Thermocouples are generally known and are used for temperature measurement
and/or control in many (industrial) processes and devices intended for these processes.
A thermocouple generally comprises two wires made from different metals,
which extend in a first direction and are in contact with one another at a measurement
point at their ends. This measurement point is referred to in the specialist field
as the "junction", a term which will be used below in the present description. The
other ends of the wires can be coupled to the terminals of the measurement instrument.
When the junction (hot weld) is heated, a small voltage can be measured across the
ends, known as the Seebeck effect. This Seebeck effect is composed of two components,
namely the Peltier voltage built up at the junction and the terminals, and the Thomson
voltage generated in the wires as a result of the temperature gradient. The wires
are generally accommodated in a thermocouple rod which comprises a protective sleeve.
The junction may be accommodated within a protective cap. A protection of this type
can also be omitted in order to obtain a faster response in the event of temperature
One use of thermocouples is for the measurement and control of temperature
in a device for treating food products with conditioned air, in particular an oven.
A device of this type typically comprises a housing with an entry for products to
be treated and an exit for treated products, a conveyor device being provided for
conveying the products from the entry to the exit. While they are being transported
in the treatment device, the products are exposed to conditioned air, such as air
at a high temperature and/or steam. Cf. for example EP-A-0 953 286. The conditioned
air is prepared in a suitable way, for example with the aid of heat exchangers,
and circulated through the device with the aid of fans.
In a practical embodiment of an oven in accordance with the abovementioned
patent application, one or more thermocouples are disposed vertically in the oven
chamber, hanging from the hood of the oven, the housing of which is composed of
a trough and a hood. When the oven is in use, the thermocouple measures the temperature
in the oven space. The measurement signal is used to control the temperature in
It has now been found that significant fluctuations, in particular
sudden drops followed over the course of time by rapid increases, occur in the measurement
signal from the thermocouple. This leads to constant changes in the temperature
control and relatively extensive deviations of the temperature in the oven space
from the desired treatment temperature. This phenomenon is observed in particular
in a humid oven atmosphere with dew points from approximately 80°C and under moderate
temperature conditions up to approximately 150°C, with the temperature higher than
or at least equal to the dew point.
It is an object of the present invention to eliminate one or more
of the abovementioned drawbacks.
More particularly, it is an object of the invention to provide a thermocouple,
in particular for use in a treatment device for treating food products with conditioned
air, which provides a more stable measurement signal.
A further object of the invention is to provide a treatment device
of this type for treating food products with conditioned air, in particular an oven,
having a thermocouple-based temperature control for maintaining a constant treatment
According to the invention, in the thermocouple of the type described
in the introduction, for this purpose the ends are bent with respect to the said
The device is based on the insight that moisture which is present
within the treatment space condenses locally, inter alia on the thermocouple, and
accumulates at the bottom end of the latter in the form of drops. In other words,
it accumulates at the position where the junction is located in the case of a vertically
disposed thermocouple with a conventional configuration. In this situation, the
thermocouple is measuring the temperature of the drops rather than the temperature
of the air. These drops cause a fall in the measurement signal. In response to this
measurement signal, the temperature control supplies more heat to the device in
order to match the measurement signal to a desired, predetermined temperature value.
Over the course of time, the drop drops off the bottom end, so that the thermocouple
is once again measuring the temperature which is actually present in the oven space,
but this temperature, as a result of the additional heat supplied, has in the meantime
risen to above the desired temperature, with the result that the temperature control
causes less heat to be fed to the device. This process of drop formation and influencing
the temperature control constantly recurs and causes the interference which has
been recorded in the measurement signal, together with temperature fluctuations.
In the thermocouple according to the invention, the ends, and therefore
the junction, are bent with respect to the first (longitudinal) direction of the
metal wires. This enables the thermocouple to be positioned in such a way that the
junction is no longer the lowest point of the thermocouple. Any drops which condense
on the junction flow directly to the lowest point, and therefore have much less
or no influence on the measurement signal. This leads to a more stable measurement
signal and, when used in a temperature control system, to a more constant temperature.
The same also applies if the humidity of the conditioned air is subject to fluctuations,
for example when using steam of varying quality.
The ends are advantageously bent through an angle of at least 90°.
It is preferable for the ends to extend in a direction opposite to
the said first direction. In this preferred embodiment, given a vertical arrangement
of the thermocouple, the junction is bent through an angle of approximately 180°
with respect to the thermocouple rod in which the wires are generally disposed parallel
to one another in the said first direction. The junction is therefore shifted upwards,
so that the measurement point is not at the lowest point of the thermocouple, and
any condensed drops have no interfering influence on the measurement signal. Even
when disposed horizontally, the thermocouple can be positioned in such a manner
that the measurement point is not at the lowest point.
