This invention relates to an improved thermowell or fluid sampling probe
for use in chemical processing vessels, pipelines and the like.
Gas sampling probes, for example the insertion type; where in use a
sample of gas has to be dynamically taken from a pipeline or large vessel are
well known but suffer from a number of problems due to the flowing nature of
the fluids to be sampled and the required length of the probe.
There are a number of problems associated with thermowell probes and
gas sampling probes for use with natural gas pipelines. For example, in
designing such probes to meet the mechanical requirements of the installation
may result in a probe that has a large volume and generates significant
turbulence; this again is incompatible with sampling requirements. Thus, such
probes typically suffer from the following drawbacks: they have a large internal
volume, which is incompatible with 'real time' analysis and environmental
considerations; they are prone to inaccurate sampling (due to turbulence); and
mechanical failure of the probe can result due to resonance failure that are a
consequence of vortex shedding. These three drawbacks are described more
fully below.
Firstly, following recognised guidelines for sampling natural gas, such as
ISO 10715:2001, which states that samples should be taken from the middle 'A
of the pipe, results in a "long" sample probe. Not only does the probe have to be
at least 'A the diameter of the pipe (pipe size is often 2ft- 4ft in
diameter/6OOmm to 1200mm) but also the length has to be sufficient to
connect the probe via a branch tee and flange or if permitted, by a threadolet.
(Normally branch flanges are the preferred connection type ). In many cases the
length of a gas sampling probe is significantly, or even hugely increased by the
requirement for a retractable and isolatable probe. In this case the probe is
connected by a branch tee, valve and flange combination.

Secondly, there is the need to consider the phenomena of vortex shedding and
the possibility that the vortex shedding frequency may coincide with the natural
frequency of the probe. Should the two coincide then it is very likely that the
probe will fail (snap off) due to resonance effects.
The combination of the two points above forces a probe design of a
fattish nature. (Normally a probe with about a 25mm (1") outside diameter).
Due to the way tubes and pipes are manufactured, it is not economical/normal,
to manufacture a tube of say I" OD (25mm) with an ID of less than 'A "
(12.5mm).
In the case of gas sampling probes the combination of the 'long' length of
the probe combined with the 'relatively' large internal diameter results in a
significant gas hold up volume in the sample probe itself. This stored gas is
often known as 'dead space' gas and has to be vented, or otherwise disposed of,
before actual gas from the pipeline can enter the analyser. The volume of stored
or 'dead space' gas within the probe is further increased by the effect of
pressure. For each bar of the pressure that the pipeline operates above
atmospheric pressure, then the real (or normal or standard) volume of gas in the
probe is increased by that ratio. For example if the internal volume of the probe
was say 0.25 litres and the pressure of the pipeline it is operating in is 40 bara
then the real (normal or standard) volume of stored or 'dead gas' within the
probe will be approximately 0.25 x 40 = 10 litres. It is not uncommon for gas
pipelines to be operating at 80 bara or even higher.
Thus, there is a problem designing a gas sampling probe with a response
time fast enough to match an associated analytical system. In such
circumstances a significant amount of gas that has to be moved out of the way
(vented) before a representative sample of the actual gas in the pipeline can be

