MECHANICAL MEASUREMENT &

METROLOGY

ERROR AND PRECAUTION IN TEMPERATURE MEASUREMENT

150053119008 – JADEJA DHARMAJEET CLASS TEACHER – MRS. JAYATI GIRISH

150053119009 – BHAVESH KATARIA LAB TEACHER – MR. NILESH PATEL

THE ERRORS IN TEMPERATURE MEASUREMENT MAY ARISE DUE TO :-

(I.) ERROR IN SENSING : INCOMPLETE IMMERSION, STRATIFICATION AND POOR MIXING OF FLUIDS, INHOMOGENEITY OF THERMOCOUPLE WIRES, RADIATION AND CONVECTION LOSSES FROM THE MEASURING ELEMENT SURFACE.(II.) ERRORS IN LEAD WIRES : PARASITIC VOLTAGE, PARASITIC THERMOELECTRIC, INSULATION BREAKDOWN AND ERRORS DUE TO LEAD WIRE JUNCTION AT DIFFERENT TEMPERATURE.(III.) ERROR IN INDICATING INSTRUMENTS : VOLTAGE DROP DUE TO CIRCUIT RESISTANCE AND LEAK CURRENT.(IV.) DYNAMIC ERROR : THE INSTRUMENT MAY NOT RESPOND TO INSTANTANEOUS TEMPERATURE CHANGES BECAUSE OF THERMAL LAG OF THE SYSTEM.

ACCURACY CAN BE ACHIEVED BY OBSERVING THE FOLLOWING PRECAUTION :-• PROPER INSPECTION OF THE FILAMENT IN A MERCURY THERMOMETER WITH REGARD TO

BREAKAGE AND SEPARATION• PROPER SELECTION OF THERMAL WALLS; EXPOSED PARTS ARE TO BE INSULATED• NON-WITHDRAWAL OF THERMOMETER FROM THE TEMPERATURE FIELD DURING READING• ADEQUATE STIRRING AND MIXING OF FLUID FOR AVERAGE TEMPERATURE MEASUREMENT • USE OF FINE WIRE THERMOCOUPLES TO HAVE A MINIMUM LAG, HIGH SENSITIVITY AND

LOW RADIATION ERROR• THERMOCOUPLES CORRECTIONS FOR LEAD RESISTANCE AND FOR COLD JUNCTION

TEMPERATURE• USE OF PROTECTING SHIELDS AROUND THE SENSITIVE ELEMENT TO MINIMISE THE

RADIATION LOSSES• THERMOMETER BULB OR THERMOMETER WELL TO BE INSTALLED WHERE THERE IS A

FAST MOVEMENT OF THE GAS OR LIQUID. IF THE BULB ACCUMULATES AN INSULATING FILM, ITS THERMAL RESISTANCE WILL INCREASE. GREATER THERMAL RESISTANCE, GREATER THE LAG.

• IT IS NOT POSSIBLE TO LAY DOWN SIMPLE RULES OF GENERAL APPLICATION AS REGARDS THE PRECAUTIONS TO BE TAKEN TO ENSURE CORRECT MEASUREMENT OF TEMPERATURE. EACH METHOD WOULD REQUIRE DETAILED CONSIDERATION IN THIS REGARD. HOWEVER, N ATTEMPT IS MADE HERE TO GENERALISE THE VARIOUS REQUIREMENTS THAT SHOULD BE MET FOR CONTACT AND NON-CONTACT METHODS.

1) CONTACT METHODS 2) NON-CONTACT METHODS

FOR CONTACT METHODS…..

• THE CONTACT BETWEEN THE TESTING BODY AND THE HOT BODY MUST BE SATISFACTORY. ACTUALLY IT MUST BE SO GOOD THAT THE EXCHANGE OF HEAT BETWEEN THEM IS RAPID ENOUGH TO PREVENT APPRECIABLE DIFFERENCES IN TEMPERATURE ARISING BETWEEN THE TWO BODIES.

• THE TESTING BODY MUST HAVE A SMALL HEAT CAPACITY SO AS NOT TO DISTURB THE TEMPERATURE CONDITIONS. THIS IS ALL THE MORE ESSENTIAL WHEN THE VARYING TEMPERATURE IS BEING MEASURED AND THE TESTING BODY SHOULD BE ABLE TO FOLLOW THE FLUCTUATIONS OF TEMPERATURE WITHOUT TOO MUCH TIME LAG.

• THE TESTING BODY SHOULD NOT REACT CHEMICALLY WITH THE HOT BODY, OTHERWISE SOME HEAT MAY BE EITHER LIBERATED OR ABSORBED AS A RESULT OF THE CHEMICAL REACTION.