In a further embodiment, the thermocouple, in particular the thermocouple
rod, is provided with a protective cap for shielding the measurement point from
moisture. Any drops which condense on the thermocouple rod and flow down the rod
are diverted away by the protective cap, so that it is impossible for them to reach
the junction under any circumstances.
The thermocouple according to the invention may be of any type known
in the specialist field with standard combinations of metals. A temperature-measuring
device equipped with a thermocouple according to the invention may comprise the
standard reference and/or compensation circuits or programs.
According to a further aspect, the invention relates to the use of
a thermocouple according to the invention for controlling the temperature in a device
for treating food products with conditioned air, in particular an oven. In this
use according to the invention, the abovementioned drawbacks of the prior art do
not arise and a stable treatment temperature is achieved.
A further aspect of the invention relates to a treatment device for
treating food products with conditioned air, in particular an oven, provided with
at least one thermocouple according to the invention. The treatment device, in particular
an oven for baking, roasting or cooking (or otherwise treating with heated and/or
humid air) food products, in particular meat products, such as chicken pieces, products
preformed from meat material, such as hamburgers, etc., generally comprises, as
has already been indicated in the discussion of the prior art, a housing, optionally
composed of more than one part, with an entry for products to be treated and an
exit for treated products. A conveyor passes the products through the treatment
space, which may be divided into a plurality of sections each with a dedicated temperature
measurement and control system, from the entry to the exit. The conveyor may be
of rectilinear configuration, as used in a tunnel oven, or of helical configuration.
The climate control can be effected in a manner which is known per se, except for
the provision of one or more thermocouples according to the invention. A preferred
embodiment relates to an oven with helical conveyor in which at least one thermocouple
according to the invention is suspended from the roof of the housing, in particular
from a removable cap thereof.
The invention is explained below with reference to the appended drawing,
- Figs. 1 and 2 show an embodiment of a thermocouple according to the invention;
- Fig. 3 shows an embodiment of an oven according to the invention.
Figs. 1 and 2 diagrammatically depict an embodiment of a thermocouple
according to the invention. The thermocouple, denoted overall by reference numeral
10, comprises a thermocouple rod 12 made from a heat-resistant, electrically nonconductive
material, in which two wires 14 and 16 are arranged parallel to and isolated from
one another in the longitudinal direction of the rod 12. The wires 14 and 16 are
made from different metals. At one end 18 and 20, the wires can be connected to
the generally copper terminals of the associated measurement device. The other ends
22 and 24 are in contact with one another and form a junction 26, the measurement
point. The thermocouple rod 12 shields the wires 14 and 16 from their ends 18 and
20, respectively, to close to the junction 26. The junction 26 is not shielded and
is in direct contact with the atmosphere whereof the temperature is to be measured.
In this thermocouple 10, the wire ends 22 and 24 are bent through 180°, so that
the junction 26 is located above the lowest point, denoted by reference numeral
30. Drops of condensation which accumulate on the bottom end of the thermocouple
10 during use flow towards this lowest point and then drop off at some time. Consequently,
these drops cannot interfere with the measurement signal from the thermocouple 10.
A protective cap 32, which shields the junction 26 and wire ends 22 and 24 from
drops of moisture which condense on the thermocouple and flow downwards during use,
is provided at the lower end of the thermocouple rod 12, above the junction 26.
This reduces the risk of interference with the measurement signal still further.
Fig. 3 shows a diagrammatic embodiment of an oven according to the
invention. The oven 40 comprises a housing 42, which comprises a trough 44 for collecting
fat, drops and the like, and a cap 46. In the cap 46 there are openings or cutouts
which are used as entry 48 and exit 49 for the food products. From the entry 48,
a conveyor 50 runs through the oven space 52 delimited by the housing 42 to the
exit 49. The conveyor 50 is in this case an endless conveyor belt, for example a
so-called grill belt, which is designed to receive the food products (not shown)
and is driven by at least one of the central columns 53. The conveyor 50 passes
through a helical conveyor path, which in a first section 54 of the oven space 52
runs upwards from the entry 48, crosses over the oven space at the top side of the
column 53 and in a second section 56 continues downwards to the exit 49. Heat exchangers
58, of which only one is diagrammatically indicated, in the head space for the cap
46 are responsible for conditioning the oven atmosphere which is circulated with
the aid of fans 60 (only one of these is illustrated) from the interior of a column
53 through outlet openings 61, over the products on the conveyor 50. In each section
54 and 56, the atmosphere can be set to a specific temperature and/or humidity level,
for example using spray means 62. In the embodiment illustrated, diagrammatically
depicted thermocouples 10 according to the invention are provided for the purpose
of temperature control; the thermocouples are suspended vertically in the columns
53 from the bottom of the cap 46.