Resistance elements come in many types conforming to different standards,
capable of different temperature ranges, with various sizes and
accuracies available. But they all function in the same manner: each has
a pre-specified resistance value at a known temperature which changes in a
predictable fashion. In this way, by measuring the resistance of the
element, the temperature of the element can be determined from
tables, calculations or instrumentation. These resistance elements are the
heart of the RTD (Resistance Temperature Detector). Generally, a
bare resistance element is too fragile and sensitive to be used in its raw form,
so it must be protected by incorporating it into an RTD.
Resistance Temperature Detector is a general term for any device that
senses temperature by measuring the change in resistance of a material.
RTD's come in many forms, but usually appear in sheathed form. An RTD
probe is an assembly composed of a resistance element, a sheath, lead wire
and a termination or connection. The sheath, a closed end tube, immobilizes
the element, protecting it against moisture and the environment to be
measured. The sheath also provides protection and stability to the transition
lead wires from the fragile element wires.
Some RTD probes can be combined
with thermowells for additional
protection. In this type of application,
the thermowell may not only add
protection to the RTD, but will also seal
whatever system the RTD is to
measure (a tank or boiler for instance)
from actual contact with the RTD. This
becomes a great aid in replacing the
RTD without draining the vessel or
Thermocouples are the old tried and
true method of electrical temperature
measurement. They function very
differently from RTD's but generally
appear in the same configuration: often
sheathed and possibly in a thermowell.
Basically, they operate on the Seebeck
effect, which results in a change in
thermoelectric emf induced by a
change in temperature. Many
applications lend themselves to either
RTD's or thermocouples.
Thermocouples tend to be more
rugged, free of self-heating errors and
they command a large assortment of
instrumentation. However, RTD's,
especially platinum RTD's, are more
stable and accurate.
RESISTANCE ELEMENT CHARACTERISTICS
There are several very important
details that must be specified in order
to properly identify the characteristics
of the RTD:
1. Material of Resistance Element (Platinum, Nickel, etc.)
2. Temperature Coefficient
3. Nominal Resistance
4. Temperature Range of Application
5. Physical Dimensions or Size Restrictions
1. Material of Resistance Element
Several metals are quite common for
use in resistance elements and the
purity of the metal affects its
characteristics. Platinum is by far the
most popular due to its linearity with
temperature. Other common materials
are nickel and copper, although most of
these are being replaced by platinum
elements. Other metals used, though
rarely, are Balco (an iron-nickel alloy),
tungsten and iridium.
The temperature coefficient of an
element is a physical and electrical
property of the material. This is a term
that describes the average resistance
change per unit of temperature from ice
point to the boiling point of water.
Different organizations have adopted
different temperature coefficients as
their standard. In 1983, the IEC
Commission) adopted the DIN
(Deutsche Institute for Normung)
standard of Platinum 100 ohm at 0oC
with a temperature coefficient of
0.00385 ohms per ohm degree
centigrade. This is now the accepted
standard of the industry in most
countries, although other units are
widely used. A quick explanation of
how the coefficient is derived is as
follows: Resistance at the boiling point
(100oC) =138.50 ohms. Resistance at
ice point (0oC) = 100.00 ohms. Divide
the difference (38.5) by 100 degrees
and then divide by the 100 ohm
nominal value of the element. The
result is the mean temperature
coefficient (alpha) of 0.00385 ohms per
ohm per oC.
Some of the less common materials and temperature coefficients are:
|Pt TC ||= ||.003902 (U.S. Industrial Standard)
|Pt TC||=||.003920 (Old U.S. Standard)
|Pt TC||=||.003923 (SAMA)
|Pt TC||=||.003916 (JIS)
||Nickel TC||=||0.00617 (DIN)
|Nickel TC||=||.00672 (Growing Less Common in U.S.)
|Tungsten TC ||=||0.0045
Please note that the temperature
coefficients are the average values
between 0 and 100oC. This is not to
say that the resistance vs. temperature
curves are truly linear over the
specified temperature range.
3. Nominal Resistance
Nominal Resistance is the prespecified
resistance value at a given
temperature. Most standards, including
IEC-751, use 0oC as their reference
point. The IEC standard is 100 ohms at
0oC, but other nominal resistances,
such as 50, 200, 400, 500, 1000 and
2000 ohm, are available.
