Application Note PA-R
The purpose of this article is to provide a review of platinum
RTD elements, internal construction, and fabrication into
complete packaged assemblies.
Platinum resistance sensing elements are generally
manufactured in four configurations. Only RdF manufactures all
four with the special processes required in each configuration:
Wire-Wound
Film
Coil
Hollow Annulus
The wire-wound, or resistor-style sensing element (see Figure 1),
is built by simply winding a small diameter platinum sensing wire
around a mandrel constructed of non-electrically conductive
material. The winding mandrel has provisions for anchoring a
larger lead-wire, which facilitates attachment to external
connections. The sensing wire is attached to the larger lead
usually by resistance spotwelding and/or high-temperature
soldering. The sensing wire is then overcoated with a
non-conductive protective coating such as ceramic cement, or
glassy coating.
The second style of sensing element — the thin film style — is
relatively new as a production element, but dominates new
general-purpose applications in place of the Figure 1 woundon-mandrel style. RdF certiified suppliers deposit a thin layer of
platinum on a ceramic substrate followed by very high
temperature annealing and stabilization. The platinum is
deposited in a resistance pattern, usually with provisions for
adjusting the final resistance by cutting the circuit in a trim area.
This element is produced in a flat configuration and may be
over-molded to other configurations. One advantage of this type
of sensing element is that greater resistance can be placed in
smaller areas than with other elements. As an example, a 1000Ω
sensor is typically manufactured no larger than 1.6mm wide x 2.6
mm long.
Figure 2. The film-type sensing element is made by depositing a thin layer
of platinum in a resistance pattern on a ceramic substrate. A layer of glass
is applied for protection.
Figure 1. The wire-wound sensing element is built by winding a small
diameter platinum sensing wire around a nonconducting mandrel
In the construction of this, or any of the element styles, it is
important to match the coefficients of thermal expansion between
sensing wire, mandrel, and coating as closely as possible. This
minimizes compressive and tensile strain on the small diameter
sensing wire. The less strain on the wire, the better the
repeatability and stability of the sensor. A good indication of the
amount of strain in a sensor is the closeness between the
temperature coefficient of the finished sensor and that of the
original unsupported sensing wire.
Platinum sensing wire is usually purchased in a hard tempered
condition. This makes winding easier, but also requires annealing
of the element to obtain the optimum temperature coefficient. This
annealing process will also affect the performance of the sensor if
it is not done properly.
These sensors are typically coated with a thin glassy coating
over the resistance grid and with heavy glassy reinforcement over
the lead attachment for mechanical and moisture protection. A
typical film type sensor is shown in Figure 2. These sensors,
being a very thin film, are susceptible to some substrate
expansion mismatch and are available with a maximum
temperature coefficient of 0.00385ohm/ohm/°C. This solid sensor,
combined with RdF‘s packaging processes, produces extremely
rugged RTD assemblies.
The third style sensing element, the coiled element, is
manufactured in several different designs, but they all have the
same basic goal — to provide a strain-free sensing element. This
is done by eliminating the constraints of a normal winding
mandrel and overcoating. This freedom minimizes the influence of
the mis-match of coefficients of thermal expansion between
materials.
These elements are constructed by starting with a helical coil of
platinum sensing wire that resembles a lightbulb filament. The
wire is inserted into the internal bores of an insulating mandrel.
Powder is packed around the coil to prevent it from shorting and
to provide vibration resistance during service. In other versions of
this element, the tangent points on the coil are cemented to the
winding element or the coil is placed on the outside of a
-1-
threaded mandrel (or the insulated inner wall of the sensor
assembly). Figure 3 shows a typical coiled element manufactured
by RdF.
Whatever strain-free method is used, it generally provides a
sensor with a higher temperature coefficient and better stability.
Of all typical designs, the RdF design, shown here, is the most
rugged. All laboratory standard platinum resistance thermometers
(SPRTs) are specialized strain-free designs.
