The temperature is to be measured on a hot pipeline, but the display must be located several meters away on an easily accessible control panel.
Electrical temperature measurement would be possible. However, in many machines and systems, a purely mechanical:
gas-actuated thermometer with remote capillary
is deliberately used for such applications.
The temperature sensor is located at the process, while the display housing is connected to the sensor via a thin capillary line.
At first, this sounds straightforward.
In practice, however, the measuring range alone does not determine whether the temperature will be indicated reliably.
At least equally important are:
- length of the capillary line,
- ambient temperature along the remote line,
- temperature at the display housing,
- bending radius of the capillary,
- mechanical load and vibration stress,
- sensor diameter and active sensor length,
- sensor mounting position,
- insertion depth,
- use of a thermowell,
- heat dissipation through the process connection and surrounding environment.
The capillary line is particularly critical.
It is not simply a mechanical connection between the sensor and the display, but an integral part of the closed measuring system.
The remote line of a gas-actuated thermometer should therefore already be planned during system design. Retrofitting it along arbitrary routes, with tight bending radii or through strongly varying temperature zones can impair mechanical reliability and, under unfavorable conditions, also the measurement quality.
How Does a Gas-Actuated Thermometer Work?
A gas-actuated thermometer uses the temperature-dependent pressure change of an enclosed gas.
In simplified form, the measuring system consists of:
- temperature sensor or stem,
- capillary line,
- pressure element or Bourdon tube in the display housing,
- pointer mechanism and dial.
The sensor, capillary and measuring element form a closed system filled with an inert gas.
If the temperature at the sensor changes, the pressure within this closed system also changes.
This pressure acts on the measuring element inside the display housing.
Its movement is transferred to the pointer via the movement mechanism.
This creates a mechanical temperature indication without any electrical auxiliary power.
An important difference compared with a bimetal thermometer
In a bimetal thermometer, the temperature-sensitive measuring element is located directly in or on the stem.
In a gas-actuated thermometer, however, the pressure change can be transmitted via a remote capillary to a display housing mounted at a distance.
This makes applications possible in which:
- the measuring point is difficult to access,
- the temperature must be read from a safe distance,
- the display instrument is installed in a control panel,
- strong process vibrations should be kept away from the pointer mechanism.
What Is the Function of the Capillary Line?
The capillary line connects the temperature sensor to the measuring element in the display housing.
It is filled with the same measuring gas as the sensor and is therefore part of the actual measuring system.
A typical remote capillary has only a very small diameter.
This keeps its internal volume as small as possible in relation to the effective volume of the sensor.
This is important because the temperature should actually be measured at the:
sensor
and not the average temperature of:
sensor + capillary line + measuring element
.
In practice, the influence of the remote line can be greatly reduced and partially compensated for by design.
Nevertheless, its length and thermal exposure cannot be chosen arbitrarily.
The specific thermometer version must therefore always match the application.
How Long May the Remote Line Be?
The maximum permissible capillary length depends on the instrument.
With an industrial gas-actuated thermometer, several meters or even considerably longer remote lines can be realized.
However, this does not mean:
the longer, the better
.
As a general rule, the remote line should only be as long as the installation actually requires.
An unnecessarily long capillary has several disadvantages:
- larger gas volume influenced by ambient temperature,
- a longer routing path that requires mechanical protection,
- more fixing points,
- greater risk of kinking or damage,
- more complex installation,
- more excess line if the installation position changes.
Depending on the version, remote lines of several tens of meters are possible for the WIKA Type 73, for example.
However, the actual required length should already be determined when ordering based on the planned routing path.
When planning the length, consider:
actual route + bends + installation path + small service/installation reserve
.
The straight-line distance between sensor and display is not sufficient for planning.
Why Does Ambient Temperature Influence the Measurement?
The gas is not located only inside the sensor.
The following also contain part of the measuring gas:
- capillary line,
- measuring element,
- connection areas.
If their temperature changes, the internal pressure of the system also changes.
The design of the gas-actuated thermometer significantly reduces this influence.
Compensation elements can, for example, be incorporated into the display housing.
Nevertheless, the following general rule applies:
The larger the volume outside the actual temperature sensor and the more strongly its temperature fluctuates, the more important the influence of ambient temperature becomes.
A typical example
The sensor measures a constant:
100 °C
.