• AS FAR AS POSSIBLE CARE SHOULD BE TAKEN TO AVOID CONDENSATION OF VAPOUR ON THE TESTING BODY SINCE THE LATENT HEAT OF CONDENSATION OR WET BULB EFFECT MAY CAUSE SERIOUS ERRORS IN THE READINGS.

FOR NON-CONTACT METHODS…..• THE BODY WHOSE TEMPERATURE IS TO BE MEASURED SHOULD BE PLACED IN

AN ENCLOSURE AT SENSIBLY THE SAME TEMPERATURE AS ITSELF.• THE BODY SHOULD NOT HAVE ANY ABNORMAL FLUORESCENT OR LUMINESCENT

EFFECT, AND IF AT ALL THEY ARE THERE THEN THE PROPERTIES OF THESE EFFECT SHOULD BE KNOWN. IN OTHER WORDS, IN ORDER THAT A CORRECTIONS MAY BE APPLIED TO THE READINGS OF THE INSTRUMENT TO GIVE TRUE TEMPERATURE, THE EMISSIVITY OF THE SURFACE ON WHICH THE INSTRUMENT IS SIGHTED AND THE TEMPERATURE OF SURROUNDINGS SHOULD BE KNOWN.

• SIMILARLY THE PRESENCE OF ANY OTHER MEDIA SUCH AS OCCASIONAL PRESENCE OF SMOKE OR DIRTY WINDOWS HAVING APPRECIABLE ABSORPTION COEFFICIENTS WOULD REQUIRE SPECIAL CALIBRATION OF THE INSTRUMENT. IT MAY BE NOTED THAT THE GLASS WINDOW CAN BE KEPT FREE DEPOSITS BY HEATING AT LEAST TO C, ABOVE TEMPERATURE OF ADJACENT GASES.

MERCURY, ALCOHOL, ETC. IN GLASS THERMOMETER• THE TEMPERATURE OF THE INTERIOR OF SOLID CAN BE MEASURED WITH A LIQUID

EXPANSION THERMOMETER BY INSERTING IT IN A HOLE, AND FILLING THE SURROUNDING SPACE WITH SOME LIQUID TO ENSURE GOOD THERMAL CONTACT.

• ERROR ARE ALSO INTRODUCED IF THE BULB AND STEM OF THE THERMOMETER ARE NOT COMPLETELY IMMERSED IN A REGION OF UNIFORM OF TEMPERATURE.

• FOR CORRECT READINGS, THE STIRRINGS OF THE LIQUID IS USUALLY NECESSARY.• THIS ERROR CAN BE MINIMISED BY PROVIDING THE PROPER SHIELDING WITH BRIGHT

METAL SCREENS, FORCED FLOW OF GAS.• THE THERMOMETER SHOULD BE SUFFICIENTLY FAR AWAY FROM ANY SURFACES OR

CORNERS WHERE IT IS DIFFICULT TO PROMOTE MIXING.• TEMPERATURE IS NOT NEARLY UNIFORM THE INSERTION OF A THERMOMETER OF

RELATIVELY HIGH HEAT CAPACITY MAY BE UPSET THE TEMPERATURE DISTRIBUTION.• CARE MUST BE TAKEN TO AVOID CONDENSATION OF VAPOUR ON THERMOMETER.

MERCURY IN STEEL, ALCOHOL ETC., IN METAL THERMOMETER• BULBS OF THE DIAL TYPE THERMOMETERS HAVE A COMPARATIVELY HIGH HEAT

CAPACITY AND THUS THESE HAVE SOMEWHAT GREATER THERMOMETRIC LAG THAN GLASS THERMOMETERS WITH DIRECTLY EXPOSED BULBS.

• THE THERMOMETRIC LAG CAN BE REDUCED BY USING BULBS OF SMALLER DIAMETER.• BULBS FOR GAS TEMPERATURE SHOULD BE FORMED WITH AN ADEQUATE LENGTH OF

NON-SENSITIVE PORTION OF PREFERABLY WITH A ‘NECK’ OF SMALL DIAMETER WHICH IS ALSO IMMERSED IN THE GAS.

PRECAUTIONS TO BE TAKEN IN THE USE OF RADIATION PYROMETER• IT GIVES CORRECT TEMPERATURE ONLY WHEN THE BODY WHOSE TEMPERATURE

IS TO BE MEASURED IS A ‘BLACK BODY’ OR THE SPECTRAL DISTRIBUTION OF THE RADIATION EMITTED BY IT IS KNOWN SO THAT NECESSARY CORRECTION COULD BE APPLIED.

• NONE OF THE RAYS PROCEEDING FROM THE BODY TO THE PYROMETER ARE CUT OFF; OTHERWISE COMPLETE ENCLOSURE WITH WALLS AT UNIFORM TEMPERATURE.