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Isotech 944 Operating Manual

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TRUE SURFACE TEMPERATURE
MEASUREMENT SYSTEM
MODEL 944
TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
Isotech North America
158 Brentwood Drive, Unit 4
Colchester, VT 05446
Phone: (802)-863-8050 Fax: (802)-863-8125
www.isotechna.com
sales@isotechna.com
The company is always willing to give technical ad vi ce and assistance where appropriate. Equally, because of the programme of continual development and improvement we reserve the right to amend or alter characteristics and design without prior notice. This publication is for information only.
Page 1 of 32
TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
Page 2 of 32
CONTENTS
TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
Page No.
EMC Information
Health & Safety Instructions
Guarantee 5
Safety Precautions
Introduction
Technical Notes 7 - 13
TTI 4 System
Front & Rear View - Figures 4 & 5
Operation
Using the PC Interface 17
Using Cal Notepad
3
4
6
6
14
15
16
18 - 19
Tutorial
Fault Finding 28
Page 3 of 32
20 - 27
TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
EMC INFORMATION
This product meets the requirements of the European Directive on Electromagnetic Compatibility (EMC) 89/336/EEC as amended by EC Directive 92/31/EEC.
T he product meets the susceptibili ty requirements of EN 50082-1, criterion B.
ELECTRICAL SAFETY
This equipment must be correctly earthed.
This equipment is a Class 1 Appliance. A protective earth is used to ensure the conductive parts cannot become live in the event of a failure of the insulation.
The protective conductor of the flexible mains cable which is coloured green/yellow MUST be connected to a suitable earth.
The Blue conductor should be connected to Neutral and the Brown conductor to Live (Line).
Warning: Internal mains voltage hazard. Do not remove the panels.
There are no user serviceable parts inside. Contact your nearest Isotech agent for repair.
Voltage transients on the supply must not exceed 2.5kV.
Page 4 of 32
HEALTH AND SAFETY INSTRUCTIONS
1. Read all of this handbook before use.
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MODEL 944
ISS.04 - 06/06
2.
3.
4.
5.
6.
7.
8.
9. If cased, do not return the apparatus to its carrying case until the unit has cooled.
10.
Wear appropriate protective clothing.
Operators of this equipment should be adequately trained in the handling of hot and cold items and liquids.
Do not use the apparatus for jobs othe r th a n those for which it was designed, ie. the calibration of thermometers.
Do no t handle the apparatus when it is hot (or cold), unless wearing the appropriate protective clothing and having the necessary training.
Do not drill, modify or otherwise change the shape of the apparatus.
Do not dismantle the apparatus without disconnecting it from the supply and leaving time for it to reach ambient temperature.
Do not use the apparatus outside its recommended temperature range.
There are no use r servi ceable parts inside. Contact your nearest Isotech agent for repair.
11.
12.
13.
Product covered by this document are described in our manual attached.
Ensure materials, especially flammable mat e ri als are kept away from hot parts of the apparatus, to prevent fire risk.
Ensure adequate ventilation when using oils at high temperatures.
Each apparatus is protected by an over temperature circuit. Please consult handbook for details.
Page 5 of 32
TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
GUARANTEE
This instrument has been manufactured to exacting standards and is guaranteed for twelve
months against electrical break-down or mechanical failure caused through defective material
or workmanship. Failure caused by misuse is not covered. In the event of failure covered by
this guarantee, the instrument must be returned, carriage paid, to the supplier for examination,
and will be replaced or repaired at our option.
FRAGILE CERAMIC AND/OR GLASS PARTS ARE NOT COVERED BY THIS GUARANTEE
INTERFERENCE WITH, OR FAILURE PROPERLY TO MAINTAIN, THIS INSTRUMENT MAY
INVALIDATE THIS GUARANTEE
RECOMMENDATION
The life of your ISOTECH Instrument will be prolonged if regular maintenance and cleaning to
remove general dust and debris is carried out.
Serial No:..........................................
Date:..................................................
Isotech North America
158 Brentwood Drive, Unit 4
Colchester, VT 05446
Phone: (802)-863-8050 Fax: (802)-863-8125
www.isotechna.com
sales@isotechna.com
The Company is always willing to give technical adv i ce and assistance where appropriate. Equally, because of the programme of continual development and improvement, we reserve the right to am end or alter characteristics and design without prior notice. This publication is for information only.