presented/introduced to the analyser/sample cylinder connected to the sample
probe. This venting process can be very damaging to the environment.
An alternative to using a pipe or tube would be to use a solid bar with a small
hole 'drilled' down the middle. However, drilling a 2, 3, 4, 5 mm diameter hole
or even larger, down the length of a stainless steel bar of typically say 0.3 to 2.0
metres long is no easy or cheap task. Additionally, the quality of the surface
finish of such a drilled hole is difficult to control which brings its own problems
to representative sampling of natural gas, especially with the higher
hydrocarbons and reactive components.
Lastly, by introducing such a large protrusion into the flowing gas creates
significant turbulence which in turn can momentarily alter the composition of
the gas. Small droplets of hydrocarbon liquid may be formed, similar to the
white vapour trails often seen behind an aeroplane (except in the case of the
aeroplane it is water droplets not hydrocarbon liquid droplets). These small
droplets not only change the gaseous phase composition but also have the
potential to absorb, momentarily, any reactive components such as hydrogen
sulphide. Therefore at the point in space (actually the point in the pipeline at the
tip or entrance to the sample probe) where the gas is sampled from, every effort
needs to be made to reduce the turbulence.
An object of this invention is to reduce the internal volume of a gas
sampling probe. Another object of the invention is to minimise or eliminate
vortex shedding induced by use of such a probe. A further object of the
invention is to minimise the turbulence at the sampling point.
In one aspect the invention provides a gas sampling probe comprising an
elongate main tubular member having an inlet end and an outlet end and a
sampling tube housed within said main tubular member; said sampling tube

extending from said inlet end to said outlet end; wherein the cross sectional area
of the sampling tube is 0.1 to 30 mm .
In another aspect the invention provides a gas sampling probe having an
elongate main tubular member having an inlet end and an outlet end and a
sampling tube housed within said main tubular member; said sampling tube
extending from said inlet end to said outlet end; wherein the main body has at
least one helical fin attached to and wound around the outer surface of said main
tubular member; or integrally formed as part of the main tubular member. The
thickness of the fin, while not being critical, is preferably in the range 0.005D to
0.2D; where D is a diameter of the main tubular member. The depth of the fin is
preferably in the range 0.05D to 0.25D: where D is the diameter of the main
tubular member.
Preferably, the gas sampling probe comprises a sampling tube housed
within said main tubular member; said sampling tube extending from said inlet
end to said outlet end; wherein the cross sectional area of the sampling tube is
0.1 to 30 mm2.
In both of the above aspects, preferably the inner surface of the sampling tube
has a surface roughness below 0.8ì roughness average (RA). Preferably, the
inner surface of the sampling tube is treated by electro-polishing in order to
reduce surface roughness. The inner surface of the sampling tube may be further
treated w ith a passivation process to reduce surface activity such as a siliconc
based chemical vapour deposition process of which Silcosteel R or Sulfinert ™
coatings are specific examples. The gas sampling probe may be fabricated using
stainless steel.
Preferably, the gas sampling probe further comprises an end member with
a smooth curved outer surface, located at the inlet end, and configured to
provide a seal between an outer surface of the sampling tube and an inner

surface of the main tubular member; the curved outer surface may
predominantly correspond to a surface formed by revolving a smooth curve
about the centre axis of the sample tube and/or tubular member. The curved
outer surface may be formed by a partial ellipsoid, partial catenoid. partial
conoid or partial paraboloid of revolution. Preferably, the smooth outer surface
has a surface roughness less than 0.4ì RA. The smooth outer surface may be
further treated with a passivation process to reduce surface activity and
particulate build up such as a silicon based chemical vapour deposition process
of which Silcosteel®-AC is a specific example.
In another aspect the invention provides a gas sampling probe comprising
an elongate main tubular member having an inlet end and an outlet end; wherein
the main body has at least one helical fin. This fin may be attached to and
wound around the outer surface of said main tubular member, or may be formed
integrally with the main tubular member.
The addition of the helical fins, of course, eliminates the requirement for
increasing the thickness and mass due to natural frequency considerations
however the fins themselves are structural and may be taken into consideration
to reduce the stresses due to the straightforward loads due to velocity etc which
would/can by itself reduce the mass of the gas sampling probe. Preferably the
probe further comprises a sampling tube housed within said main tubular
member; said sampling tube extending from said inlet end to said outlet end.
Preferably, the sampling probe has a hemispherical inlet end. The fluid inlet of
the sampling probe may be located on the surface of the inlet end of the probe,
where surface conditions are controlled. A sampling tube may pass throughout
the whole length of the probe. Preferably, helical fins are provided on the
exterior portion of the probe, that in use. lies within the flowing stream of the
gas.