However, the capillary first runs through an area at:
20 °C
and then for several meters directly beside a hot pipeline with an ambient temperature of:
80 °C
.
The remote line is therefore additionally heated.
In a configuration that is not designed for this, the indication may be influenced.
The ambient conditions should be considered particularly carefully if:
- the capillary line is very long,
- large temperature differences exist along the line,
- the display housing becomes very hot or very cold,
- high measurement accuracy is required.
Correctly Planning the Capillary Routing
The shortest geometric connection is not automatically the best routing path.
Wherever possible, the remote line should:
- be protected against mechanical damage,
- not be routed directly along hot pipelines,
- not unnecessarily pass through strongly varying temperature zones,
- be kept away from moving machine components,
- not be routed over sharp edges,
- remain accessible for inspection and installation.
Routing directly beside the following is particularly unfavorable:
- steam lines,
- exhaust pipes,
- furnaces,
- heating elements,
- hot motor or machine housings.
It should also be avoided that one part of the capillary is exposed to intense sunlight while another remains permanently in the shade if this creates large and changing temperature differences.
If thermal loads cannot be avoided, compatibility with the intended remote line should already be checked during instrument selection.
Observe the Capillary Bending Radius
A capillary line may be bent.
However, it must:
not be kinked.
The permissible minimum bending radius depends on the specific capillary and its protective covering.
A bend that is too tight can:
- deform the cross-section,
- damage the capillary tube,
- create a local mechanical weak point,
- in extreme cases, cause leakage in the closed measuring system.
A damaged capillary cannot be repaired like a normal electrical cable simply by reconnecting two ends.
The measuring system has a defined gas filling and must be treated as one complete unit.
The bending radius specified by the manufacturer for the specific thermometer type must therefore be observed.
During installation, bends should be formed smoothly and without sharp rebending.
Mechanically Protect the Capillary Line
The relatively thin remote line is often more sensitive to mechanical damage than the robust thermometer housing.
Hazards may arise, for example, from:
- tools,
- maintenance work,
- moving covers,
- sharp sheet-metal edges,
- rubbing against machine components,
- foot traffic,
- transport activities.
In such areas, an additional protective sheath or spiral protection may be useful.
This increases mechanical robustness, but may also change:
- the required bending radius,
- the outside diameter,
- the maximum possible line length,
- the flexibility of the line.
The protection should therefore already be specified when ordering and not improvised afterwards.
Correctly Secure the Remote Line
A capillary line should not hang loosely over long distances.
On the other hand, it must not be crushed or constricted by its fixing points.
A suitable fastening system should:
- guide the line securely,
- have no sharp edges,
- not exert excessive pressure on the capillary tube,
- avoid vibration and rubbing movement,
- not unnecessarily restrict thermal expansion.
Particular attention should be paid to the transition areas:
sensor → capillary
and:
capillary → display housing
.
No permanent bending, tensile or torsional forces should be introduced at these points.
What Should Be Done with Excess Capillary Length?
A factory-made capillary line should not simply be shortened.
This would open the closed measuring system and destroy the gas filling.
A certain amount of installation reserve must therefore be accommodated through suitable routing.
Very tight coils should be avoided.
Instead, the excess line can be routed and secured in:
large, stress-free loops
.
The permissible minimum bending radius must also be observed here.
However, very large additional lengths should not be ordered as a precaution if they would only have to be coiled up in the installation afterwards.
The better solution is a remote line dimensioned as closely as possible to the actual requirement.
Choose the Correct Sensor Position
Even a perfectly routed capillary line cannot compensate for an unfavorable sensor position.
The temperature sensor must measure the temperature of the actual medium of interest.
In a pipeline, it should therefore extend as far as possible into the relevant flow area.
Unsuitable positions may include, for example:
- edge zones with low flow,
- dead zones,
- areas immediately adjacent to heated outer walls,
- positions directly downstream of a mixing point where the medium is not yet homogeneous.
The correct installation point always depends on the process.
The temperature at a pipe wall can, for example, differ significantly from the temperature at the center of the pipe.
Ensure Sufficient Insertion Depth
The sensor has a thermally effective or active area.
This area should be exposed as completely as possible to the temperature being measured.
If the insertion depth is too small, heat conduction occurs between:
- process medium,
- process connection,
- pipe wall or vessel wall,
- ambient environment.