Page 6 of 32
TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
SAFETY PRECAUTIONS
1. Please read this manual before operating the instrument.
2. All warnings, notes, etc. within this manual must be observed.
3. The TTI 4 system is maintenance free. Servicing should be carried out by qualified personnel only.
4. Disconnect power before servicing or removing parts.
5. When using the TTI 4 any operating conditions that cause damage to objects or danger to human beings must be avoided.
6. The measuring results of the TTI 4 system not only depend on the correct functioning of the instrument itself but also depend on various other conditions, such as the calibration of the surface sensor.
Therefore, the results must be verified by an expert before further steps based on those results are taken.
7.The compensated surface sensor is often and by its nature HOT. Appropriate precautions must be observed when handling the sensor.
Page 7 of 32
TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
INTRODUCTION
Surface temperature measurement with contact thermometers has always been a problem because of the heat flux from the surface being measured affects the surface temperature, and the temperature as indicated by the measuring sensor.
The following article describes the problems and the principle of operation of the Isotech True Temperature Indicator 4.
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TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
TRUE SURFACE TEMPERATURE MEASUREMENT
Summary:
The mechanism of surface tem p e rature sources of error are described. These all have to do with conduction of heat away from the surface being measured. By thermally compensating for these conduction mechanisms a new and novel device is described th a t will measure true surface temperature. This device, already in production and use has reduced errors by up to
95%.
Introduction:
As soon as a measuring device is attached to a surfa ce he at flows from the hot to the cold surface.
This for a hot surface means that the surface is cooled locally, and that the measuring device is heated. The rate a t which heat is transferred depends on the thermal contact between
surface and measuring device. Often silicone grease or other conducting material is introduced, to help conduction. The smoothness of the surface also affects the heat flow.
Wh a t wo ul d b e ideal is a surface sensor that generated an equal and opposite heat flux reflected back into the surface.
This would eliminate all the errors associated with surface temperature measurement.
Causes of measurement errors:
There are three main problem areas in determining the surface temperature of a solid which is in equilibrium with the surrounding atmosphere, which we shall call the first, second and third partial error.
The first partial error results from the loading effect on the object being measured by bringing a cold measuring probe into contact with it. The heat flow from the surface of the object in contact with the probe increases from its equilibrium value as flux is drawn up the probe. The result is a disturbance of the temperature field and a drop in surface temperature at that point. This error is particularly severe with non-metallic objects where the low thermal conductivity of the object results in large temperature field disturbances.
The non-ideal contact conditions between the probe and the object surface result in a thermal resistance at the interface between the two. T h e in te nse heat flux up the probe has to pass through this resistance, and a temperature drop is experienced, resulting in the second partial error.
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The third partial error is caused by the temperature drop with distance from the surface of the object being measured to the sensitive point of the probe which i s re sp on si ble for the thermometer readings. Due to the physical construction of the probes, this distance, /, is non­zero. Figure 1 shows the effects of these three partial errors on the reading of the contact thermometer.
These three errors can be represented by the following equations [1]:
1.
R 1= -
2.
2= -
1
3. 3= -
where:
qd = nett heat flux density up probe
qT = total heat flux density up probe
T = total heat flux up probe
RT = radius of the probe
b and T = thermal conductivity of the object probe respectively
W
= contact resistance
c

W
T
T
b
c.T

(
.
F
). q
m
d
q
T
F
and
= constant
m
From these equations it can be seen that all three errors are proportional to either the heat flux or the heat flux density that flows up the measuring probe d u e to the te m perature gradient between object and probe. If this h e a t flux can be reduced, then it follows that the errors of measurement will also be reduced.
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Figure 1 ­Temperature Profile through Object and Measureme nt Probe.
TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
A THERMALLY COMPENSATED PROBE
The intense heat flux that flows up the probe when brought into contact with the object being measured is as a result of the large temperature difference between the hot object and the cold probe. If this temperature gradient can be reduced the heat flux, and hence the three partial
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errors, will be reduced.
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A schematic of a thermally co mpen sa t ed p r o b e i s s h o w n i n figure 2. A heater element i s w o u n d aro u nd th e probe near the sen si ng tip, and is used to raise
Figure 2. Schematic of a thermally compensated probe.