Preferably, the internal bore of the sampling tube has a special surface
treatment such as electro polishing and/or for critical analysis conditions either
the Silcosteel or Sulfinert ™ surface coating.
Preferably, the hemispherical end with controlled surface conditions is
treated with the Silcosteel®-AC surface coating.
In another aspect the invention provides a gas sampling probe comprising an
elongate main tubular member having an inlet end and an outlet end; wherein
the main body has at least one helical fin. This fin may be attached to and
wound around the outer surface of said main tubular member, or may be formed
integrally with the main tubular member. Preferably the probe further comprises
a sampling tube housed within said main tubular member; said sampling tube
extending from said inlet end to said outlet end.
Preferably, the gas sampling probe comprises a sampling tube housed within
said main tubular member; said sampling tube extending from said inlet end to
said outlet end; wherein the cross sectional area of the sampling tube is 0.1 to 30
ram'.
Preferably, the sampling probe has a hemispherical inlet end. The fluid
inlet of the sampling probe may be located on the surface of the inlet end of the
probe, where surface conditions are controlled. A sampling tube may pass
throughout the whole length of the probe. Preferably, helical fins are provided
on the exterior portion of the probe, that in use, lies within the flowing stream of
the gas.
Preferably, in use the longitudinal axis of the gas sampling probe is be
inclined at an angle to the axis of a pipe or conduit carrying fluid that is to be
sampled; where a is in the range 90° to 45°. The gas sampling probe of the
invention is preferably used as part of a retractable sampling probe system; thus
in use, allowing retraction of the sampling probe, at least in part, out of the flow

of fluid to be sampled. Preferably, only the last 'A of the portion of the probe
that lies within the flowing fluid has helieal fins. However, often helieal fins
will extend along most or all of the portion of the probe that lies within the fluid
(low from which samples are to be taken.
In another aspect the invention comprises a method of using a gas
sampling probe according to the above mentioned aspects. In use the
longitudinal axis of the gas sampling probe may be inclined at an angle to the
axis of a pipe or conduit carrying fluid that is to be sampled; where a is in the
range 90° to 45°. The gas sampling probe of the invention is preferably used as
part of a retractable sampling probe system; thus in use, allowing retraction of
the sampling probe, at least in part, out of the flow of fluid to be sampled.
Preferably, only the last V- of the portion of the probe that lies within the
flowing fluid has helical fins. However, often helical tins will extend along
most or all of the portion of the probe that lies within the fluid flow from which
samples are to be taken.
In their simplest form thermowells comprise a tube, sealed at one end and
with a fitting at the other end to facilitate attachment to the wall of a pressure
vessel, pipeline etc.. Such a device typically allows a temperature sensor, such
as a thermocouple, to be inserted within the thermowell tube. The thermoweli
thus allows the sensor to be in reasonably close thermal contact to a fluid the
temperature of which is to be measured; it also protects the sensor from direct
contact with this fluid and so avoids mechanical damage to the probe.
When thermowells or gas sampling probes are used in certain applications, such
as high pressure or high velocity pipelines, a known problem is deformation or
fracture of the probe in response to cyclic stresses induced in the probe as a

result of fluid flow. This is a particular problem at high velocities and can result
from vortex shedding from points around the probe.
An object of the present invention is to provide a thermowell that is less
susceptible to this type of damage. A further object of this invention is to
minimise or eliminate vortex shedding induced by use of the probe.
For a thermowell to provide a good and fast response, to allow temperature
changes of the fluid being measured by the temperature sensor contained within
the thermowell "quickly and accurately ", the thermowell should preferably be
of the thinness section possible and preferably of the minimum mass possible.
The requirements of designing a thermowell to resist both the straightforward
loads due to velocity etc and to design it so that the natural frequency is "away"
from any vortex shedding frequency is incompatible with this requirement.
In one aspect the invention comprises a thermowell having an elongated
tube with one or more helical fms wound longitudinally around at least part of
the outer surface of said tube. The addition of the helical fins, of course,
eliminates the requirement for increasing the thickness and mass due to natural
frequency considerations however the fins themselves are structural and may be
taken into consideration to reduce the stresses due to the straightforward loads
due to velocity etc which would/can by itself reduce the wall thickness and
therefore the mass of the thermowell.
Preferably, the tube is substantially circular in cross section; more
preferably the tube is cylindrical in shape. The tube may be closed at one end, in
which case the closure is preferably curved or flat in shape; and is more
preferably hemispherical is shape. Preferably, there are 2, 3. 4, 5 or 6 helical
fins.