The measured temperature may then lie somewhere between the process temperature and ambient temperature.
Example
The medium has a temperature of:
180 °C
.
However, the sensor only extends a few millimeters into the pipeline, while the process connection and surroundings are significantly cooler.
Heat is conducted away from the actual measuring area through the metallic stem.
The indication may therefore be too low even though the thermometer itself is operating correctly.
The required minimum insertion depth therefore depends on:
- sensor diameter,
- sensor design,
- temperature range,
- process connection,
- thermowell used, where applicable.
Avoid Heat-Conduction Errors
Heat conduction is one of the most important installation influences in mechanical temperature measurement.
It occurs when heat is transferred between the process and the environment through the sensor or connection.
Particularly critical conditions include:
- very short insertion lengths,
- small pipe diameters,
- large temperature differences between process and environment,
- massive metallic process connections,
- low flow velocity of the medium.
A greater insertion depth often improves thermal coupling to the process.
In small pipelines, it may be useful to install the sensor:
- at an angle,
- in a pipe bend,
- or against the direction of flow
provided this is mechanically and process-technically permissible.
The objective is always to ensure that the active sensor area is surrounded as completely as possible by the medium whose temperature is to be measured.
Influence of a Thermowell
In many process systems, the temperature sensor is not installed directly in the medium, but in a:
thermowell
.
This offers several advantages.
For example, the sensor can be removed without directly opening the process.
The thermowell also protects the sensor against:
- process pressure,
- flow forces,
- corrosive media,
- mechanical stress.
Thermally, however, it creates an additional barrier between the medium and the sensor.
This can increase the response time.
For fast measurement, the following are therefore relevant, among other things:
- suitable inside diameter of the thermowell,
- suitable sensor diameter,
- minimal unnecessary air gaps,
- sufficient insertion depth,
- appropriate thermowell design.
The thermowell must therefore not be selected solely based on the process thread.
Mechanical strength and thermal behavior must be considered together.
What Influences Response Time?
The indication of a mechanical thermometer does not respond instantaneously to a temperature change.
The response time depends, among other things, on:
- sensor diameter,
- sensor mass,
- heat transfer from the medium to the sensor,
- flow velocity,
- type of medium,
- use of a thermowell,
- dimensions of the thermowell.
A thin sensor directly exposed to a flowing liquid can respond significantly faster than a massive thermowell in slowly moving air.
A sluggish indication should therefore not automatically be attributed to an excessively long capillary line.
The complete thermal arrangement from the process medium to the temperature-sensitive section of the sensor must be considered.
Correctly Mount the Display Housing
The advantage of a remote line is precisely that the display can be positioned independently of the actual measuring point.
This freedom should be used.
Wherever possible, the display housing should be positioned so that it is:
- easy to read,
- accessible,
- protected against excessive ambient temperature,
- mechanically stable,
- subject to as little vibration as possible.
A frequently unfavorable solution is, for example, to use a remote thermometer and then mount the display housing directly above a very hot process pipeline.
The remote line provides the opportunity to select a thermally and mechanically more favorable mounting location.
Vibration and the Remote Line
Strong vibrations place particular stress on the mechanical pointer mechanism of a thermometer.
A remote line makes it possible to separate the display housing from a strongly vibrating measuring point.
For example, the sensor can be installed on a:
- pump,
- compressor,
- motor,
- vibrating pipeline
while the display housing is mounted on a separate bracket or control cabinet.
In this case, however, the capillary line itself must also be routed so that there is no continuous bending movement at its transition points.
A freely vibrating capillary section directly at the sensor or housing can cause long-term mechanical stress.
Define the Capillary Length Before Ordering
The remote line of a gas-actuated thermometer is not a typical accessory that can be freely adapted on site.
It is part of the closed measuring system.
Before ordering, at least the following information should therefore be established:
- Measuring or indication range.
- Process temperature.
- Process medium.
- Process connection.
- Sensor diameter.
- Required insertion length.
- Position of the display housing.
- Actual routing path between sensor and display.
- Required capillary length.
- Ambient temperature at the display housing.
- Temperature conditions along the capillary.
- Required mechanical protection of the remote line.
- Vibration exposure.
- Thermowell required, if applicable.