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ISS.04 - 06/06
the temperature of the probe to the same as that of the object being measured. When the probe is brought into contact with the object, the temperature field at the object surface is no longer disturbed, as the difference between the heat flux drawn up by the probe, H1, an d th e he a t supplied by the probe, H2, is equal to the equilibrium heat flux, H3, that would f l o w into the atmosphere if the probe were not present.
Results of a thermally compensated probe and two standard probes is shown in figure 3. The reduction in heat flux up the probe resulted in a vast reduction in the measurement errors. Furthermore, the temperature field of the object was not disturbed, and thus a measure of the true, unloaded surface temperature could be made.
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Figure 3:
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The Practical Realization
To be useful, the practical realization of the concepts described above needs to be of similar dimensions to a standard surface temperature measuring probe.
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A diagram of the device is shown below:
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ISS.04 - 06/06
935-14-81
Also now commercially available is a control and temperature indicator package.
Conclusion
A practical embodiment of the device is now commercially available.
N.B. This Author wishes to acknow l e d g e the contribution to this article of B.D. Foulis of N.M.L. from w hom much of the material is deriv ed.
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TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
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TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
TTI 4 SYSTEM
Description:
Comprises a temperature compensated surface probe and an indicator/controller.
The probe is fragile by its nature and must be handled with care.
Generally no oil or grease is necessary between probe and surface. Introducing any compound will affect the results and may prevent proper operation of the probe (see Tutorial).
There are 3 connections between the probe and the controller/indicator.
Two type N thermocouple plugs and one heater plug. Connect all three into the rear panel of the controller/indicator.
Connect the controller/indicator to the mains and switch on.
The controller is a temperature deferential device and its purpose is to heat the compensated probe to the same temperature as the su rfa ce b e i ng measured. The units displayed are
somewhat arbitrary and the controller is trying to achieve 0.0 for ideal compensation.
The indicator indicates the temperature of the probe. It is a type N thermocouple and will be ac cura te within either the normal type N thermocouple uncertainties or as quoted on t h e calibration report, if one has been ordered.
See figures 4 & 5.
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Figure 4 - Front View
TTI 4 HANDBOOK
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Figure 5 - Rear View
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TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
OPERATION
Operation is very simple. In an ambient with no air movement, place the probe at right angles onto the surface to be measured.
The probe MUST be positioned on the surface before pow ering up the TTI 4.
In general DO NOT use conducting liquids (see Tutorial) on the surface. For best results using a TTI 4 and surface probe measurement system the surface should be clean and smooth.
The outer sheath of the probe has a small clear area near the tip, this is to allow the user to position the tip flat against the surface to give maximum contact.
At all times during use the probes measuring junction should be flat against the surface to be measured. This can be achie ve d using very slight pressure, however the weight of the probe is ideal to provide sufficient contact for it to work correctly. Variable pressure will as with hand held measurements cause the compensation and hence the reading to vary, so arrange for the support to be a clamp rather than holding the probe by hand.
The TTI 4 can now be powered up.
Next wait until the probe temperature has stabilised and the controller is reading approximately
0. The reading will then be the true temperature of the surface.
As an experiment remove the heater plug from the rear of the controller/indicator and see the error when the probe is not compensated.
If the reading is unstable then using the 'auto tune' at 'set point' may improve this - see controller instructions. However, this is not advised if the instrument has been calibrated.
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TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
Using the PC Interface
The TTI 4 includes an RS422 PC interface and a special converter cable that allows use with the a standard RS232 port. When using the unit with an RS232 port it is essential that this converter cable is used. Replacement cables are available from Isotech, part numbe r ISO-232-432. A fu rth er lead is available as an option, Part Number ISO-422-422 lead which permits up to 5 instruments to be daisy chained together.
The benefit of this approach is that a number of calibration baths may be connected together in a "daisy chain" configuration - and then linked to a single RS232, see diagram.
Note: The RS 422 standard specifies a maximum lead length of 1200M (4000ft). A true RS422 port will be required to realise such lead lengths. The Isotech conversion leads are suitable for maximum combined lead lengths of 10M that is adequate for most applications.
Connections
For RS232 use simply connect the Isotech cable, a 9 to 25 pin converter is included to suit PCs with a 25 pin serial converter.
