Thermocouples

Thermocouples

Thermocouples - types, principles and temperature ranges:

One of the most common industrial thermometer is the thermocouple. It was discovered by Thomas Seebeck's in 1822. He noted that a voltage difference appeared when the wire was heated at one end. Regardless of temperature, if both ends were at the same temperature there was no voltage difference. If the circuit were made with wire of the same material there was no current flow.

Schematic of a thermocouple

A thermocouple consists of two dissimilar metals, joined together at one end, and produce a small unique voltage at a given temperature. This voltage is measured and interpreted by a thermocouple thermometer.



The thermoelectric voltage resulting from the temperature difference from one end of the wire to the other is actually the sum of all the voltage differences along the wire from end to end
Thermocouples can be made from a variety of metals and cover a temperature range 200 ^oC to 2,600 ^oC. Comparing thermocouples to other types of sensors should be made in terms of the tolerance given in ASTM E 230.

Base metal thermocouples

Thermocouple	Maximum Temperature (oC)
	Continuous	Spot
Copper-Constantan	400	500
Iron-Constantan	850	1,100
Chromel-Constantan	700	1,000
Chromel-Alumel	1,100	1,300
Nicrosil-Nisil	1,250	-
Tungsten-Molybdenum*	2,600	2,650

* Not used below 1250 ^oC.

Advantages with thermocouples

• Capable of being used to directly measure temperatures up to 2600 ^oC.
• The thermocouple junction may be grounded and brought into direct contact with the material being measured.

Disadvantages with thermocouples

• Temperature measurement with a thermocouple requires two temperatures be measured, the junction at the work end (the hot junction) and the junction where wires meet the instrumentation copper wires (cold junction). To avoid error the cold junction temperature is in general compensated in the electronic instruments by measuring the temperature at the terminal block using with a semiconductor, thermistor, or RTD.
• Thermocouples operation are relatively complex with potential sources of error. The materials of which thermocouple wires are made are not inert and the thermoelectric voltage developed along the length of the thermocouple wire may be influenced by corrosion etc.
• The relationship between the process temperature and the thermocouple signal (millivolt) is not linear.
• The calibration of the thermocouple should be carried out while it is in use by comparing it to a nearby comparison thermocouple. If the thermocouple is removed and placed in a calibration bath, the output integrated over the length is not reproduced exactly.

Thermocouple Types

Thermocouples 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.



Some of the thermocouple types have standardized with calibration tables, color codes and assigned letter-designations. The ASTM Standard E230 provides all the specifications for most of the common industrial grades, including letter designation, color codes (USA only), suggested use limits and the complete voltage versus temperature tables for cold junctions maintained at 32 ^oF and 0 ^oC.

There are four "classes" of thermocouples:

• The home body class (called base metal),
• the upper crust class (called rare metal or precious metal),
• the rarified class (refractory metals) and,
• the exotic class (standards and developmental devices).

 The home bodies are the Types E, J, K, N and T. The upper crust are types B, S, and R, platinum all to varying percentages. The exotic class includes several tungsten alloy thermocouples usually designated as Type W (something).

Instrument	Temperature Range		Accuracy
	Recommended (oF)	Maximum (oF)
Type J probes	32 to 1336	-310 to 1832	1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater
Type K probes	32 to 2300	-418 to 2507	1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater
Type T probes	-299 to 700	-418 to752	0.9 to 3.6oF or 0.4% of reading above 32oF, whichever is greater
Type E probes	32 to 1600	32 to 1650	1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater
Type R probes	32 to 2700	32 to 3210	2.5oF or 0.25% of reading, whichever is greater
Type S probes	32 to 2700	32 to 3210	2.5oF or 0.25% of reading, whichever is greater

Temperature Conversions

• ^oF = (1.8 x ^oC) + 32
• ^oC = (^oF - 32) x 0.555
• Kelvin = ^oC + 273.2
• ^oRankin = ^oF + 459.67

