calibration of thermometry instruments

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Calibration of Thermometry Instruments

Nihal GunasekaraSri Lanka

Bangladesh BEST Programme

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Essential Features of Calibration

• Fixed Point Calibration – Primary Level Primary Laboratories –NPL or NML

• Comparison Calibration – Secondary Level Secondary Level Laboratories


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Fixed Point Calibration

• Fixed Point Calibration – ITS-90 Fixed Points• Medium – Fixed Point Bath or Furnace• Advantage - Very Small Uncertainty


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Comparison Calibration

• Comparison – Secondary Standard• Medium – Calibration Bath or Dry Well• Uncertainty – Larger than Fixed Point Calibration


Calibration Hierarchy ITS 90

Fixed Points

Standard Platinum Resistance Thermometers Upto 1000 °C

Working Standard Platinum Resistance Thermometers

upto 1000 °C

Platinum Resistance Thermometers

Liquid in glass ThermometersDial Thermometers

Thermocouples

Thermocouples Type R & S upto 1500 °C &

Thermocouples Type B upto 1700 °C

Thermocouples upto 1700 °C

Thermocouples upto 1000 °C Temperature BathsOvens/Incubators

Furnaces

Black Body Radiation

Source

Optical Pyrometers

upto 3500 °C

Furnaces upto 3000 °C


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Calibration of Laboratory Equipment• Laboratory Ovens – Mainly used for sterilization of

Glassware

Normally upto 300 °C

• Laboratory Incubators - Mainly used for growing microbial cultures

Normally upto 50 °C


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Performance Testing of Ovens/Incubators –Three Point Method

Check the following before commence the test• Power supply and its socket• Temperature range/ indicator of the oven / incubator• Location of the built-in temperature sensor of the

oven / incubator• Ventilation levels• Any other indicators, if available


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Selection of Test Temperatures• Select at least three test points which are approximately

equally spaced covering the working range• If only one test point is requested select that test point• Oven with the range 50 °C to 250 °C select 50 °C, 100 °C,

150 °C, 200 °C and 250 °C • Select test point which are not within the ± 15 °C of the

requested points by the customer• If the incubator is used for one/two test point/s select

that/both test point/s


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Placement of Temperature Sensors• Place the temperature sensors in the “upper”,

“middle”, “lower” and “near temperature sensor “

Upper

Middle

Lower

Near sensor


Steady-state Condition

Stability “Obtain the readings after stabilized the temperature setting and ensure that the set temperature is stable before commence the record readings”

.


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Stability Conditions


150

80

100

Time (min.) t1 t2

Temperature (° C )

t1 - t2 is the temperature stabilized period at 150 °C

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Data Recording

• Record the maximum / minimum of the built in temperature indicator.

• Record the temperature from all four test sites at 30 seconds time intervals during the “test interval” (i.e. one hour or three cycles of the controller, which ever is the longer), after stabilization.

• Record ambient temperature and ambient relative humidity near the oven (i.e. about 1m away from the oven).

• Repeat the above steps for each of the test temperature settings of the oven.

• Record all data in a data recording sheet.


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Performance test results for an oven/incubator (three point)Specification:180 ±1oC

Temp. setting

(oC)

Value of the indicator

(oC)

Measured mean value(oC)

Exp.Unce.(oC)k=2

Min Max Near the

sensor

Upper area

Middle area

Lowerarea

180.0 180.0 179.9 180.1 180.5 180.3 181.1 ±0.3

Uncertainty component=Exp.Unc/2


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Performance Testing of Ovens/Incubators – Multi- Point Method

Definitions :• Enclosure temperature: mid-range value of the temperatures obtained from all relevant sites within the enclosure throughout the test interval.

• Measured enclosure : enclosure temperature computed from the measured temperatures. It is equal to half the sum of the maximum and minimum indicated temperatures

• Indicated enclosure temperature : enclosure temperature computed from indicated temperatures. It is equal to half the sum of the maximum and minimum indicated temperatures


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Definitions :• Spatial variation: difference between the mid-range of all measured temperatures obtained at one site and that at another site for those sites which give the greatest difference.

• Measured spatial variation: spatial variation computed from measured temperatures

• Indicated spatial variation: spatial variation computed from indicated temperatures


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Definitions :• Temporal variation: maximum value of the temperature range obtained for each standard site through out the test interval.

