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Liquid-in-Glass Thermometer Calibration

PRINCIPLE OF OPERATION

SCHEMATIC DIAGRAM

Liquid-in-Glass Thermometer is a thermometer consisting of liquid in a glass tube. Calibrated marks on the tube allow the temperature to be read by the length of the liquid within the tube, which varies according to the heat given to it. Liquid-in-glass thermometers are normally calibrated by compare with SPRT by fixing the temperature. The immersion of liquid-in-glass thermometer is according to type of liquid-in-glass thermometer. Total immersion thermometer is immersed to within a few divisions of the top of the column of the thermometric liquid, leaving the gas-filled section and a few millimeters of thermometric liquid exposed. In case of partialimmersion, thermometer is immersed to a fixed distance from the bottom of the bulb to the immersion line. In addition, an emergent-stem thermometer should be attached to thermometer for monitoring the surrounding air temperature.

TRACEABILITY CHART

MATHEMATICAL MODEL

Measurement range of liquid-in-glass thermometer is -40 oC to 250 oC.

MEASUREMENT RANGE

MEASUREMENT SETUP

Measurement Range Uncertainty-40 oC to 110 oC 0.02 oC>110 oC to 250 oC 0.05 oC

Liquid-in-glass thermometer and the SPRT are placed in calibration bath. SPRT should be immersed at least 15 times of diameter of SPRT + length of sensor of SPRT and connected to the indicator.

Mathematical model of liquid-in-glass thermometer measurement ist90 = tx + Cx + CRfP

Determination of Cx (and CRfP):Cx= (ts-tx) - (tRfP-txRfP) + ts + tI + tDS + tDI + tRX + tUni + tSta + tSt + Cstem

where:Cx = reduce correction;tS = temperature reading indicated by SPRT;tx = reading of UUC;CRfP = tRfP - tXRfP ;

tRfP = temperature of reference point or ice point;tXRfP = reading of the UUC at the reference point or ice point;tS = correction due to standard thermometer;tI = correction due to indicator;tDS = correction due to drift of standard thermometer;tDI = correction due to drift of indicator;tRX = correction due to resolution of thermometer under calibration

which is equal to 1/10 of subdivision;tUni = correction due to spatial temperature difference between

reference thermometer and UUC;tSta = correction due to temporal temperature difference between

reference thermometer and UUC;tSt = correction due to short term stability of UUC at ice point

measurement;CStem = stem correction (Cstem= 0, for total immersion and Cstem= C2 for

partial-immersion).

ITS-90

Standard Platinum Resistance ThermometersPlatinum Resistance Thermometers

(Reference Standards)

General Measurement Instruments Digital Thermometers

Liquid in Glass Thermometers

Platinum Resistance ThermometersIndustrial Platinum Resistance Thermometers

(Working Standards)

NIMTFixed Point Cells & Apparatus

Ag961.78

Al660.323

Zn419.527

Sn231.928

In156.5985

Ga29.7646

WTP0.01

Hg-38.8344

Ag961.78

Al660.323

Zn419.527

Sn231.928

In156.5985

Ga29.7646

WTP0.01

Hg-38.8344

ITS-90Defined Fixed Point Cells

(National Standards or International Standards)