ASTM Standards Related to Thermocouples

• E 207-00...Method of Thermal EMF Test of Single Thermo element Materials by Comparison with a Secondary Standard of Similar EMF-Temperature Properties
• E 220-02 Standard Test Method for Calibration of Thermocouples By Comparison Techniques
• E 230-98e1..Temperature Electromotive Force (EMF) Tables for Standardized Thermocouples
• E 235-88(1996)e1..Specification for Thermocouples, Sheathed, Type K, for Nuclear or Other High-Reliability Applications
• E 452-02..Test Method for Calibration of Refractory Metal Thermocouples Using a Radiation Thermometer
• E 574-00..Specification for Duplex, Base-Metal Thermocouple Wire with Glass Fiber or Silica Fiber Insulation
• E 585/E 585M-01a ..Standard Specification for Compacted Mineral-Insulated, Metal-Sheathed, Base Metal Thermocouple Cable
• E 601-81(1997)..Test Method for Comparing EMF Stability of Single-Element Base-Metal Thermocouples Materials in Air
• E 608/E 608M-00. Standard Specification for Mineral-Insulated, Metal-Sheathed Base-Metal Thermocouples
• E 696-00 Standard Specification for Tungsten-Rhenium Alloy Thermocouple Wire
• E 710-86(1997) Standard Test Method for Comparing EMF Stabilities of Base-Metal Thermo elements in Air Using Dual, Simultaneous, Thermal-EMF Indicators
• E 780-92(1998) Standard Test Method for Measuring the Insulation Resistance of Sheathed Thermocouple Material at Room Temperature
• E 839-96 Standard Test Method for Sheathed Thermocouples and Sheathed Thermocouple Material
• E 988-96(2002) Standard Temperature-Electromotive Force (EMF) Tables for Tungsten-Rhenium Thermocouples
• E1129/E1129M-98 Standard Specification for Thermocouple Connectors
• E 1159-98 Standard Specification for Thermocouple Materials, Platinum-Rhodium Alloys and Platinum
• E 1350-97(2001) Standard Test Methods for Testing Sheathed Thermocouples Prior to, During and After Installation
• E 1652-00 Standard Specification for Magnesium Oxide and Aluminum Oxide Powder and Crushable Insulators Used in the Manufacture of Metal-Sheathed Platinum Resistance Thermometers, Base Metal Thermocouples, and Noble Metal Thermocouples
• E 1684-00 Standard Specification for Miniature Thermocouple Connectors
• E 1751-00 Standard Guide for Temperature Electromotive Force (emf) Tables for Non-Letter Designated Thermocouple Combinations
• E 2181/E 2181M-01 Standard Specification for Compacted Mineral-Insulated, Metal-Sheathed, Noble Metal Thermocouples and Thermocouple Cable 

 
 Article Source: www.engineeringtoolbox.com 

Read more...

Temperature : Basics

Temperature:

A short introduction to temperature:

Temperature (sometimes called thermodynamic temperature) is a measure of the average kinetic energy of a systems particles. Temperature is the degree of "hotness" ( or "coldness"), a measure of the heat intensity.

When two objects of different temperatures are in contact, the warmer object becomes colder while the colder object becomes warmer. It means that heat flows from the warmer object to the colder one.

Degree Celsius (oC) and Degree Fahrenheit (oF) :

A thermometer can help us determine how cold or how hot a substance is. Temperatures in science (and in most of the world) are measured and reported in degrees Celsius (oC). In the U.S., it is common to report temperature in degrees Fahrenheit (oF). On both the Celsius and Fahrenheit scales the temperature at which ice melts (water freezes) and the temperature at which water boils, are used as reference points.

           • On the Celsius scale, the freezing point of water is defined as 0 oC, and the boiling point of water is defined as 100 oC.

           • On the Fahrenheit scale, the water freezes at 32 oF and the water boils at 212 oF.

 

On the Celsius scale there are 100 degrees between freezing point and boiling point of water, compared to 180 degrees on the Fahrenheit scale. This means that 1 oC = 1.8 oF.

Thus the following formulas can be used to convert temperature between the two scales:

tF = 1.8 tC + 32 = 9/5 tC + 32

tC = 0.56 (tF - 32) = 5/9 (tF - 32)

where

tC = temperature (oC)

tF = temperature (oF)

Kelvin - K :

Another scale (common in science) is Kelvin, or the Absolute Temperature Scale. On the Kelvin scale the coldest temperature possible, -273 oC, has a value of 0 Kelvin (0 K) and is called the absolute zero. Units on the Kelvin scale are called Kelvins (K) and no degree symbol is used.

Because there are no lower temperatures than 0 K - the Kelvin scale does not have negative numbers.

A Kelvin equal in size to a Celsius unit:

1 K = 1 oC

To calculate a Kelvin temperature, add 273 to the Celsius temperature:

tK = tC + 273.16 

 

Degree Rankine - R :

In the English system the absolute temperature is in degrees Rankine (R), not in Fahrenheit:

tR = tF + 459.67 

Temperature Sensors - Comparing Types:

Comparing advantages and disadvantages of thermocouples, RTDs and thermistors temperature sensors :

Attribute	Thermocouple	RTD	Thermistor
Cost	Low	High	Low
Temperature Range	Very wide -350oF +3200oF	Wide -400oF +1200oF	Short to medium -100oF +500oF
Interchange ability	Good	Excellent	Poor to fair
Long-term Stability	Poor to fair	Good	Poor
Accuracy	Medium	High	Medium
Repeatability	Poor to fair	Excellent	Fair to good
Sensitivity (output)	Low	Medium	Very high
Response	Medium to fast	Medium	Medium to fast
Linearity	Fair	Good	Poor
Self Heating	No	Very low to low	High
Point (end) Sensitive	Excellent	Fair	Good
Lead Effect	High	Medium	Low
Size/Packaging	Small to large	Medium to small	Small to medium

Article Source : www.engineeringtoolbox.com

Read more...