Q:
What is Differential?
Q: What is the differential of your
thermostats?
Q: Can a special differential be requested?
Q: What is a snap action thermostat?
Q: How are your thermostats tested?
Q: Why do my test results differ from
the published operating temperature?
Q: What is the appropriate method to
test your thermostats?
Q: What is a CREEPER type thermostat?
Q: How does a PTCR electrical self-hold
thermal protector work?
Q: What is a thermal fuse and how
does it work?
Q: What is a thermal protector?
Q: What is a control thermostat?
Q: How do I calculate the resistance
of a thermistor?
Q: Do your products have agency approvals?
Q:
What is Differential?
The
differential or hysteresis is the difference in
temperature, measured in degrees Celsius, between
the point where the thermostat first operates and
the level where the contacts return to their original
state. ^ Top
Q:
What is the differential of your thermostats?
The published differential of our thermal protector
line (most Microtherm products) is 30°C ±15°C
below the set temperature. Since our products are
designed to be a protector they have been given
a rather large hysteresis which provides a significant
amount of cooling prior to resetting thus eliminating
the repetitive cycling on and off at a possibly
damaging or dangerously high temperature.
The differential of the Control Thermostats can
be selected as described in the brochure and is
specified in the part number. ^ Top
Q:
Can a special differential be requested?
Yes, in some cases a specific differential range
can be supplied through a selection process however
a minimum differential of 10°C must be maintained
in order to insure a strong snap of the bimetal
disc. Limitations do exist in the results obtainable
as we can only select thermostats from within the
actual range of temperatures which appeared on our
test reports. Usually, a lower set temperature will
have a smaller differential and a high set point
will have a larger differential. ^
Top
Q:
What is a snap action thermostat?
The snap action thermostat is the result of the
design of the bimetallic disc which is formed in
order to hold its original shape until the specified
transition temperature and then switches the position
of the contacts quickly. Also a differential or
hysteresis is built into the design which eliminates
the rapid and repeated on/off chattering or telegraphing
characteristic of creeper type thermostats.
^
Top
Q:
How are your thermostats tested?
Each Cantherm/Microtherm product is tested before
leaving our factory. Depending on the particular
model switch that you have purchased, they have
either been tested for response temperature on a
sample batch basis based on AQL 2.5 level I, or
tested on a 100% basis. The pill type thermal protectors
(F, B, T) are tested 100% and each device is selected
and categorized into the appropriate temperature
range and tolerance; the A and D series thermal
cutouts are batch tested as per the above level
of ANSI Z1.4 – 1993 (was MIL-STD-105-E).
The methods vary depending on the type but both
the pill and open type thermostats are currently
tested in an air oven equipped with 800 individually
computer monitored positions from which we record
the set and if required the reset temperature of
each device. This oven is also used to perform the
sample testing on the A & D types. The pill
type thermostats can also be tested using specially
designed machinery which monitors the contact state
after passing over each of a series of temperature
controlled plates. ^
Top
Q:
Why do my test results differ from the published
operating temperature?
Often the perceived operating temperature of a thermostat
in the actual application will differ from the published
temperature range because of thermal lag. This is
the result of the thermal mass of the device and
the thermal resistance of the insulation which slows
down the reaction of the bimetal disc. The bimetal
disc always operates at roughly the same temperature,
however the temperature inside the thermostat and
outside may be significantly different and time
must be allowed for the heat to penetrate and leave
the housing. The faster the temperature rises and
the larger the total temperature difference from
the beginning of the heating cycle to the specified
operating temperature the more lag will be experienced.
^ Top
Q:
What is the appropriate method to test your thermostats?
In order to standardize results and to be able to
compare one thermal switch to another, the industry
has adopted a standard rate of temperature change
which ranges from 0.25°C to 1.0°C change
per minute. The larger and the more thermally resistive
the package the slower the temperature rise rate
must be. After the rise from room temperature a
10 – 15 minute soak at the lower threshold
of the published range of operation is required
in order to allow temperatures to stabilize. After
the soak the specified temperature rise rate must
be used and the contacts monitored closely to determine
the correct operating temperature. ^
Top
Q:
What is a CREEPER type thermostat?
