	N	Constantan

Note P denotes Positive terminal. N denotes Negative terminal

Basic Value Curves for Thermocouples
The curves below is a plot of Thermal voltages of the various types of thermocouples highlighted in the table above against temperature:

Thermocouple problems:
Because thermocouples can be used in high temperature environments, it is possible that the extension wires can be damaged by excessive heat. If a short circuit develops in the wires, it may not be possible to detect. The sensing equipment will no longer be measuring the temperature at the sensing junction, but instead will measure the temperature at the short.

If a new thermocouple has been installed but does not make contact with the thermowell then an air gap is introduced which affects response time and can have a temperature variation from the actual temperature. A thermopaste can be used, and should only be applied at the tip where the temperature measurement occurs. The insertion depth is also a factor, as the deeper the insertion the more accurate the measurement. Thermopaste can make up for some shortness in length, but is limited if the shortfall is too great.

Replacing thermocouples in thermowells can be very challenging. It is important to check that the bore of the thermowell is cleaned. During the changeover or just over time, it is possible (and therefore probable!) that material may accumulate at the bottom of the well, which can insulate the thermocouple from the sheath and prevent heat transfer.

Another problem is when the new thermocouple is of a different mass to the old one. This can affect the response time and, although it may not affect the accuracy, it can affect the stability in a closed loop system.

Grounding can be another problem, where the accuracy and response can differ between grounded and ungrounded devices.

Failure Modes of Thermocouple Sensors:
An open circuit in the thermocouple detector means that there is no path for current  flow, thus it will cause a low (off-scale) temperature reading.

A short circuit in the thermocouple detector will also cause a low temperature reading because it creates a leakage current path to the ground and a smaller measured voltage.