Explicación de los sensores de temperatura 1. RTD´s= Resitance Temperature D 2. Thermocuoples=Termocoples

1. RTD´s
Thermocouples
Recopila:
Ing. Hugo Posada

2. Qué es un RTD?
RTD stands for Resistance Temperature Detector. RTDs are
sometimes referred to generally as resistance thermometers. The
American Society for Testing and Materials (ASTM) has defined
the term resistance thermometer as follows:
Resistance thermometer, n. - a temperature-measuring device
composed of a resistance thermometer element, internal
connecting wires, a protective shell with or without means for
mounting a connection head, or connecting wire or other fittings,
or both. [Vol. 14.03, E 344 - 02 § 3.1 (2007).]
An RTD is a temperature sensor which measures temperature
using the principle that the resistance of a metal changes with
temperature. In practice, an electrical current is transmitted
through a piece of metal (the RTD element or resistor) located in
proximity to the area where temperature is to be measured. The
resistance value of the RTD element is then measured by an
instrument. This resistance value is then correlated to
temperature based upon the known resistance characteristics of
the RTD element.

3. Cómo funciona un RTD?
RTDs work on a basic correlation between metals and temperature. As the
temperature of a metal increases, the metal's resistance to the flow of
electricity increases. Similarly, as the temperature of the RTD resistance
element increases, the electrical resistance, measured in ohms (Ω), increases.
RTD elements are commonly specified according to their resistance in ohms at
zero degrees Celsius (0° C). The most common RTD specification is 100 Ω,
which means that at 0° C the RTD element should demonstrate 100 Ω of
resistance.
RTD elements are typically in one of three configurations: (1) a platinum or
metal glass slurry film deposited or screened onto a small flat ceramic
substrate known as "thin film" RTD elements, and (2) platinum or metal wire
wound on a glass or ceramic bobbin and sealed with a coating of molten glass
known as "wire wound" RTD elements. (3) A partially supported wound
element which is a small coil of wire inserted into a hole in a ceramic insulator
and attached along one side of that hole. Of the three RTD elements, the thin
film is most rugged and has become increasingly more accurate over time.

4. Porqué un RTD tiene 2, 3, 4 alambres de conexion?
A simple rule of thumb is that the more wires an RTD has the more accurate it is. The entire
RTD assembly is not platinum. Among other issues, constructing an RTD in that manner
would for most purposes be prohibitively expensive. As a result, only the small RTD element
itself is made of platinum.
Three wire RTDs are the most common specification for industrial applications. Three wire
RTDs normally use a Wheatstone bridge measurement circuit to compensate for the lead
wire resistance as shown below.

5. Como se conecta un RTD en un equipo de medición de temperatura?

6. Qué es un termocople?
The American Society for Testing and Materials (ASTM) has defined the term thermocouple
as follows:
Thermocouple, n. - in thermometry, the sensor of a thermoelectric thermometer, consisting
of electrically conducting circuit elements of two different thermoelectric characteristics
joined at a junction. [Vol. 14.03, E 344 - 02 § 3.1 (2007).]
A thermocouple occurs when any two different kinds of metals joined at a junction are
exposed to a temperature gradient. When the two different metals are exposed to a
temperature gradient they generate a very small electrical charge, commonly measured in
millivolts, that correlates to the temperature to which the elements are exposed. This
phenomenon is sometimes referred to as the Seebeck effect.

7. In the United States, different letter and color code designations are defined for each
thermocouple type by the ANSI/ASTM E 230 standard. European standards are set by the IEC
which uses different color code designation for thermocouples but largely sticks with the
same letter designations

8. 1. Type J Thermocouple (Most Common): This thermocouple consists of an Iron and a Constantan leg and is perhaps
the most common thermocouple in use in the United States. The bare Type J thermocouple may be used in vacuum,
reducing, oxidizing and inert atmospheres. Heavier gauge is wire recommended for use above 1000 deg. F since the
iron leg of this thermocouple oxidizes rapidly at high temperatures.
2. Type K Thermocouple (Most Common Real Hot): This thermocouple consists of a Chromel and an Alumel leg. This
thermocouple is recommended for oxidizing or inert atmospheres up to 2300 deg. F. Cycling above and below 1800
deg. F is not recommended due to EMF alteration from hysteresis. This thermocouple is fairly accurate and stable at
high temperatures.
3. Type N Thermocouple (A Newer, Better Type K): This thermocouple consists of a Nicrosil and a Nisil leg. This
thermocouple is recommended for the same range as a Type K. It has better resistance to degradation due to
temperature cycling, green rot and hysteresis than the Type K and is typically very cost competitive with the Type K.
4. Type T Thermocouple (Most Common Real Cold): This thermocouple consists of a Copper and a Constantan leg. It
may be used in vacuum, oxidizing, reducing and inert atmospheres. It maintains good resistance to corrosion in most
atmospheres and high stability at sub-zero temperatures.
5. Type E Thermocouple (Most Common Power Application): This thermocouple consists of one Chromel leg and one
Constantan leg. This thermocouple is not subject to corrosion in most atmospheres. The Type E also has the highest
EMF per degree of any standard thermocouple type. However, this thermocouple must be protected from sulfurous
atmospheres.
6. Type B, R & S Thermocouples (Most Common Real, Real Hot): Platinum & Rhodium Thermocouples.
Recommended for use in oxidizing or inert atmospheres. Reducing atmospheres may cause excessive grain growth
and drift in calibration of these thermocouples. Types R & S may be used up to 1480 C. Type B may be used up to
1700 C.
7. Type C Thermocouple (For the Hottest of Environments): Tungsten and Rhenium thermocouple. Recommended for
use in vacuum, high purity hydrogen or pure inert atmospheres. May be used at extremely high temperatures (2316
C). This thermocouple, however, is inherently brittle.

10. Como se conecta un TERMOPAR
en un equipo de medición de temperatura?

11. REFERENCIAS.
http://www.jms-se.com/rtd.php
http://www.jms-se.com/search.php?q=what+is+a+thermocouple
http://www.omega.com/