Integrity in Temperature Calibration

WELCOME TO THEFOURTH ISSUE OF “HIDDEN GEMS”.

In this issue of Hidden Gems we introduce the microK and

focus on IndustrialResistance Thermometers

and Immersion depth.

www.isotechna.com

In This Issue

Free CD:Encyclopaedia of

Thermometry, Part 4

Isotech’s microKThermometer

PRT Sensing Length, Immersion Depth and

Stem Conduction Explained

Calibration Laboratory News - Uncertainties

Lowered Again !

Metrology Web Links

Two Special Offers !

1

04MAR’ 07

New Type of Precision Thermometer Offers Unrivalled Accuracy and Stability

Isothermal Technology Limited (Isotech) has introduced a new type of precision thermometer, which it claims will set new world standards for accuracy and stability. Designed for a wide range of highly accurate industrial and scientific calibration applications, the instrument uses a completely new measurement technique to achieve accuracies better than 0.4 parts per million (ppm) - equivalent to 0.0004°C - when used with a standard platinum resistance thermometer (SPRT).

Designed and manufactured in Britain, microK is believed to be the only instrument of its type in the world that will work to this precision with all three commonly used sensors, platinum resistance thermometers (PRTs), thermocouples and thermistors. In terms of long term stability, microK achieves literally zero drift for resistance ratio measurements and an annual drift of only 3 ppm for voltage measurements.

While utilising sophisticated software and specially developed circuitry, the microK is easy to use and has a comprehensive range of built-in features. It utilises a 6.4-inch VGA colour touch screen as the main user interface, providing a number of graphically presented user options and controls. The underlying operating system is Microsoft Windows CE™ facilitating direct reading of temperature for all sensor types, data logging, export of data into Microsoft Excel™ applications and graphing facilities.

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Measurement ‘Best Practice’ guidelines recommend the use of two separate reference thermometers when carrying out temperature calibrations. For that reason, the designers of the microK have incorporated three measuring channels, enabling users to achieve best practice without having to invest in additional multiplexers. The instrument also includes ‘keep-warm’ current sources to maintain the power in a PRT when it is not being measured. This feature effectively eliminates uncertainty resulting from power coefficients. A key to achieving such high accuracies and stability is the use of a specially designed analogue to digital converter (ADC) within the instrument’s measurement circuits. In conjunction with a digital signal processor (DSP), this uses a unique adaptation of existing advanced sigma-delta ADC techniques to achieve linearity of better than 0.4 ppm. A further major benefit of this ADC technique is its low noise performance, with electronic noise reduced by a factor of 32 over conventional sigma-delta ADC circuits.

A further break with convention is that the instrument uses no mechanical switches, relays or potentiometers whatsoever (other than the main power on/off switch), relying on solid state switching to route voltage and current signals internally. Comparative instruments in this class have used high quality mechanical relays for this purpose, a technique which inherently introduces inaccuracies arising from thermal EMFs, and degrades reliability. By using the latest semiconductor technology, performance has been enhanced, component counts have been reduced and reliability considerably improved.

The microK range comprises two instruments, offering a choice of measurement accuracy: The microK 400 is accurate to 0.4ppm over the whole temperature range of SPRTs with R0 2.5, equivalent to 0.1mk (0.0001°C) at 0°C. With thermocouple sensors the voltage uncertainty is equivalent to 0.01°C for Gold-Platinum thermocouples. The microK 800 is accurate to 0.8ppm over the whole temperature range of SPRTs with R0 2.5, equivalent to 0.2 mK at 0°C. With thermocouple sensors the voltage uncertainty is the same as the microK 400 model.

The two instruments in the microK range offer performance characteristics and features which are simply not available elsewhere. Comparable instruments available Internationally do not achieve the same accuracy or stability (zero drift characteristics with SPRT measurements are not obtainable in any other instrument), do not support the same variety of sensors, and offer considerably less operational features. As a result, the ‘Cost of Ownership’, a key feature of growing International importance, has been considerably reduced.

22

microK

KEY FEATURES

Accuracy ±0.4ppm.

Zero drift for PRTs.

PRT, thermocouple and thermistor sensing.

1s measurement time.

Keep-warm currents.

Internal Standards

ISOTECH’S microK PRECISION THERMOMETER

If you would like further information, a quotation or

a visit from one of our sales engineers please

contact Isotech.

JoanneWilkinsonusing themicroK.

Detailof the

microKconnections.

