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What is the meaning of the "curly" symbol close to the double up arrows (Positive Temperature Coefficient, PTC) thermistor symbol? enter image description here. (Ref.1, Ref.2)

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    \$\begingroup\$ I do not see any omega symbol. Do you mean theta symbol? \$\endgroup\$
    – arnisz
    Commented Aug 14, 2021 at 19:40
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    \$\begingroup\$ Omega indicates it is a sensor that changes on resistance. || Theta and two arrows indicates change in resistance with temperature. Two Up arrow indicate PTC. One up and one down arrow indicate NTC. \$\endgroup\$
    – Russell McMahon
    Commented Aug 20, 2021 at 12:28

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That's not an omega at all; it's a script theta, ϑ, a symbol used here for temperature. You may be more familiar with the upright form of the character, θ, commonly used in mathematics to name angles, or the capital form, Θ, commonly also used as a symbol for temperature.

The double up arrows indicate that an increase in temperature corresponds to an increase in resistance (i.e. it's a PTC thermistor); if it was an NTC thermistor, one arrow would point up and the other down to indicate that an increase in temperature corresponds to a decrease in resistance.

RTDs are always PTC thermistors because they work over a greater temperature range and, as I recall, have better linearity. The reason also likely has something to do with the fact that platinum, as a metal, has a positive temperature coefficient, and platinum is used for RTDs for its resistance to corrosion even at high temperatures. But NTC thermistors can be preferred for certain applications; I believe they tend to have greater sensitivity, for instance.

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The symbol does not appear with the thermocouples (TC) which produce a voltage to indicate temperature. It appears with the thermistors, which are variable resistors, to indicate that their output is indeed a variable resistance (omega being a symbol for ohms).

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    \$\begingroup\$ That's a ϑ, a theta, not an omega (Ω or ω) at all. \$\endgroup\$
    – Hearth
    Commented Aug 14, 2021 at 20:11
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    \$\begingroup\$ @Hearth I don't agree. It looks exactly like the omega symbol in your comment. \$\endgroup\$
    – Barry
    Commented Aug 14, 2021 at 23:01
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    \$\begingroup\$ There's a Ω in the image, but the question is about the symbol "close to the double up arrows", which is ϑ. \$\endgroup\$
    – Hearth
    Commented Aug 15, 2021 at 0:03

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