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HNL Temperature Switches - Selection
HNL manufacture a wide range of temperature switches with both electrical outputs and pneumatic outputs. These include ATEX certified temperature switches suitable for use in hazardous areas.
Series 700 is a mechanical temperature switch - the Series 800 temperature switch is an electronic device that also functions as a temperature indicator and transmitter. Both are available
with aluminium or stainless steel housings and either direct or capillary mounting.
Choose from the following list of temperature switches:
Series 700 - HNL's most popular temperature switch
Mechanical Temperature Switching / Wide selection of Ranges and Outputs / Electrical & Pneumatic outputs
Series 800
Electronic Temperature Switching / Electrical output only / Indication & 4-20mA transmission
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Series 700 Temperature Switches (T)
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Series 700 Standard
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Series 700 EExd
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Series 700 Pneumatic
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Series 800 Temperature Switches
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Series 800 Smartstat
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HNL Temperature Switches - Glossary
ATEX
Capillary System
Direct Mounting
Electrical Output Switch
Hazardous Area
Pneumatic Output Switch
Pneumatic Temperature Switch
Range Screw
Range Spring
Reset
Scale Plate
Setpoint
Switch Housing
Temperature Switch
Thermowell
'U' Length
ATEX
ATEX covers the EU directives for explosion protection in industry. ATEX gets its name from the French title of the 94/9/EC directive:
Appareils destinés à être utilisés en ATmosphères EXplosibles.
Capillary System
Used to isolate the temperature switch housing from the temperature bulb. The housing is wall mounted using the bracket supplied, the
bulb fits into the thermowell and a capillary, typically from 1.5 to 4.0 metres in length connects the two. The capillary may be
armoured to provide greater mechanical protection.
Direct Mounting
A direct mount temperature switch comes with a 1/2" male thread above the bulb allowing it to be screwed directly into the thermowell.
Electrical Output Switch
A single-pole double-throw (S.P.D.T.) microswitch that can be used to make or break an electrical circuit when the temperature switch
operates. A microswitch with gold contacts is normally used on low voltage, low current applications (i.e. intrinsically safe circuits),
and for higher voltages/currents silver contacts are used. Most microswitches have a fixed switching differential, on some it is
adjustable and on others the microswitch must be manually reset with a pushbutton. Where double-pole double-throw (D.P.D.T.) switching
is required it is achieved using two S.P.D.T. microswitches ganged together to operate simultaneously. By setting the two switches at
different positions two switch setpoints can be achieved on the same temperature switch.
Hazardous Area
Area in which an explosive atmosphere exists or may exist. Explosive atmospheres may be caused by flammable gases, vapours or
combustible dusts. Hazardous areas are classified into zones depending upon the likelihood of an explosive atmosphere occurring and
its persistence if it does. Equipment designed for operation in hazardous areas must comply with the ATEX directive.
Pneumatic Output Switch
A pneumatic valve used to feed or vent a pneumatic circuit when the temperature switch operates. Pneumatic output switches can have
either a fixed switching differential, an adjustable switching differential or manually reset with a pushbutton.
Pneumatic Temperature Switch
A temperature switch having a pneumatic output.
Range Screw
The range screw is used to adjust the setpoint of the temperature switch. It does this by changing the
amount of compression on the range spring. The range screw on Series 700 switches is similar to a capstan and
adjustment is carried out using a tommy bar supplied with the switch.
Range Spring
A pre-compressed spring, the force generated by this determines the temperature at which the switch operates.
Reset
The reset (or switching differential) is the difference in the rising and falling temperatures at which the output switch operates. For
a fixed differential output switch this is typically about 1% to 3% of the switch range. For an adjustable differential output switch it
may be adjusted from about 5% to 12% of the switch range.
Scale Plate
An indicator to show the approximate setting of the temperature switch, as a percentage of the switch range.
Setpoint
The temperature at which the temperature switch is required to operate. A temperature switch may be set to operate on either a rising
temperature (high level alarm) or a falling temperature (low level alarm). Ideally the range of the switch should be chosen such
that the setpoint lies between 25% to 75% of this range.
Switch Housing
The enclosure containing the electrical or pneumatic output switch. For an electrical output one or more tapped connections
(usually M20 x 1.5) are provided to allow the cable to be brought into the housing via a suitable gland. For a pneumatic output two
or more bulkhead connections are provided for the output connections. Switch housings are available in either aluminium (epoxy painted)
or stainless steel.
Temperature Switch
A device designed to monitor a process temperature and provide an output when a set temperature (setpoint) is reached. A mechanical
temperature switch does this by applying the process temperature to a sealed bulb containing gas or liquid/vapour to create a
pressure. This pressure acts upon a diaphragm to generate a force which is compared to that of a precompressed range spring.
Thermowell
A thermowell (or thermopocket) is generally used to accomodate the bulb of the temperature switch. It is screwed (or flanged) into
the vessel or pipe, the temperature of which is to operate the switch. This allows the switch to be removed for calibration or
replacement without risk of process leakage.
'U' Length
The 'U' length is the rigid stem immersion length of a thermowell into a process and is required when calculating the bulb length
of a temperature switch. This distance is typically measured from the tip of the thermowell to the start of the thread.
The required bulb length can be calculated as the 'U' length + 'T' length (distance from the start of the thread to the top of the
thermowell) less 20mm. (e.g. if 'U' = 170mm and 'T' = 50mm, bulb length = 170 + 50 - 20 = 200mm).
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