• Measured temporal variation : temporal variation computed from measured temperatures

• Indicated temporal variation : temporal variation computed from indicated temperatures


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Definitions :• Maximum measured temperature : the highest measured temperature obtained during the test interval

• Minimum measured temperature : lowest measured temperature obtained during the test interval

• Overall variation: difference between the maximum and minimum measured temperatures.


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Definitions :• Test site: location at which a test sensor is positioned for the purpose of temperature measurement during testing of the enclosure.

• Mid-range value: half the sum of maximum and minimum values of a set of temperatures.

• Temperature range: difference between the maximum and minimum values of a set of temperatures.


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Definitions :• Test interval: interval of time to which the steady state characteristics. The test interval will commence after stability has been established and shall be of a sufficient period of time to characterize the temporal variation.

• Apparatus required : A temperature data logger and a set of suitable thermocouples to cover the required temperature range of the incubator.


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Inspection:• Power supply and its socket• Temperature range/ indicator of the oven or

incubator• Location of the built-in temperature sensor of

the oven or incubator • Ventilation levels• Any other indicators, if available


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Selection of Test temperatures - Oven

Select at least three test points which are approximately equally spaced covering the working range

If only one test point is requested select that test point

Oven with the range 50 °C to 250 °C select 50 °C, 100 °C, 150 °C, 200 °C and 250 °C

Select test point which are not within the ± 15 °C of the requested points by the customer

If the incubator is used for one/two test point/s select that/both test point/s


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Selection of Test temperatures- Incubator

Select the temperature settings designed for the incubator


Performance test results for a laboratory oven/incubator (multi point)

Temperature variationTemperature specificationGrade of an oven/incubatorPerformance test results


Temperature variation inside a chamber

T1

T2

T3

T2

T4

T5

T6

T7


Temperature Specification

T± ΔT oC

Ex: 100 ±2.5 oC


Grade of an oven/incubatorG = 8- 6.64log[100Rm/100+D]

Where; G: Grade of the oven Rm: Maximum overall variation (Rm=2 ΔT)

D : Difference between oven temperature and ambient temperature

N = 3 + 3(G)0.6(V)0.2 Where; N: Number of standard test sites

V: Volume of the oven


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Performance test results

Test point:100 ± 2.5oC (temperature specification)

Rm, Maximum overall variation=5oCD= 100-20oCG=5 N = 9


Grade of an oven/incubator

Grade (G) Max. permissible overall variation (Rm ) °C

D = 30 D = 100

4 5.2 8.0

6 2.6 4.0

8 1.3 2.0

9 0.92 1.4

12 0.33 0.5

14 0.16 0.25


The above approximate values are provided as examples for guidance only

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Calculation and Derived Quantities

The mid range temperature for the sensor = ½ ( t max + t min)

Temperature range of the sensor = ( t max - t min )

Let for specific test temperature : T max is the largest t max value obtained from N test sites T min is the smallest t min value obtained from N test sites

OVERALL VRIATION = T max – T min

MEASURED ENCLOSURE TEMPERSTURE = ½ (T max + T min )


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T/C Position Maximum(oC)

Minimum(oC)

Difference(oC)

Mid range(oC)

Exp.Unc(oC), k=2

1 100.9 100.8 0.1 100.8

±0.3

2 100.5 100.3 0.2 100.4

3 99.9 99.8 0.1 99.8

4 99.0 98.8 0.2 98.9

5 100.2 100.0 0.2 100.1

6 100.5 100.2 0.3 100.4

7 100.2 100.0 0.2 100.1

8 97.9 97.4 0.5 97.6

9 100.8 100.7 0.1 100.6

Indicator value 100.6 100.5 0.1 100.6

Indicated Enclosure temperature = 100.6 oC

Measured enclosure temperature = 99.2 oC

Spatial variation = 3.2 oC

Temporal variation = 0.5 oC

Overall variation=3.5oC Grade = 6

Calibration ResultsBangladesh BEST Programme

ESTIMATION OF UNCERTAINTY

Prepare the Uncertainty Budget as per Uncertainty of Measurement GUM JCGM 100 :2008

Calculate the CMC value for the scope of accreditation (not mote than 02 significant figures)

The reported uncertainty in your report should always be greater than CMC value

Bangladesh BEST nProgramme


Thank you

calibration of thermometry instruments

Documents

c thermocouples upto

test temperature settings

temperature indicator

set temperature

c thermocouples type

c laboratory incubators

record ambient temperature

cfurnaces upto