A suitable capillary length should be planned and ordered – it should not be adapted during installation by cutting it or by using improvised tight coils.
Practical Example: Temperature Display 8 m from the Process
In a production plant, the temperature of a hot liquid in a pipeline is to be measured.
The measuring range is:
0...200 °C
.
The pipeline is located behind a machine enclosure and is difficult to access during operation.
The temperature should therefore be read from the operator side of the machine.
The direct distance is:
approx. 6 m
.
Step 1: Determine the actual routing path
Because of the machine design, the capillary line cannot be routed directly.
It must:
- first run upwards,
- along a cable and piping section,
- around a machine enclosure,
- and then to the display panel.
The actual routing path is therefore approximately:
7.5 m
.
Step 2: Allow a small installation reserve
A remote line is selected that covers the actual routing path with a reasonable installation reserve.
A significantly longer line is avoided.
Step 3: Avoid hot areas
One originally planned section would run for several meters directly parallel to a hot steam line.
Instead, the capillary is routed at a distance along a thermally more favorable path.
Step 4: Define mechanical protection
In the area of a maintenance opening, there is a risk that the line could be damaged during service work.
A mechanically protected version is therefore specified in this section.
Step 5: Install the sensor correctly
The sensor is positioned so that its active area extends sufficiently into the process flow.
Step 6: Mount the display housing
The display housing is installed at a location that is:
- easy to read,
- low in vibration,
- thermally moderate.
This ensures that the advantages of a gas-actuated thermometer with a remote line are actually utilized.
Systematically Investigate Measurement Deviations
If a remote thermometer displays an unexpected value, it should not immediately be assumed that the thermometer needs to be recalibrated.
The entire measuring point should be investigated.
Step 1: Check the reference temperature
First, a suitable reference thermometer is used to determine the actual temperature at the sensor location.
Step 2: Check the sensor position
Is the sensor inserted deeply enough and is the active area actually located in the medium?
Step 3: Check the thermowell
Is the sensor fully inserted?
Is there an unusually large air gap or an incorrectly dimensioned thermowell?
Step 4: Check the capillary line
Check for:
- kinks,
- crushing,
- abrasion points,
- tight bending radii,
- unusually hot or cold line sections.
Step 5: Check the display housing
Is the housing within the permissible ambient conditions?
Is it exposed to strong vibration or a local heat source?
Step 6: Compare the measuring system
If the deviation remains under controlled conditions, the complete thermometer assembly can be checked against a suitable temperature reference.
Because the sensor, capillary and measuring element form one complete measuring system, the assembly should be considered as a whole.
Common Planning and Installation Errors
- Ordering the remote line based on straight-line distance: The actual routing path with bends and detours is longer.
- Selecting an unnecessarily long capillary: The excess line must later be accommodated in a complicated and thermally unfavorable way.
- Shortening the capillary afterwards: This opens the closed measuring system.
- Using too small a bending radius: The capillary tube can be deformed or damaged.
- Kinking the capillary: A damaged remote line is not a normally repairable cable fault.
- Routing the remote line directly along a hot pipeline: The capillary environment is heated unnecessarily.
- Routing the capillary unprotected across a maintenance opening: Mechanical damage is likely.
- Clamping the line too tightly: The fixing can deform the thin capillary.
- Insufficient sensor insertion: Heat conduction distorts the measurement.
- Inserting only the sensor tip into the process: The active area is not completely brought to process temperature.
- Using an excessively large thermowell: A large air gap increases thermal response time.
- Mounting the display housing directly on a vibration source: The advantage of the remote line is not utilized.
- Ignoring ambient temperature: Especially long capillary lines are more sensitive to unfavorable thermal conditions.
- Automatically attributing a slow response to capillary length: The thermowell, sensor dimensions and heat transfer are often more important causes.
Suitable Gas-Actuated Thermometer
For mechanical temperature measurement with a remotely located display, the WIKA Type 73 gas-actuated thermometer in the F73 version with remote capillary is one suitable option.
Type 73 is intended for industrial applications including:
- chemical and petrochemical industries,
- oil and gas industry,
- power engineering,
- machine and plant construction,
- vessel and apparatus construction,
- water and wastewater technology.