RS422 Connections Pin 4 Tx+ A 5 8 9 Rx- B 1
Using the Interface
The models are supplied with Cal NotePad as standard. This easy to use package is compatible with MS Windows 9x. A handbook for Cal NotePad can be found on the first installation disk in
Connection
Tx- B Rx+ A
Common
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TTI 4 HANDBOOK
MODEL 944
ISS.04 - 06/06
Adobe PDF format. If required a free Adobe PDF reader can be downloaded from, www.adobe.com.
CAL NOTEPAD
Cal Notepad can be used to log and display val ues from the equipment and an optional temperature indicator.
Minimum System Requirements
CNP requires Windows 95 / 98, a minimum of 5Mb of free hard drive space and free serial ports for the instruments to be connected.
Dev elopment
CNP was developed by Isothermal Technology using LabVIEW from National Instruments.
License
Use of the Cal NotePad software program "CNP" is as granted in this license agreement. In using the CNP software the user "licensee" is agreeing to the terms of the license. You must rea d and understand the terms of this license before using CNP. 1, This license permits licensee to use CNP software on a single computer. The user may make copies for back up and archival purposes freely as long as the software is only ever in use on a single computer at any one time. Please enquire about multi-user licenses. 2, CNP is protected by international copyright laws and treaties. CNP must not be distributed to third parties. 3, CNP must not be reversed engineered, disassembled or de-compiled. Licensee may transfer the software to a third party provided that no copies or upgrades of CNP are retained. 4, It is the responsibility of the user to ensure the validity of al l sto red results and printed certificates. Isot he rm a l T e chnology Ltd accept no responsibility for any errors caused by inappropriate use, incorrect set up or any other cause; including defects in the software. 5, Limited Warranty. Isothermal Technology warrants that CNP will perform substantially as described in this manual for a period of 90 days from receipt. Any distribution media will under normal used be guaranteed for a period of 90 days.
NO OTHER WARRANTIES, EXCEPT AS STATED ABOVE. The software and documentation is provided "as is" without warranty of an y ki n d and no other warranties (either expressed or implied) are made with regard to CNP. Isothermal Technology does not warrant, guarantee or make any representations regarding t he use or results of the use of the software or documentation and does not warrant that the operation of CNP will be error free.
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In no event will Isothermal Technology, its employees, agents or other associated people be liable for direct, in d irect, incidental or consequential damages, expenses, lost profits, business interruption, lost b u siness information or other damages arising out the use or inability to use CNP. The license fee reflects this allocation of risk.
CNP is not designed for situations where the results can threaten or cause injury to humans.
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Installing Cal NotePad
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MODEL 944
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1.
2. Click on the START button on the task bar, select RUN, type A:\SETUP (Where A: is your
3.
4
Starting Cal NotePad
From a Standard Installation:
Click the START button
Highlight PROGRAMS
Sel ec t I so te c h
- Se l e c t Cal pad
Insert CNP DISK 1 into the disk drive
drive letter) then click OK Follow the prom pts which will install the application and necessary LabVIEW run time
support files. Should you ever need to uninstall the software then use t he Add/Remove Programs
option from the Control Panel.
Protocol
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ISS.04 - 06/06
The instruments use Modbus (3216 controller)
If required, e.g. for writing custom software the technical details are available from our website at, www.isotech.co.uk/refer.html
TUTORIAL
Calibrating Contact Surface Temperature Sensors using the Isotech Small Hotplate Model 983 (50°C to 350°C) and the calibrated Surface Probe System Model 944 (30°C to 350°C).
INTRODUCTION
Surface temperature measurement has always been a problem.
Perhaps the problem is best expressed by Ni chols & White in their book `Traceable Temperatures' ISBN 0471938033. They say:
A surface, because it is the interface between 2 syst e m s, does not have a temperature as such. A user who wants to know a `surface temperature' should define his measurement problem more exactly - see Appendix 1 for the full text.
One major problem wi th all contact surface measurements, except the compensated probe of the Model 944 (TTI 4), is that the act of making the measurement significantly affects the temperature of the surface being measured, due to the heat flow from the surface to the probe.
Ideally all contact thermometers should be the compensated probe typ e wh ere no heat flows between surface and probe. Currently these are the only surface measurement probes, which because their thermal compensation enables them to measure surface temperature with minimum affect to the surface temperature, qualify to be `calibrated' in the accepted sense of the word.