4.Temperature Range of Application
Depending on the mechanical
configuration and manufacturing
methods, RTD's may be used from
-270oC to 850oC. Specifications for
temperature range will be different, for
thin film, wire wound and glass
encapsulated types, for example.
5. Physical Dimensions or Size Restrictions
The most critical dimension of the
element is outside diameter (O.D.),
because the element must often fit
within a protective sheath. The film type
elements have no O.D. dimension.To
calculate an equivalent dimension, we
need to find the diagonal of an end
cross section (this will be the widest
distance across the element as it is
inserted into a sheath).
The industry standard for platinum RTD's according to IEC-751 is + /- 0.12% (of resistance) at 0°C, commonly referred to as Class B accuracy. This will provide an accuracy of + /- 0.3°C at 0°C, which is quite good if you compare it to the + /- 2.2°C of a standard Type J or K thermocouple. But as the temperature increases, so does the permissible deviation due to the variations possible in the TC. So, not only do we have the possible + /- .3°C offset at 0°C, but also the probability that the TC is not equal to 0.00385. This could account for a permissible deviation of up to + /- 4.6°C at a maximum temperature of 850°C . But that's still better than the K thermocouple, which could be off by as much as + /- 6.4°C, and even more for the Type J, which is not recommended at this temperature. Because a well manufactured RTD will have high repeatability (relative to the application), Class B accuracy is generally sufficient unless there is a need for better interchangeability; or when measuring change of temperature; or if you know that you have special accuracy requirements.
When Class B accuracy is not quite enough, the International Electrotechnical Commission (IEC) offers us Class A accuracy, which permits + /- 0.15°C at 0°C and much tighter control of the TC. To ensure this control, the single ice point calibration acceptable for Class B sensors will not suffice. The IEC therefore states in section 4.2.2 of Standard 751. "The test for Class A thermometers shall be carried out at two or more temperatures suitably spaced over the stated working range".
The minimum and maximum temperatures of the stated working range are conve-nient points to chose and will ensure Class A accuracy, but will at the same time tend to drive up the cost of the sensor. It is more practical to look the application. If need to control the process most closely at 37°C, for example, choose a range from 0°C to 50°C This will fulfill your requirements without needlessly increasing costs or manufacturing restrictions. But remember, when specifying a Class A RTD, you must always include the working range at which it must perform to this accuracy.
Another word on Class A and Class B RTD's. These are IEC designations of accuracy. Although conforming to the use of a 0.00385 TC, the ASTM has its own designations of Grade A and Grade B that differ slightly from the IEC permissible deviations.
Of course, classes A and B or grades A and B cannot cover every possible accuracy specification desired. Then you need to spell out your requirements for the applications engineer. If your SPC/SQC charts indicate that you need to control a specific process within + /- 0.5°C at 250°C, even a Class A RTD will not do the job. As we discussed earlier, you may not actually need the accuracy at this point, but rather the repeatability. But if you believe that starting with an accurate sensor is the first step toward tight control of the process, request an accuracy of + /- 0.5°C at 250°C, or over the range of 200°C to 300°C. This is not unrealistic for a well-made RTD, although it requires special selection of the sensing element at this temperature. Keep in mind that this special selection will generally result in a longer delivery time and higher price tag on the RTD. Conversely, not all applications require even Class B accuracy. If you need to know only, "Is it hot or is it not?" can generally appreciate some savings by requesting a less accurate sensor that will still suit your needs.
All too often the specification will read something like "Accuracy within + /- 1.0%". My question: "Percent of what?" If it is meant to be percent of indicated value we need to clarify a few things. There are four primary temperature scales in use today. Kelvin and Rankine, which are absolute temperature scales, and Celsius and Fahrenheit, which are not. Let's take the freezing point of water in Celsius for example. What is + /- 1.0% of temperature accuracy at 0°C? A perfect reading? Possible, but not likely. If we were reading this in Fahrenheit the tolerance would be + /- 0.32°F; in Kelvin, it would be + /- 2.73°K, which equals + /- 2.73°C. So which is right? None. The specification was poorly written.It is acceptable, however, to use percent for % F.S. if you clearly state what the scale will be.
Or we can say percent of resistance at a given temperature, as the IEC does for the nominal resistance of a Class B RTD; 100 ohm,+ /- 0.12% at 0°C. Aside from these cases, it is generally better to state your requirements in terms of temperature tolerance in degrees over the temperature range where it is actually required.