The thermowell tube may have an external diameter in the range 3 to 75
mm. The length of the tube is preferably in the range 10 to 1800mm. In use, the
length of tube inserted within flowing fluid is preferably in the range 10 to
1500mm. The tube preferably has an internal bore the internal diameter of
which is in the range 1 to 25 mm.
The thermowell of the invention is preferably used as part of a retractable
thermowell system; thus in use, allowing retraction of the thermowell, at least in
part, out of the flow of fluid. This is difficult for conventional thermowells
owing to the thickness and mass considerations noted above. A retractable
thermowell would have to be thicker and of a greater mass than a fixed
thermowell; to resist both types of loads noted above. The reduction in
thermowell mass resulting from adding the helical fins allows a retractable
design. A retractable thermowell also provides useful benefit for: a) easier
service and maintenance, b) easier calibration, c) change out without
interruption to the process.
The measurement of flow (or more correctly mass per unit time) of a
fluid requires that the primary flow measurement signal is corrected for both the
actual temperature and pressure of the fluid being measured. In the case of
temperature measurement this normally means that a thermowell is placed in the
fluid stream adjacent to the primary flow measurement signal (Generally the
rules, "Standards and codes of Practice", require that the temperature is
measured from the middle third of a pipeline etc.) Nearly all primary flow-
measuring devices require a steady/uniform flow pattern upstream and
downstream of the device in order to produce an accurate primary flow signal.
Adding a thermowell (that protrudes at least to the middle third of the pipeline)
adjacent to the primary flow measuring device is incompatible with this
requirement as it produces a disturbance in the flow pattern and therefore

reduces the accuracy of the primary flow measurement signal. Because the
addition of helical fins to a thermowell provides a much more stable flow
pattern around the thermowell than one without, and because the thermowell
may well be of a smaller diameter (does not have to be designed to cater for
vibration due to vortex shedding) the flow disturbance is much reduced and
therefore using a thermowell with helical fins will allow for a more accurate
signal from the primary flow measuring device.
In its basic form a thermowell provides two functions:
1. It provides a protection, support and attachment means where a primary
temperature measuring device can not be placed directly at a desired position
into the medium whose temperature is to be measured.
2. It provides a means of transferring the temperature of the medium to the
primary tteemmppeerraattuurree mmeeaassuurriinngg ddeevviiccee;;
iiddeeaallllyy wwiitthh t the minimum thermal lag (delay in reaching
temperature equilibrium with medium)
These requirements are generally in conflict with one another. In many
cases this means that a relatively massive thermowell (sleeving etc.) is required
to support/protect the temperature measuring device. This results in a significant
thermal lag, which is particularly disadvantageous when measuring fluctuating
temperatures; such a system will tend to measure the (time) average temperature
and not respond to short-term transients.
Thus, a further object of the present invention is to provide a thermowell
that in use allows a temperature measuring device housed within to react to
rapid changes in the fluid temperature being measured and thereby reduces the
disadvantages of thermal lag.
In one aspect the invention comprises a thermowell having a first portion having
an elongated tube having an inlet end and an outlet end and means of holding at

