Slide 1
A thermometer first measures its own temperature
A contact thermometer responds to its own temperature. Good thermal coupling helps it approach the temperature of the material being measured.
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Heat can also travel along a probe’s sheath and wires toward its surroundings. Adequate immersion reduces this competing path. A shallow probe can therefore report a mixture of thermal influences even when its electronic conversion is accurate.
Slide 2
Sunlight can make a sensor hotter than the air
An air-temperature sensor in direct sunshine can warm above the surrounding air. Placement and shielding determine which influences reach it.
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A weather enclosure must block unwanted radiation while allowing air to circulate. Sealing a sensor in a protective box can defeat that purpose by restricting airflow. The installation should expose it to the air being sampled while limiting heat from nearby surfaces.
Slide 3
Measuring current can heat a resistance thermometer
A resistance thermometer needs measuring current. That current produces heat inside the sensing element and can raise its temperature above the surroundings.
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At unchanged resistance, doubling current produces four times the electrical heating. The resulting temperature error also depends on how readily heat escapes. A self-heating correction measured in flowing liquid may not apply unchanged when the same probe is installed in air.
Slide 4
A changing temperature takes time to reach the reading
A temperature sensor cannot follow every sudden change instantly. Its size, packaging and thermal contact influence the time needed to respond.
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A larger sensor can respond more slowly yet be less susceptible to self-heating. Electrical isolation can also slow a thermocouple junction’s response. Selecting a rugged or isolated assembly therefore involves a timing tradeoff when brief temperature changes are the quantity of interest.
Slide 5
An electrical probe can likewise change its circuit
An electrical probe becomes part of the circuit it touches. Its resistance and capacitance can change the voltage waveform being investigated.
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High input resistance reduces steady loading, but it does not remove capacitive loading during fast changes. A low-capacitance probe can preserve sharper transitions. An apparently slow signal may consequently reflect the measuring setup as well as the original circuit.
Sources
- Guide on Secondary Thermometry: Industrial Platinum Resistance Thermometers
NIST / Consultative Committee for Thermometry
- How Do You Measure Air Temperature Accurately?
NIST
- Measuring Temperature with Thermocouples, RTDs, and Thermistors
NI
- How to Minimize Probe Loading with Low Capacitance Probes
Tektronix