Because there are many thousands of uncompensated surface tempe rature measuring devices in existence these can not be ignored, and are often sent for `calibration'.
A preferred word to `calibration' for these devices would be `simulation' or `evaluation', ie. we would simulate the normal contact conditions of the surface measuring device.
A few examples will help to explain:
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2.
TUTORIAL
Another major factor af fe cti ng measurement is airflow and hence stem conduction. Used in windy conditions outside for example, the stem conduction of the surface measuring probe will be many times that of the same probe in still air.
Although, as far as I know, there are no methods written down for contact surface temperature probe calibration, the following should give enough information to evaluate such a probe.
If the surfac e probe is used without thermally conducting liquid, on, for example, the curved rough surface of a cast iron pipe, then a piece of similar ma te ri a l sho uld be
placed in the hot plate recess to simulate the actual users situation.
Suggested method of contact surface temperature probe performance evaluation:
1.
2. Add any change of surface material supplied by the customer to simulate use.
3.
4. Connect the 944 probe onto the surface close to the unknown probe.
5.
6. Move both probes and repeat.
The above tests should be performed in still air.
7.
Set the temperature of the hot plate to the value agreed with the customer.
Support the unknown probe vertically onto the surface, using conducting liquid - or not ­as agreed with the user. The probes weight should b e u sed to give the necessary
contact to the surface.
Allow time to stabilise and record hot plate controller temperature, 944 temperature and unknown probe temperature. Take a number (n) of readings say 3n10 to obtain the
mean value and the deviation from the mean.
Switch on a fan about 1 metre away from the test apparatus and wait for the system to re-stabilise. Note readings as in 5 and 6 above.
8.
Change temperature if required and repeat steps, 5, 6 and 7.
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9.
NOTE
A proposal by Dr. M. De Groot has been put before the European Community to better define Surface Temperature Measurement. Isothermal Technology is the only UK body involved in this study.
TUTORIAL
APPENDIX 1
SURFACE TEMPERATURES
The results should give a calibration under stated conditions plus variations in readings due to different user conditions.
A surface, because it is the interface between 2 syst e m s, does not have a temperature as such. A user who wants to know temperature should define his measurement prob l e m m ore exactly. For example:
Is he interested in the amount of heat dissipated by the surface? How hot the surface feels to the touch - will it burn fingers? The amount of radiation emitted by the surface? The temperature of one of the systems near the surface?
Each of these measurement problems has a different answer and each would give a di ff erent "surface" temperature for the same surface. In general there are 2 classes of techniques used to measure surface temperature.
NON-CONTACT METHODS
Infrared and optical pyrometers measure the temperature by determining the amount of energy radiated from the surface. Generally the emissivity of the surfa ce needs to be known, though the adve n t of "two colour" pyrometers allows the temperature to be found without knowledge of the emissivity.
Commonly used mounting techniques for surface thermometers
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TUTORIAL
Coloured paints or crayons which change colour at specified temperatures can be applied to the surface as temperature indicators. For many the colo u r ch a ng e is irreversible, and they are, therefore, "once only" techniques. They may also change the temperature profile of the surface due to the insulation effects and different emissivities.
Several measurement techniques have been developed that establish a heat balance between the thermometer and the surface. The thermometer is slowly heated to the same temperature
as that of the surface, with the only contact between the 2 being through the intervening warm air. These thermometers have found application where large moving surfaces are involved.
CONTACT M ETHODS
The problem of obtaining sufficient immersion depth, when surface temperatures are measured, is by itself difficult. When combined with the large temperature gradients usually found on one or both sides of the surface, accurate measurements of surface temperature by con tact methods become almost impossible.
Most surface thermometers are based on thermocouples, PRT's or thermistors.
To obtain a good immersion depth, the thermometer should be "thermally tied" to the surface for several centimetres. Another approach is to bed the thermometer under the surface, ie. approach the surface from the side with the smallest temperature gradients.
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BIBLIOGRAPHY
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1.
2. Some more modern development in industrial temperature measurements can be found
3. Thermometer applications are covered by articles in TMCSI.
4.
An older text, still in print, offers a comprehensive look at many industrial temperature measurement problems. It is "Temperature measurement in engineering", by H.D. Baker,
E.A. Ryder, and N.H. Baker, Wiley, Vol. 1. 1953, Vol. II 1961.
in "Advances in instrumentation", Proceedings of the ISA Conference held annually.