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Hidden Gems Issue 4

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contactus

Isotech North America158 Brentwood Drive, Unit 4

Colchester, VT 05446

Telephone

(802) 863-8050

Fax(802) 863-8125

Email

sales@isotechna.com

Website

www.isotechna.com

Encyclopaedia of ThermometryPart Four

DON’T MISS OUT - CONTACT US TO BE SURE OF GETTING ISSUE No. 5 3

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For your free copy of theEncyclopaedia of Thermometry

visitwww.isotechna.com

The CD Library now contains:

! Fundamentals of Thermometry Part 1 Temperature Scales, Part 2 Fixed Points, Part 3 Standard Platinum Resistance Thermometers, Part 4 Standard Thermometers, Bridges and Measurements

! Practical Calibration of Thermometers on the ITS-90

! Platinum Resistance Thermometers as Interpolation Standards for the ITS-90

! Standard Platinum Resistance Thermometers calibrations on the ITS-90; how to specify and order

! International Equivalence of thermometer calibration, testing and certification

! Improvements in Metrological Apparatus

! Evaluation of the Gallium Melting Point by a two cell comparison

! The Platinum Resistance Thermometers of C.H. Meyers (Historic Reprint)

! Coiled Filament Platinum Resistance Thermometers

! Open Cell, Sealed Cells, Slim Cells

! The Heat Pipe and its use in Thermometer Calibration

! Uncertainties in Temperature Measurement

! The Gallium Watchdog (in quality assurance of measurement)

! New Developments and Discoveries - high temperature platinum resistance thermometers

! Review: About Words - available metrology vocabularies

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Hidden Gems Issue 4

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ITS90MICROK

Platinum resistance thermometers, PRTs or RTDs are widely used in industrial and secondary lab calibration applications.

They are capable of

! High Accuracy

! Low Drift

! Wide Operating Ranges

The temperature sensing element is usually at the bottom of the thermometer sheath. The sensing element can be of a film construction or of a wire wound construction. Good quality precision probes use wire wound detectors for the best stability and operating ranges.

The length of the detector can vary considerably with lengths from 6 to 30mm being commonly used but can also be as long as 50 to 75mm for special applications and standards.

When selecting or calibrating a thermometer it is important to know the sensing length and to ensure that all of the sensing length is adequately immersed in the system being measured.

For example, when calibrating a thermocouple against a PRT in a Dry Block it is important that all of the thermometer sensing lengths are in a uniform zone of the calibration volume. Isotech Dry Blocks are evaluated with regard to and meet the requirements of EA10/13 Guidelines on the Calibration of Dry Block Calibrators published by the European Cooperation for Accreditation, see the Web Links on page 6.

PRT Sensing Length

The temperature detectors used in Isotech probes are

manufactured by our sister company Thermal

Developments International, TDI.

TDI has manufactured over 6 million wire wound

temperature detectors. For more information visit

http://www.t-d-i.co.uk

To avoid large errors when calibrating temperature sensors it is very important to ensure that the thermometer under test is sufficiently immersed into the calibration equipment.

Figure 1 shows a typical industrial platinum resistance thermometer, IPRT. It has a sensing length of 15mm and is immersed just half the sensing length. It is immediately clear the thermometer is not sufficiently immersed.

In Figure 2 the sensor has been immersed all of the sensing length but it is still evident that it is not immersed deeply enough to reach the same temperature as the Dry Block.

Heat conduction along the thermometer sheath gives rise to “stem conduction” errors. Stem conduction is most significant when immersion is small and when the temperature difference between the thermometer and ambient is great.

When selecting a Dry Block, consider the diameter of the probes being calibrated, the sensing length of the probes and the temperature range.

Isotech application specialists have a great deal of experience with the calibration of all types of thermometer and are available to discuss your application and offer advice in the appropriate selection of equipment.

Dry Blocks from Isotech are available with immersion depths from 120 to 300mm.

Further reading, Temperature Calibration; Depths of Immersion: John P. Tavener available online, http://www.isotech.co.uk/pdfs/immersion.pdf

Immersion Depth and Stem Conduction

4

15mm

15mm

30mm

Figure 1

Figure 20

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Hidden Gems Issue 4

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5

A Dry Block for True Metrology Applications

Calibrate from50 to 700°C

Excellent Temperature Uniformity

Uses three zone control for flat vertical profile

Calibrates probes too long for other Dry Blocks

Immersion Depth to 285mm

Total Flexibility for Industry and Secondary

Laboratories

Can be used with fixed point cells, accessories for surface sensors and

Isotech Medusa 3

Quick DataThermocouple Information to IEC 584-3Type Conductors

+/-Sheath Insulation andPositive Conductor Colour

E Nickel Chromium / Copper Nickel VIOLET

J Iron / Copper Nickel BLACK

K Nickel Chromium / Nickel Aluminium GREEN

N Nickel Chromium Silicon / Nickel Silicon PINK

R Platinum / Platinum Rhodium 13% ORANGE

S Platinum / Platinum Rhodium 10% ORANGE

T Copper / Copper Nickel BROWN

Platinum Resistance Thermometer IdentificationRED WHITE RED WHITERED RED WHITERED WHITE

Uncertainties Lowered, Again !

