By Matthias Muziol
Platinum thin-film sensors
are available in a variety of forms as shown.
Thin-film sensors for precise temperature readings
In recent generations of household
appliances, there has been a noticeable increase in the use of electronic
circuits. The use of such control circuits has led to increased appliance
functionality, a reduction in the use of resources, and effective and
lasting savings in household costs. Exact temperature measurements combined
with carefully designed electronic controllers extend the lifetime of
both an appliance and its components. For precise temperature measurement,
the electronic control circuit requires the signal from an accurate temperature
sensor. Platinum-temperature sensors based on thin-film technology provides
this accuracy, making them ideal for a wide variety of domestic applications.
Structure and properties
Today’s platinum temperature
sensors are comprised of structured and encapsulated platinum thin-film
elements, and are created under clean-room conditions. This is the stage
at which the reproducibility of the method is tested with a view to subsequent
large-scale production. The platinum-metal structure is deposited on an
aluminium-oxide ceramic substrate and then covered with a thin layer of
glass. The bond pads for creating electrical connections to the meander-shaped
platinum structure are also sealed using a special glass-ceramic paste.
(Fig. 1)
Platinum thin-film sensors
are available in a broad variety of forms. Depending on the proposed
application, the sensor may be unhoused, encapsulated in ceramic, configured
as a surface mounted device (SMD) or located within a plastic housing
with rigid pins.
Platinum sensors exploit the
fact that the electrical resistance of platinum metal is temperature dependent.
For each type of sensor there are specified tolerance ratings, which dictate
the measurement precision to be expected when that type of sensor is used.
Advantages
The economic large-scale production
of platinum temperature sensors is now possible due to the application
of modern manufacturing techniques based on thin-film technology. This
development has led to price levels, which were previously the exclusive
domain of NTC elements.
By modifying individual process
stages, a broad range of variants differing in structure, resistance values
and geometry can be realized. Besides classical temperature-sensing applications,
platinum combination sensors also enable other physical quantities to
be monitored. Such combination sensors are used, for example, as air mass
meters in car engines. Further, the high level of automation in the production
process permits extremely precise miniature sensors to be manufactured
which are capable of meeting the exacting demands required by physical,
biochemical and medical applications. By fine tuning the sensors properties
to meet these requirements, even mass markets can be supplied with these
advanced technological sensor devices.
High tech in the home
Given this situation, it is
not surprising that products based on platinum thin- film technology have
been finding increasing use in the domestic environment. With the growing
use of electronic-control systems in the new generation of domestic appliances,
platinum temperature sensors have been finding increasingly widespread
use in ovens where they have replaced electromechanical regulators, such
as capillary tube and solid expansion thermometers and NTC thermistors.
Typical sensor applications in the food preparation sector are shown in
Fig. 2.
Even conventional hotplates
can be fitted with a continuously variable temperature control. The encapsulated
platinum sensor is positioned so that it is in contact with the cast-iron
plate from below and thus able to register the temperature of the hotplate.
Normally the power supplied to the plate is regulated by a stage switch.
The increased control sensitivity is made possible by incorporating electronic
circuitry capable of interpreting and acting upon the sensor signals.
Similar technology is also
being used in induction hotplates and radiant heating elements and in
the silicon nitride hotplates currently under development and which are
characterized by the excellent heat transfer between the heat source,
the sensor and the sauce pan. Gas hotplates could also be provided with
the latest in domestic temperature sensing technology by fitting them
with electric temperature- control circuitry.
Intelligent process control
based on platinum temperature sensors and tailored electronics provides
an effective method of improving both kitchen safety and the user friendliness
of kitchen appliances, and also contributes to reducing energy consumption.
The growth in the use of platinum
sensor technology is to be found, above all, in ovens. The sensors are
used to fulfill two main functions. The sensors measure the temperature
in the oven during the baking process. The precise temperature measurement
achievable contributes effectively to reducing hunting oscillations. The
electronic control systems now in use have also made it possible to store
different heating profiles, which the oven can use to react intelligently
to the different types of food being baked. By exploiting this type of
technology there is no longer a need to cook by trial and error nor to
have to adapt the baking process to the quirks and peculiarities of a
particular oven.
The use of catalytic converters
in modern domestic ovens means that odors in the kitchen arising from
baking and roasting are now a thing of the past. The catalytic converters
are fitted into the oven’s venting shaft and convert unpleasant
smelling substances into CO, CO2 or other non-odorous gases. To provide
the best possible control of the catalytic odour eliminator, platinum
temperature sensors are employed to ensure that the catalyst is working
in its optimum operating range, typically 500°C-550°C. An analogous
function is fulfilled by sensors that are used to control the pyrolytic
cleaning process. This involves heating the entire oven to a temperature
of around 550°C, a temperature high enough to make oven cleaning with
the usual chemical cleaners superfluous. The high temperature achieved,
pyrolyses any organic compounds present and the ash residues can then
be easily removed from the oven by brushing. Platinum high-temperature
sensors provide a reliable means of monitoring and regulating the temperature
and thus safeguard the safe and complete pyrolysis of food deposits in
the oven. Economic considerations also make platinum temperature sensors
attractive to users, enabling high temperatures to be measured at an affordable
system price. In contrast to conventional thermoelements, platinum sensors
do not demand complicated analysis electronics and are insensitive to
electromagnetic field influences.
Applications
Fig. 3 presents a number of
platinum sensors currently being used in kitchen appliances. The probe
with the flange and the long stainless-steel housing was developed for
use in ovens. Also available are models equipped with high-voltage insulation,
which can be built-in if demanded by the control electronics. The shorter
sensor with the tapered form is a customized high temperature probe, which
was built to order for a client. On the right of Fig. 3 is a sensor designed
for measuring surface temperatures. The sensor, which is integrated into
a plastic housing, is easily mounted via the bore hole. Besides these
special probe geometries, a universal sensor in a ceramic housing is also
available. Situated in the center of the picture is an SMD component on
a long ceramic plate that is used for temperature measurements up to 750°C.
In addition to the designs shown, Heraeus Sensor-Nite is able to develop
and manufacture sensor geometries and housings that are specially tailored
to meet specific customer requirements.
Future prospects/outlook
In principle platinum sensors
can be used for applications in the temperature range -200°C to +1,000°C.
In the kitchen, temperatures typically ranging between 200°C and 600°C
are measured. The ability to accurately measure lower temperatures, opens
the possibility of using platinum sensors in fridges, freezers, washing
machines and tumble dryers. Given these prospects, it is clear that the
end of the road leading to the intelligent kitchen and household has not
yet been reached.
Matthias Muziol is a design
engineer at Heraeus Sensor GmbH, Kleinostheim, Germany. |
