ISA Type Designation

Temperature measurement

1.Expansion Thermometer

These thermometers work on the principle of dimensional changes. Increase in length of metals corresponding to temperature Change in length is quite small. So some scheme are used to magnify the change in length as under

i. Constant Volume gas ThermometerThese thermometers employ helium below 32F, hydrogen below 32 and 212F, and nitrogen below 212F. The gas is maintained at constant volume, and, by the ideal-gas law, the pressure is directly proportional to temperature

ii. Mercury-in Glass Thermometer

Mercury in glass thermometer is an example of a filled thermometer that works on the expansion principle.The volume coefficient of expansion of mercury is about eight times that of glass. Due to the difference in coefficient, the mercury rises up the capillary in the stem to indicate temperature.

In the industrial mercury in-glass thermometer a thermal well is provided for the purposes of preventing breakage and providing a sealing means at the point of installation. It is generally of brass or steel, although cast iron, Monel, stainless steal, and aluminum are sometimes used. For use in ovens and air ducts, perforated metal guards are employed for protection against breakage.2. Bimetallic thermometer

It measures temperature by means of the differential thermal expansion of two metals. The bimetallic strip consists of a bonded composite of two metals. One of the metals is usually a Copper alloy and the other Invar, a Nickel steel with low thermal expansion coefficient. A temperature change will cause the bimetallic strip to deflect and this deflection can be related quantitatively to the temperature change. The deflection with temperature is nearly linear, depending mainly on the coefficients of linear thermal expansion.

Thermocouples

In a thermocouple two dissimilar thermo elements so joined as to

produce a thermal emf, when measuring and reference junctions are

Measuring junction

Measuring Junction is that junction of Thermocouple which is subjected to the temperature to be measured.Reference junction

Reference Junction is that junction of Thermocouple which is at the known temperature or which automatically compensated for its temperature.

The desirable properties of thermocouples for industrial use are:

Relative large thermal emf.

Precision of calibration.

Resistance to corrosion and oxidation.

Linear relation of emf to resistance.

The five most commonly used thermocouples are:

Copper-Constantan

Iron-Constantan

Chromel-Alumel

Platinum-platinum, 13% rhodium.

Platinum- platinum, 10% rhodium.

Thermal wells

For the protection of thermocouples, thermal wells are provided, which generally are made in the form of a tube with a closed end installed. The thermocouple is inserted inside the tube.

The mechanical properties, which must be considered in the selection of a thermal well, are:

Resistance to corrosion and oxidation.

Resistance to mechanical shock

Resistance to thermal shock

Resistance to gas leakage

Mechanical strength

The materials used for thermal wells are, Platinum, firebrick, Mullite, silicon carbide, calorized iron and Nichrome.

Thermocouple comparison Table

ISA Type DesignationPositive WireNegative WireMillivolts per Deg.FRecommended Temp Deg. FScale LinearityAtm Environ RecommendedFavourable PointsLess Favourable Points

Numbers=PercentageMinMax

BPt70-RH30Pt94-Rh6.0003-.006323380Same as for type R coupleInert or slow oxidizing____

EChromelConstantan.015-.042-3201830GoodOxidizingHighest emf/Deg FLarger Drift than other base metal couples

JIronConstantan.014-.035-3201400Good; nearly linear from 300-800ReducingMost Economical__

KChromelAlumel.009-.024-3102500Good; most linear of all T/COxidizingMost linearMore expensive than T & J

RPt87-Rh13Platinum.003-.00803100Good at high temps. Poor below 1000 Deg FOxidizingSmall size, fast responseMore expensive than Type K

SPt90 -Rh10Platinum.003-.00703200Good at high temps. Poor below 1000 Deg FOxidizingSmall size, fast responseMore expensive than Type K

TCopperConstantan.008 - .035-310750Good but crowed at low endOxidizing or reducingGood resis. To corrosion from moistureLimited temp

YIronConstantan.022 - .033-2001800About same as J typeReducing - Not industrial standard

Resistance thermometers

The resistance thermometer operates from the change of electrical resistance of a substance with temperature. Usually this substance is a metal, (whose resistance increases with temperature) but nonmetallic materials called semiconductors have also been used. The industrial resistance thermometers nearly always employs platinum, copper, or nickel. A material suitable for use as a resistance-thermometer element should have, first a continuous and stable relationship between rsistance and temperature, and, second, a relatively high thermal coefficient of resistance.

Resistance-temperature characteristics of metal can be expressed as

R = Ro(1+T)

R = resistance, ohms

Ro = resistance, ohms at 0C

T = temperature, C

= constant

Common materials used for resistance thermometers are given below-

Material

Range

Platinum

-260 to 1000 0C

Copper

-200 to 260 0C

Nickel & Balco

-100 to 230 0C

(70% Ni / 30% Fe)

Tungsten

-100 to 2500 0C

Thermistors

Thermistors are simply resistive elements. These are available with -ve or +ve coefficients. -ve temperature coefficient thermistors are used for temperature sensing. Thermistors are commercially available in the form of beads, rods, and flakes.

The -ve resistance-temperature relationship can be expressed as

R= Ro e ( (1/T-1/To)

R = resistance at the temperature T

T= absolute temperature

Ro= resistance at T0

(= material constant

Radiation Temperature Measurement

Radiation temperature measuring devices are used primarily for temperatures above 2000F.

But radiation means of temperature measurement may be used at low temperatures as long as a sufficiently sensitive element is employed to detect the small amounts of radiation.

In radiation temperature-measuring devices, three types of pyrometers are used

1. Radiation Pyrometers

A radiation pyrometer is a no contact instrument that measures the el radiation energy emitted from a body and infers its temperature from the detected radiation.

2. Optical Pyrometer

In optical pyrometer we measure the spectral radiant intensity of the radiated energy from a heated body at a given wavelength to measure the temperature of the heated body.

3. Photoelectric Pyrometer

Photoelectric methods possess great advantages in speed, since the response of a photosensitive cell to radiation is immediate. In photoelectric pyrometers, a photovoltaic cell directly produces an emf proportional to the amount of radiation received.

APPROXIMATE RANGE AND ACCURACY OF VARIOUS TEMP.MEASURING ELEMENTS

TypeRange Deg FAccuracy Deg F

GLASS THERMOMETER

MERCURY FILLED -38 TO 7600.5 to 2

MERCURY AND NITROGEN FILLED -38 TO 10000.5 to 10

ALCOHAL FILLED -95 TO 1501 to 2

PRESURE - GAUGE THERMOMETERS

VAPOUR - PRESSURE TYPE20 TO 4002 to 10

LIQUID OR GAS FILLED -200 TO 10002 to 10

BIMETALLIC THERMOMETER -100 TO 20000.5 to 25

THERMOCOUPLES

BASE MATEL -300 TO 20000.5 to 20

PRECIOUS METAL -300 TO 28000.5 to 20

RESISTANCE THERMOMETER -400 TO 18000.005 to 5

THERMISTORS -150 TO 500Depends upon aging

PYROMETERS

OPTICAL1400 UP20 to black body conditions

RADIATION1000 UP20 to 30 black body conditions

FUSION1100 TO 3600as low as 20 or 30 under optimum conditions