the outlet end a second portion that in use houses a primary temperature
measuring device. Preferably, the second portion comprise an open lattice or
frame that extends axially away from said outlet end. The lattice/frame may
comprise a plurality of similar helically wound fins; wound around a common
axis. In another embodiment, the second portion may comprise a tip. made at
least in part, of higher thermal conductivity material and/or comprising a thinner
wall member than said first portion; and attached to the outlet end of the first
portion. Preferably a thermally insulating element is provided between the first
and second portion or between the second portion and the primary temperature
measuring device.
Preferred embodiments of the invention will now be described with
reference to the following diagramatic figures in which:
Figure 1 shows a side view of a gas sampling probe according to a first
embodiment of the invention;
Figure 2 shows a diametric sectioned view corresponding to Figure 1 ;
Figure 3 is a more detailed section of the hemispherical inlet end shown in
Figure 2;
Figure 4 shows a section of an example of a retractable gas sampling probe,
according to a second embodiment of the invention;
Figure 5 shows a side view of a thermowell according to a third embodiment of
the invention;
Figure 6 shows a side view of a thermowell according to a fourth embodiment
of the invention;
Figure 7 shows a side view in cross section of a thermowell according to a fifth
embodiment of the invention; and
Figure 8 shows a side view of a thermowell according to a sixth embodiment of
the invention.

Figure 1 shows a side view of a gas sampling probe according to a first
embodiment of the invention. The gas sampling probe 10 comprises an elongate
main tubular body 12 with an inlet end 14 and an outlet end 16. A flange 18 is
attached to the main body 12 near the outlet end 16. This is a conventional
flange that in use allows the probe to be attached in a fluid tight manner to the
system being sampled. Main body 12 comprises an upper tubular portion 20 that
is integral with a slightly smaller diameter lower portion 22. The difference in
diameter between the upper portion 20 and lower portion 22 may be such as to
allow several helical tins 24 to be attached in a streamline fashion: that is such
that the radial extension of the lower portion 22 plus fin 24 fairly closely
corresponds to the external radius of the upper tube portion 20. It should be
noted that while a plurality of fins is preferred it is not essential to have three
fins; for example two or four fins may be used.
Figure 2 shows a diametric sectioned view corresponding to Figure 1. It
can be seen that main body 10 has a constant diameter bore 30. The main body
member 10 has a wall thickness selected to provide the structural strength
required of the probe in use. A sampling tube 32 is positioned within bore 30,
preferably along the central axis of bore 30. Sampling tube 32 is held in place
by an end member 34. The sampling tube is preferably constructed from
stainless steel, and preferably has an internal diameter of 0.05 to 5 mm; and
more preferably a diameter in the range 2 to 4 mm. The sampling tube 32 has a
wall thickness selected to provide the structural strength required of the probe in
use. Preferably the sampling tube has a wall thickness in the range 0.2 to 2 mm.
Figure 3 is a more detailed section of the hemispherical inlet end shown
in Figure 2 Preferably, end member 34 takes the form of a hemispherical insert
and is sealed within the lower portion 22 by a circumferential weld 38. The
surface finish 40 of the hemispherical insert 34 is machined to give a surface

roughness of less than 0.4Ì. R A; this reduces local turbulence and help prevent
the build up of particulates and contaminants from the process on surface 40.
Preferably, the surface finish 40 is further smoothed by the application of the
Silcosteel®-AC surface coating or the like. The inlet end of sampling tube 32 is
sealed into the hemispherical insert 34 by means of a circumferential weld 40.
The internal surface of the sampling tube 32 is preferably treated, with an
electro-polishing treatment, to reduce surface roughness; and for critical
analysis conditions may be further treated with either the Silcosteel * or
Sulflnert ™ surface coating or the like. Sampling tube 32 may comprise PTFH
or a similar inert material; such as PVDF, in which case weld 40 would be
replaced by an appropriate adhesive bond.
Figure 4 shows a section of an example of a retractable gas sampling
probe, according to a second embodiment of the invention. In this embodiment
main body 12 is not directly fixed to a flange 50 but rather is fixed to a flange
by an adustment/retraction means 52. This adjustment means can be any of
several known to the skilled artisan; for example it may comprise a threaded
tube 54 fixed at one end to flange 50 through which the main body 12 passes;
tube 54 having fluid sealing means 56; for example an O-ring seal. Adjustment
means 52 further comprises an arm member that comprises cylindrical portion
6o and arm portions 62. Cylindrical portion 60 has a threaded bore that in use
co-operates with the outer thread of tube 54 to allow the position of the probe 10
to be adjusted in an axial direction.
The use of the helical fins 24 and small bore lining tube 32 to such
retractable probes is generally more beneficial than to fixed probes because they
generally have longer unsupported probe lengths making it more susceptible to
the effects of vortex shedding and the probe itself is much longer making the
internal volume that much greater.