British Standards Institute publishes a Code of Practice which is in several parts, not all available, and some are currently in need of re vi si on, but contains much useful
information. BS1041 "Code for Temperature Measurement".
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TUTORIAL
The most difficult immersion problems occur when making measureme n ts of ai r and surface temperatures. For air-temperature measurements the effective diameters of probes may be as large as ten times the actual diameter of the probe; a probe requiring 10 diameters immersion in the calibration bath may require more than 100 diameters immersion in air.
immersio n le n g th by one or two diameters to see if the reading changes. As a crude approximation about 60% of the total error is eliminated each time the immersion is increased by one effective diameter. In some cases it may be practical to estimate the true temperature from a sequence of measurements at different immersions.
Two solutions to the problem of surface temperature measurement: (a) attaching a length of the probe to the surface can approximate immersion. In some cases insulation may be helpful in reducing heat losses by radiation or convection, although it can cause
the surface to become hotter; (b) approaching the surface from the side which has the least temperature gradient will give the least error.
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TUTORIAL
RADIATION ERRORS AND SHIELDING
Heat can be transferred by any of three mechanisms:
TTI 4 HANDBOOK
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.
. Convection - for example, heat is transferred by the movement of air or other fluids;
.
Radiation is one of the most insidious sources of error in thermometry. We often fail to recognise the physical connection between the radiant source and the thermometer and overlook it as a source of error. Radiation errors are a particular problem in air and surface thermometry where there is nothing to obscure or shield the source, and where the thermal contact with the object of interest i s al ready weak. Examples of troublesome radiant sources include lamps, boilers, furnaces, flames, electrical heaters and the Sun.
A particularly common problem to watch for is the use of incandescent lamps whe n re a d i n g therm ometers. If you must use a lamp, then use a low-power fluorescent lamp which will radiate very little in the infra-red portion of the spectrum.
With more difficult measurements, such as air and surface temperatures, anything at a different temperature which has a line-of-sight to the thermometer is a source of error. This includes cold objects such as freezers which act as radiation sinks and absorb radiation emitted by the thermometer. To put things in perspective, remember that at room temperature anything radiates (and absorbs from its neighbours) about 500 watts per square metre of surface area, so the radiative contact between objects is far greater than we would expect intuitively. In a room near a large boiler a mercury-in-glass thermometer may exhibit an error of several degrees.
Conduction - for example, heat is conducted along a metal bar;
Radiation - for example, heat is radiated by lamps, radiant heaters, and the sun.
There are two basic strategies when you are faced with a measure m e nt that may be affected by radiation. Firstly, remove the source; and secondly, shield the source. Removing the source is obviously the most effective strategy if this is po ssi b l e . However, the thermometry is very often required in association with the source, particula rl y in temperature-control applications. In these cases it may be possible to change the source in a way which will give an indication of the magnitude of the error.
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TUTORIAL
If you are unable to remove the radiatio n source then shielding is the only resort. A typical radiation shield is a highly reflective, usually polish e d , me t al tube which is placed over the thermometer. The shield reflects most of the radiation away from the thermometer and itself. The shield wi l l usu a l l y reduce the error by a factor of about 3 to 5. The change in the thermometer reading when the shield is deployed will give a good indication of the magnitude of the error and whether more effort is required. Successive shields will help but will not be as effective as the first. Suitable trial shields are clean, shiny metal cans and aluminium foil.
T h e disadvantage of using a radiation shield in air-temperature measurements is tha t t he m ovement of air around the thermometer is greatly restricted, further weakening the th e rma l contact between the ai r an d th e thermometer. The problem is compounded if the shield is warmed by the radiation and conducts the heat to the stagnant air inside the shield. Therefore, to be effective the shield must allow free movement of air as much as possible. In some cases a fan may be needed to improve thermal contact by drawing air over the sensor, and to keep the shields cool. Note that the fan should not be used to push the air as the air will be heated by the fan motor and friction from the blades.
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FAULT FINDING
The TTI 4 system is maintenance free, servicing should be carried out by qualified personnel only.
The probe by its nature is made from fine wire, and after a lot of use one of these wires may break; most probably in the plug. The length of the cable to the controller is not critical and so a broken wire may be stripped and reconnected.
When a replacement semi standard thermocouple is required the 944 mu st be returned to Isotech.
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