Thermometry

Hero’s Thermoscope

Instrument Shelter Louvered sides Painted white Bottom ~4 feet above ground Instruments at 1.5 meters (~5 feet) above ground Door opens away from sun Mounted above grassy surface (or natural ground cover)

Today, many shelters are for individual sensors and may be artificially ventilated. –Natural — Artificial

Errors of All Thermometers Transient Errors –Time Response Wind Speed Errors –Compression / rarefaction of air past the sensor produces heating / cooling of the air. Not usually a problem unless dealing with rapidly moving platforms (aircraft, rocket) –Insufficient airflow past sensor (e.g. in instrument shelter)

Radiation Errors –Absorption of radiation from sun, surroundings (reflected) can potentially produce the highest errors. Conduction Errors –Conduction of energy through the thermometer supports and electrical wires.

Broad Classification of Thermometers A. Mechanical B. Electrical

A. Mechanical thermometers 1. Thermo-Expansion of a liquid Liquid in glass thermometers 2. Deformation type thermometers Bimetallic thermometer Bourdon thermometer

1. Thermo-Expansion of a liquid If we assume that the capillary tube is in the shape of a cylinder, then the volume expansion and change in length of the liquid column can be written as:  d = Coefficient of volume expansion

Immersion of Thermometer Partial immersion - immerse to mark Total immersion - immerse all of liquid Complete immersion - immerse all of thermometer

Errors of the Liquid-in-glass Thermometers Common to Mercury and Spirit Thermometers –Elastic Errors Reversible - due to sudden change in temperature Irreversible - due to slight contraction of glass over time. Causes zero point to be raised. –Emergent Stem Error Not a problem in meteorology. Always use complete immersion thermometer.

–Parallax Error Incorrect alignment of eye with mercury, or spirit, level. –Instrument Error Incorrect calibration of thermometer –Breaking of Liquid Column More common in spirit filled thermometers

Additional Errors of Spirit Thermometers Adherence of spirit to glass Secular changes in spirit due to polymerization of organic compounds –Leads to decrease in volume and lowering of zero point

Maximum Thermometers

Minimum Thermometer

2. Deformation Type Thermometers Bimetallic Thermometer –Two bars of different metal welded together. –Due to different coefficients of expansion, a change in temperature causes difference in length of expansion according to:

If  1 >  2 then metal strip A will have a greater expansion than metal strip B. The strip will start to bend.

Bimetallic Thermometers are not accurate enough for meteorology. Usually have a slow time response.

Often used as: –Dial Thermometers –Thermostat switches