Figure 5 shows a thermowell according to a third embodiment of the
invention. The thermowell 110 comprises an elongated tube 112 with an
internal bore (not shown) and sealed with a hernispherically shaped cap 118 at
one end. The other end of tube 112 is connected via a flange 114 to temperature
probe inlet 116. Tnlet 116 comprises a short tube through which a temperature
probe such as a thermocouple or thermistor may be inserted into the internal
bore of tube 112 such that the sensing element of the probe is near the bottom of
the internal bore and so in close thermal proximity to end cap 118.
Tube 112 further comprises three helically arranged fins 120a, 120b, 120c
each fin being of width W and depth d. In this case the fins trace a three
dimensional curve round and simultaneously advancing along a cylinder.
However, tube 112 may have a shape other than a cylinder; for example it may
have a somewhat conical portion. The fins are shown extending along the entire
length of elongated tube 112; however; the fins may alternatively extend only
part way along the length of tube 112. The fins 120 may be integrally formed
with or attached to tube 112.
It has been found that in use such fins may reduce or eliminate vortex
shedding from the thermowell; this is a significant benefit as such vortex
shedding can result in cyclic forces that will damage the thermowell, or even the
temperature sensor itself: especially if the period of such cycles is at or near the
resonant frequency of the thermowell. While the fin preferably has a cross
section with a sharp edge; for example a rectangular cross section other shaped
cross sections are possible; for example the cross section may have a
semicircular outer portion. Preferably the width (W) of the fin is in the range
.0.005D to 0.2D, where D is the external diameter or width of the tube.
Preferably, the depth of the fin (d) is in the range 0.05D to 0.5D. The pitch of
each helical fm is preferably in the range D to 2OD, more preferably 2D to 1OD

and most preferably 3D to 7D. It has been found that fms having dimensions
within these ranges are particularly effective in reducing or eliminating such
vortex shedding.
Figure 6 shows a fourth embodiment of the invention. In this embodiment
the thermowell 210 comprises a cylindrical tube 212 with a flat closed end 218
at one end of the tube and a threaded 214 hexagonal connector 216 at the other.
Again connector 216, threaded portion 214 and tube 212 have an internal bore
(not shown) that in use accommodates a temperature sensor. Tn this embodiment
three helical fins 220a, 220b and 220c are attached or formed to the outer
surface of tube 212.
Fig. 7 shows a thermowell where the tip 310 of the thermowell, which is
the active portion in providing the measurement/thermometry requirements, is
made of a higher conductivity material than the main body 320. Further tip 310
may be made of a thinner section material than the main body 320. Ideally tip
310 is thermally separated or partially thermally separated from main body 320
by a thermal barrier 330. Tip 310 is attached to main body 320 by means such
as screwing, gluing, soldering, welding or any appropriate method suitable for
the application.
Fig. 8 shows a thermowell were the measurement/thermometry
requirements are provided by a capsule 410, containing the primary temperature
measuring device (not shown) which is held, supported and attached, to the
containment means, by main body 420. In this case the main body 420 is of an
open lattice structure allowing the medium whose temperature is to be measured
to be in thermal contact with the capsule 410. Preferably, thermal capsule 410 is
thermally separated or partially thermally separated from main body 420 by a
thermal barrier 430. In this embodiment the means of transmitting the measured
temperature from the primary measuring device contained in capsule 410 may

be a conduit or cable 440 which is sealed/connected to main body 420 at a
distance from capsule 410 thereby reducing conductivity loss.