We have continued to invest heavily in our UKAS calibration laboratories in terms of staffing, training, research and new procedures. As a result we are now able to finalise with UKAS a new schedule with lower uncertainties at the ITS-90 fixed points of Aluminium and Silver, the benefit being smaller uncertainties when we calibrate both ITS-90 fixed point cells and high temperature SPRT calibrations.

Our Accredited uncertainties at these points are now smaller than the scope of other Accredited laboratories and smaller than most National Measurement Institutes, (NMI) second only to NIST.

Our schedule can be viewed here,

If you need small uncertainties at high temperatures, compare our accreditation schedule with our competitors.

We have also simplified and streamlined our calibration services offering a choice of services to suit all your requirements. Call us for advice. The calibration lab contacts are shown on page 3.

http://www.isotech.co.uk/lab

Calibration Laboratory News

The negative conductoris White.

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6

SPECIALOFFERSTwo very special offers for

Hidden Gems Readers.

If you purchase one of the Dry Blocks with extended

immersion depth, Oceanus or Medusa and quote reference “HG04a” we will include the storage case free of charge.

We know from your feedback how much Hidden Gems with its CD-ROM is appreciated and we intend to keep to the publishing it twice a year.

In addition we are launching our eNewsletter to be published several times a year with details of special offers, news and technical information. We hate spam as much as you do and will not share your address with other parties.

Sign up here, www.isotech.co.uk/enews

New for 2007

Metrology Web Links

If you purchase a microK we will include a wireless

keyboard, mouse and 1Gb USB Pen Drive. Quote

reference “HG04b”.

These offers are for a limited time only. Please contact Isotech for further details.

ITS-90 Thermocouple Database: NIST have a comprehensive online database with tables for thermocouples and coefficients

Did you know you can download Techniques for Approximating the ITS-90 and Supplementary Information for the ITS-90

The BIPM organized an international comparison of water triple point cells, the report by the Consultative Committee for Thermometry (CCT)

Look for references to the Isotech Jarret Water Triple Point Cell from Hidden Gems 3. Independent confirmation of the Isotech / Jarrets cells high purity and closeness to V-SMOW. EA10/13 Guidelines on the Calibration of Dry Block Calibrators

Isotech: ITS-90 Information: microK:

http://srdata.nist.gov/its90/main/

http://www.bipm.org/en/publications/its-90.html

http://www.bipm.org/utils/common/pdf/final_reports/T/K7/CCT-K7.pdf

http://www.euromet.org/docs/calguides/EA-10-13.pdf

www.isotechna.com www.its-90.com www.microk.co.uk

How can we help you ?If there is a topic that you would like to hear about in future issues, please let us know. We will do our best to help! Our contact details are on page 3.

Request received from last issue

A: The “Callendar-Van Dusen” equation describes the relationship between resistance (R) and temperature (t).

For temperatures from -200°C to 0°C the equation is of the form

2 3“R(t) = R(0)[1 + A(t) + B(t) + (t - 100)C(t) ]

For temperatures above 0°C it becomes

2R(t) = R(0)(1 + A(t) + B(t) )

The values for A, B and C are specified in IEC 751, BS EN60751: 1996 as,-3 -1A = 3.9083 x 10 °C

-7 -2B = -5.775 x 10 °C-12 -4C = -4.183 x 10 °C

The coefficients can be determined for an individual probe by calibration, many laboratory temperature indicators (e.g Isotech TTI-7) can be programmed with these co-efficients. Free software that can calculate temperature from resistance with different co-efficient values can be downloaded from,

For temperature greater than 0°C it is necessary to have a resistance value at 0°C and two other points, for temperature below 0°C a further value is needed. An online calculator is available on our website,

Q: What is the “Callendar-Van Dusen” equation and why is it relevant?

www.isotech.co.uk/prtcalc

http://www.isotech.co.uk/prtcalc-web.html

Isotech eNewsletter

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