A creeper thermostat is generally manufactured with
a blade type bimetal which has not been pre-formed
as is a disc type. Calibration is achieved by either
bending the bimetal or adjusting the pressure of
the contacts until the desired result is achieved.
Essentially a creeper has almost no differential
and once the operating temperature has been reached
it will switch off and on at or near that temperature.
This is ideal for controlling an electric blanket
but their usefulness as a thermal protector is marginal
since it will maintain the device at the high temperature
limit possibly resulting in deterioration of the
product. The successive cycling on and off of the
power could damage sensitive electronics or pose
a hazard to the operator of a piece of equipment.
^ Top
Q:
How does a PTCR electrical self-hold thermal protector
work?
A PTCR thermal protector has a tiny PTC (positive
temperature coefficient) ceramic heater imbedded
into its body, it is connected in parallel with
the contacts of the switch and as long as the switch
is closed it does not heat. When the contacts of
the protector open due to an external heat source
the PTC heater begins to heat the bimetal disc which
prevents it from reaching its reset temperature.
As long as electrical power is available, the contacts
will remain open in dead air down to -25°C.
The contacts will reset a short time after the electrical
power is removed from the product. Therefore the
PTCR protector acts like a manual resetting device
by permanently removing the power from a potentially
dangerous appliance without the inconveniences of
having to push a button to reset it. ^
Top
Q:
What is a thermal fuse and how does it work?
A thermal fuse is a single operation device similar
to a current limiting fuse which is sensitive to
surrounding temperatures instead of the current
passing through it. The small low current type are
made using a fusible link material whereas the higher
capacity fuses are equipped with an organic or wax
pellet which hold the contacts together by compressing
a spring. When their temperature threshold is reached
the link or the organic pellet melts thus breaking
the electrical circuit.
Care must be taken during transportation, storage
and installation that the device not be subjected
to temperatures near the operating temperature.
Special attention must be taken during soldering
of the leads.
It is also critical that the Tc (continuous temperature)
not be exceeded as it could cause deterioration
of the link or pellet and lead to eventual failure
of the device. ^ Top
Q:
What is a thermal protector?
A thermal protector is a device which has been designed
to protect an apparatus or appliance against an
excessively high temperature. They are typically
used in electric motors, transformers and heaters
to prevent overheating but can be installed in any
type of equipment that is sensitive to high temperatures
or to prevent damage, fire or personal injury. Because
of the nature of the applications thermal protectors
provide significant cooling before they will reset
and allow the power to circulate again. ^
Top
Q:
What is a control thermostat?
A thermal controller is a precision thermostat which
is designed to maintain the temperature of a device
or enclosure within a pre-determined range. The
contact configuration, the differential/hysteresis
and the tolerances of both the set and reset temperatures
can be specified within published parameters.
Contacts are available to open or close on temperature
rise or fall thus eliminating the need to depend
on the reset temperature of a lesser device to perform
your primary task. This is ideal for applications
where a contact must operate on temperature fall
in order to prevent freezing or similar outcome.
^ Top
Q:
How do I calculate the resistence of a thermistor?
B
or beta value is the thermal exponent of the NTC
thermistor, which is defined as the ratio of the
difference between the napierian logarithms of the
zero power resistance at two temperatures to the
difference between the reciprocals of the two temperatures.
You can calculate the RT2 using this formula:
Here: B=3380 T1=25 RT1=10Kohm
^ Top
Q:
Do your products have agency approvals?
Yes, all Microtherm thermal protectors sold by Cantherm
have UL, CSA and VDE approvals along with certifications
by other agencies such as BEAB, CB, and others.
Since Microtherm is the registered owner of the
certifications they will all appear under the name
of Microtherm GmbH, a search at the agency in question
will list all of the approvals. The file numbers
are listed on the individual product brochures or
can be obtained by calling the Cantherm sales department.
^ Top
Simply perform a search for Microtherm GmbH at any
of the following agency links:
UL: http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/geosrch.html
CSA: http://directories.csa-international.org/
VDE: http://www.vde.com/VDE_PI_en/Zertifizierungsregister/Online_Search.htm
FAQ's
8415
Mountain
Sights Avenue, Montreal (Quebec), H4P 2B8 Canada