WE CLAIM:
1. A thermowell or gas sampling probe comprising an elongated tube with
one or more helical fins wound along and around at least part of the outer
surface of said tube,
2. A thermowell or gas sampling probe according to Claim 1 wherein the
tube is substantially circular in cross section.
3. A thermowell or gas sampling probe according to Claim 2 wherein the
tube is cylindrical in shape.
4. A thermowell or gas sampling probe according to any of Claims 1-3
where the fin has a cross section with a sharp edge.
5. A thermowell or gas sampling probe according to Claim 4 wherein the fin
has a rectangular cross section.
6. A thermowell or gas sampling probe according to any of Claims 1-4
wherein the fin has a polygonal shaped cross section.
7. A thermowell or gas sampling probe according to any of Claims 1-6
wherein the width of the fin is in the range 0.005D to 0.2D. where D is the
external diameter or width of the tube.
8. A thermowell or gas sampling probe according to any of Claims 1-7
wherein the depth of the fin is in the range 0.05D to 0.5D, where D is the
external diameter or width of the tube.

9. A thermowell or gas sampling probe according to any of Claims 1-8
wherein the pitch of the helical fin is in the range D to 2OD. where D is the
external diameter or width of the tube.
10. A thermowell or gas sampling probe according to any of Claims 1-9
wherein the depth of the fin is in the range 0.05D to 0.5D5 where D is the
external diameter or width of the tube.

11. A thermowell or gas sampling probe according to any of Claims 3-10
wherein the tube has an internal diameter in the range 1 to 50 mm.
12. A thermowell or gas sampling probe according to any of Claims 1-11
wherein the length of the tube is in the range 10 to 3000 mm.
13. A thermowell or gas sampling probe according to any of Claims 1-12
wherein the external diameter or maximum width of the tube is in the range 3 to
100 mm.
14. A thermowell or gas sampling probe according to any of Claims 1-13
wherein only the last 'A of the portion of the probe that lies within the flowing
fluid has helical fins.
15. A thermowell according to any of Claims 1-14 wherein the tube is closed
at one end.
16. A thermowell according to Claim 15 wherein the closure is hemi-
spherical in shape.
17. A thermowell dependant or independent of Claim 1 comprising a first
portion having an elongated tube with an inlet end and an outlet end and means

of holding at the outlet end a second portion that in use houses a primary
temperature measuring device.
18. A thermowell according to Claim 17 wherein the second portion
comprises an open lattice or frame that extends axially away from said outlet
end.
19. A thermowell according to Claim 18 wherein the lattice/frame comprises
a plurality of similar helically wound fins; wound around a common axis.
20. A thermowell according to Claim 17 wherein the second portion
comprises a tip member made at least in part of higher thermal conductivity
material and/or comprising a thinner wall member than said first portion; and
attached to the outlet end of the first portion.
21. A thermowell according to any of Claims 17 to 20 wherein a thermally
insulating element is provided between the first and second portion, or in use
between the second portion and a containment member of a primary
temperature measuring device.
22. A gas sampling probe comprising an elongate main tubular member
having an inlet end and an outlet end and a sampling tube housed within said
main tubular member; said sampling tube extending from said inlet end to said
outlet end; wherein the cross sectional area of the sampling tube is 0.1 to 30
mnr.
23. A gas sampling probe according to Claim 22 wherein the surface
roughness of the inner surface of the sampling tube is below 0.8 ì RA.