Figure 3. The coiled element sensor, made by inserting the helical
sensing wires into a packed powder-filled insulating mandrel, provides a
strain-free sensing element.
The last style, the hollow annulus sensing element, uses an
open-ended metal winding mandrel which increases fluid contact
and lowers thermal mass to provide a faster time response. In a
typical design, shown in Figure 4, the winding mandrel —
fabricated from weldable corrosion resistant metal — is machined
to eliminate the internal mass. The winding area is coated with an
insulating material and the sensing wire-wound to a
predetermined value. The element is then covered with a coating
of insulating material. This sensor has a thin external metal
sheath welded over the winding area when it is used with no
additional housing.
Figure 4. The hollow annulus-type element is made by winding platinum
sensing wire around a hollow corrosion-resistant metal mandrel. The
entire unit is coated with an inuslating material.
This element has the advantages of being completely sealed and
having an extremely fast time response, but it is the most
expensive of the four types. The large winding diameter enables
high resistance sensors to perform optimally in cryogenic fluid
applications.
Each of the four sensing elements discussed has advantages
and disadvantages. Before purchasing an RTD assembly, it is
advisable to know what type of element the manufacturer is
intending to provide and its limitations.
The sensing elements are very seldom used by themselves and
must be packaged to interface with existing hardware and
withstand service conditions. Due to thousands of applications for
RTDs, there are an infinite number of RTD assembly
configurations. This section gives a brief description of the most
simple and commonly used constructions.
The assembly process begins with connections between the
element leads and larger intermediate or external leads. These
connections are generally made by welding, brazing or soldering.
Welding is preferable because it keeps the junction metallurgically
pure and avoids the introduction of additional materials that may
generate an EMF output, as in a thermocouple.
Intermediate leadwires are often used due to the length of the
sensor assembly or the temperature profile exceeding the
capabilities of the external leadwire (see Figure 5). Intermediate
leadwires are usually solid as opposed to stranded for external
wires. Solid wires are chosen because they are easier to handle,
and are not subject to flexing. Common materials used for
intermediate leads are: nickel, copper, constantan, nickel-plated
or clad copper (most common), stainless steel clad copper, or
platinum. The intermediate leadwires are then insulated with
ceramic tubing, fiberglass sleeving, or organic sleeving if the
temperature permits. External leadwires may be specified by the
customer, but are usually stranded, plated copper conductors
insulated with Teflon or fiberglass.
Figure 5. Intermediate lead wires in a sensor assembly are typically solid
wire and are welded to the element leads. Sensor elements must be
packaged to interface with other hardware and to withstand service
conditions.
The subassembly must then be placed into a protective housing
(shown as a tubular sheath in Figure 6). This housing is designed
to physically mate with the process and protect the subassembly.
After being placed into the sheath, the subassembly is packed
with a fine grit metal oxide powder, typically aluminum oxide or
magnesium oxide. This acts as an electrical insulator while
increasing thermal conductivity between the sensing element and
the process. The powder also supports the subassembly and
protects against damage from mechanical vibration or shock.
These oxides are hygroscopic and must be protected from the
environment, particularly in humid areas. If the powder is allowed
to absorb moisture, a high-impedance parallel shunt across the
sensing element will result and produce a low erroneous output.
Sealing RTD assemblies is one of the biggest problems faced by
manufacturers today. Most industrial sensors are sealed with
epoxy potting compounds poured into the open end of the sheath.
A satisfactory seal can be achieved if adequate attention is given
to the proper cleaning and preparation of the surfaces, proper
mixing and curing of the compound, and the penetration of potting
to a reasonable depth. (See Figure 6).
Ceramic cement is another sealant for assemblies with service
temperatures higher than the recommended range for epoxy. The
cement is usually impregnated with a silicone fluid for moisture
resistance. A more secure but also more expensive method for
sealing an assembly is a glass-to-metal or a ceramic-to-metal
seat, accomplished by welding a header or connector to the
sensor housing.