Particularly relevant for applications with remote capillary are:
- mechanical temperature measurement without electrical auxiliary power,
- version with spatially separated display housing,
- various capillary lengths,
- various process connections,
- various sensor diameters and insertion lengths,
- optional mechanical protection for the remote line,
- stainless steel design for industrial operating conditions.
The exact version should be selected based on measuring range, installation point, required remote line and the thermal and mechanical ambient conditions.
Further gas-actuated, bimetal, resistance and electronic thermometers as well as temperature sensors can be found under Temperature Measurement Technology at ICS Schneider.
Conclusion
A gas-actuated thermometer with remote capillary enables purely mechanical temperature indication at a considerable distance from the actual measuring point.
The major advantage is the spatial separation of:
temperature sensor
and:
display
.
However, the capillary line is part of the gas-filled measuring system and must therefore be planned carefully.
An unnecessarily long remote line, large temperature differences along the capillary, bending radii that are too tight or unprotected routing can impair the reliability of the measuring point.
The actual sensor installation is at least equally important.
The active area of the sensor must extend sufficiently into the medium being measured. Insufficient insertion depth can cause significant measurement deviations due to heat conduction.
If a thermowell is additionally used, its thermal inertia must be taken into account when considering the required response time.
The remote line also makes it possible to move the display housing away from hot, difficult-to-access or highly vibrating measuring points.
A reliable measuring point is therefore not achieved solely by selecting the correct temperature range. The decisive factor is the complete system consisting of sensor, insertion depth, process connection, capillary length, routing path, ambient temperature, mechanical protection and mounting location of the display.
FAQ: Gas-Actuated Thermometers with Capillary Line
How does a gas-actuated thermometer with remote capillary work?
The sensor, capillary line and measuring element form a closed, gas-filled measuring system. If the temperature at the sensor changes, the internal pressure changes. This pressure acts on a measuring element inside the display housing and moves the pointer via a mechanical movement.
Does a gas-actuated thermometer require a power supply?
A purely mechanical gas-actuated thermometer does not require electrical auxiliary power for temperature indication. This is an advantage for simple local or remote mechanical displays.
How long can the capillary line of a gas-actuated thermometer be?
The maximum possible length depends on the manufacturer and instrument type. With the WIKA Type 73, remote lines of several tens of meters are possible depending on the version. Nevertheless, the line should only be as long as the installation actually requires.
Does capillary length influence measurement accuracy?
It can be relevant because measuring gas is also present inside the capillary. The longer the line and the more strongly its ambient temperature changes, the more important the ambient-temperature influence becomes. The remote line must therefore suit the application and the required accuracy.
May a capillary line be shortened?
No. A factory-filled remote line should not be shortened on site. The sensor, capillary and measuring element form a closed measuring system with a defined gas filling.
May the capillary be bent?
Yes, but only while observing the minimum bending radius specified by the manufacturer. The line must not be kinked, crushed or routed over sharp edges.
Can excess capillary be coiled?
A small installation reserve can be routed in sufficiently large, stress-free loops. Very tight coils should be avoided. It is better to specify the remote line as closely as possible to the actual routing path when ordering.
May the capillary line be routed next to a hot pipeline?
Where possible, strong continuous additional heating should be avoided. The capillary is part of the gas-filled measuring system, so large and changing ambient temperatures must be considered, particularly with long remote lines.
Why does the thermometer indicate too low a temperature despite correct calibration?
A common cause is insufficient insertion depth. If the active sensor area is not sufficiently heated by the process medium, heat may be conducted away through the stem and process connection to the surrounding environment.
How deeply must the sensor be inserted?
The active or temperature-sensitive area should be exposed as completely as possible to the temperature being measured. The specific minimum insertion depth depends on sensor diameter, measuring range and connection design and should be taken from the instrument data.
Does a thermowell slow down temperature measurement?
Yes. A thermowell can increase response time because the heat must first pass through the thermowell and then into the sensor. Design, wall thickness, fit and medium all influence this effect.
Can a remote capillary help in applications with strong vibration?
Yes. The sensor can remain at the vibrating measuring point while the more sensitive display housing is mounted in a lower-vibration location. The capillary itself must also be routed securely.
What is particularly important when ordering a gas-actuated thermometer with remote capillary?
In addition to the temperature range, the sensor dimensions, process connection, insertion length, required capillary length, display mounting location, ambient temperatures and, where necessary, mechanical protection of the remote line should be specified.