24. A gas sampling probe according to Claim 22 or 23 wherein the inner
surface of the sampling tube has been treated by electro-polishing in order to
reduce surface roughness.
25. A gas sampling probe according to any of Claims 22 to 24 wherein the
sampling tube comprises stainless steel.
26. A gas sampling probe according to any of Claims 22 to 24 wherein the
sampling tube comprises PTFE or a derivative thereof.
27. A gas sampling probe according to Claim 26 wherein the inlet of the
sampling tube is bonded to the main tubular member using an adhesive.
28. A gas sampling probe according to any Claims 22 to 27 further
comprising an end member with a smooth curved outer surface, located at the
inlet end of the probe, and configured to provide a seal between an outer surface
of the sampling tube and an inner surface of the main tubular member.
29. A gas sampling probe according to Claim 28 wherein the curved outer
surface predominantly corresponds to a surface formed by revolving a smooth
curve about the centre axis of the sampling tube and/or tubular member.
30. A gas sampling probe according to Claim 28 or 29 wherein the curved
outer surface is formed by a partial ellipsoid, partial catenoid, partial conoid or
partial paraboloid of revolution.
31. A gas sampling probe according to any of Claims 28 to 30 wherein the
smooth outer surface has a surface roughness less than 0.4ì RA.

32. A gas sampling probe according to any of Claims 22 to 31 wherein the
inner surface of the sampling tube is further treated with using a passivation
process to reduce surface activity.
33. A gas sampling probe according to Claim 28 wherein the smooth surface
of the end member is further treated with using a passivation process to reduce
surface activity and particulate adhesion.
34. A gas sampling probe according to Claim 32 or 33 wherein the
passivation process is a silicone based chemical vapour deposition process.
35. A gas sampling probe according to Claim 34 wherein the silicone bases
chemical deposition process provides a Silcosteel , Sulflnert or Silcosteel®-AC
coating.
36. A gas sampling probe dependent or independent of any of Claims 22 to
35 comprising an elongate main tubular member having an inlet end and an
outlet end; wherein the main tubular body comprises at least one helical fin.
37. A gas sampling probe according to Claim 36 further comprising a
sampling tube housed within said main tubular member; said sampling tube
extending from said inlet end to said outlet end.
38. A gas sampling probe comprising an elongate main tubular member
having an inlet end and an outlet end and a sampling tube housed within said
main tubular member; said sampling tube extending from said inlet end to said
outlet end.
39. A method of using a thermowell or gas sampling probe according to any
preceding claim.

40. A method according to Claim 39 wherein means are provided to, in use,
allow retraction of the sampling probe, at least in part, out of the flow of fluid to
be sampled.
41. A method according to Claim 39 wherein in use the longitudinal axis of
the gas sampling probe is inclined at an angle a to the axis of a pipe or conduit
carrying tluid that is to be sampled; where a is in the range 45° to 90°.

A technique for indirectly determining the temperature
of a fluid includes directing at least a portion of the fluid
around the outside of a thermally conductive body so as
to produce a substantially uniform temperature therein.
The fluid temperature is then determined by measuring
the resulting temperature within the body. In one embodiment,
the fluid is directed around the body by enclosing
the body in a shell which has an inlet opening,
an outlet opening, and at least one passageway connecting
the inlet opening to the outlet opening. In an embodiment
which is particularly useful for gas turbine
applications, the body is located inside of the outer
casing of the turbine and is connected to a guide tube
which extends through the outer casing in such a manner
that the means employed for measuring the temperature
within the body is insertable into and removable
from the body through the guide tube. For gas turbines
having two or more turbine wheel stages, the inlet
opening may be connected in flow communication with
a wheelspace of a relatively forward one of the stages,
and the outlet opening may be connected to a wheels-
pace of a stage located downstream of the forward
stage, so that the pressure differential created between
the stages during turbine operation causes at least a
portion of the gas located in the forward wheelspace to
pass into the inlet opening, through the connecting
passageway, and out of the outlet opening.