-2©2003 RdF Corporation • 23 Elm Avenue, Hudson, NH 03051-0490 USA • TEL 603-882-5195 • 800-445-8367 • FAX 603-882-6925 • www.rdfcorp.com
Figure 6. After the sensor assembly is encased in a protective sheath and
surrounded by a fine grit metal oxide insulating powder, it is sealed with
an epoxy potting compound or ceramic cement.
Commonly used sheath materials for industrial sensors and
their recommended service are as follows:
• Brass. Used for low-temperature commodity applications in mild
environments such as air or water temperature measurements.
• 304 stainless steel. Good corrosion-resistant characteristics
and adequate for most common corrosive agents encountered in
industry; service temperatures up to 900°F; advantages for
machined housings.
• 316 stainless steel. The most corrosion resistant of the
common stainless steels; increased corrosion resistance to
non-oxidizing acids such as sulphuric, phosphoric, and acetic;
service temperature up to 900°F; standard on RdF industrial
probes and capsules.
• Inconel 600. Better high-temperature corrosion resistance than
stainless steels; pre-oxidized sheaths for temperatures up to
1500°F; optional on RdF industrial probes.
Now that the sensor is built it must be able to mate with the
process where the measurement is to be made. Generally, this is
done using standard pipe threads. This standardizes process and
conduit connections and reduces the number of different types of
threads used in a plant. For example, a direct immersion sensor
assembly for most applications will have a 1/2 NPT thread.
In processes where it is not practical or possible to open the
process line to replace sensors, a thermowell is used. The
thermowell is permanently installed in the system and a springloaded sensor whose length is designed to fit into the thermowell
is installed. The sensor length is designed to bottom in the
thermowell with a predetermined spring force. This contact
increases thermal conductivity to the thermowell and supports the
sensing sheath for protection from process vibration. The sensor
can now be removed from the thermowell for replacement or
calibration without disrupting the process. RdF’s exclusive
springload design interaces with all 1/2 NPT hardware, offers
failsafe spring loading directly on the end of the probe sheath,
and prevents probe rotation due to vibration that can damage
connection leads.
Thermowell materials vary from carbon steel to Hastelloy,
depending on process fluid and service temperatures. Figure 7
illustrates the three common thermowell mounting configurations:
threaded, flanged, and socketweld. Connections between the
thermowell and the connection heads are typically made using
pipe nipples. The pipe nipple length varies depending on the
insulation material requirements and clearances in the plant. It is
also common in some applications to use a union/nipple
extension which assists installation. The pipe union permits easier
removal of the sensor assembly and provides a method for
orienting the conduit connection in a particular direction.
Connection heads are generally used as protection for sensor
leadwire exit and as a junction point for leadwires going to other
instrumentation (readout or control). Connection heads have
screw-on covers, internal terminal boards, and female pipe
threads for conduit and process connections.
Standard connection heads are usually:
• General purpose. Cast iron or aluminum heads with screwed
on gasketed covers; designed to withstand normal plant
conditions including a washdown of equipment and generally will
meet NEMA 4 requirements; standard size is approximately 4 x 4
in.
• Explosion proof. Also cast connection heads with screwed-on
covers; designed to be used in areas with explosive or
combustible materials and often certified by Factory Mutual.
RdF’s explosion proof head is smaller than others: approximately
2.75 x 2.75 in. with a custom terminal block leaving good cavity
volume and up to eight connection posts to fit most applications.
• Specialty connection heads. A variety of plastic or stainless
steel connection heads available for use in sanitary applications
or near corrosive processes such as chemical processing for
plating.
Figure 7. The common thermowell mounting configurations are the
threaded, socketweld and flanged thermowell
©RdF, 2003, updated excerpts.
Original Publishing SENSORS, March 1985, AN ADVANSTAR
PUBLICATION