What is the meaning of the "curly" symbol close to the double up arrows (Positive Temperature Coefficient, PTC) thermistor symbol? 
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(Ref.1, Ref.2)
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1\$\begingroup\$ I do not see any omega symbol. Do you mean theta symbol? \$\endgroup\$– arniszCommented Aug 14, 2021 at 19:40
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2\$\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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2 Answers
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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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\$\begingroup\$ Re: "RTDs are always PTC thermistors...". RTDs are "Resistive Temperature Devices" and exhibit a positive temperature coefficient because they're made of very fine conductor wound on an insulating substrate like ceramic and metals increase in resistance as temperature rises. They are not thermistors. \$\endgroup\$ Commented Oct 9 at 15:20
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\$\begingroup\$ Thermistors are semiconductors whose resistance changes non-linearly with temperature and yes, those do come in NTC and PTC flavours and PTC thermistors are often used for over-current protection as solid-state fuses whereas the NTC devices are used for temperature measurement in conjunction with electronics to linearize their output. \$\endgroup\$ Commented Oct 9 at 15:20
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\$\begingroup\$ @ProfessorV By what reasoning do you say that thermistors have to be semiconductors? My understanding has always been that a thermistor is a resistor with a specified, well-controlled temperature characteristic regardless of what it's made of. \$\endgroup\$– HearthCommented Oct 9 at 23:56
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\$\begingroup\$ @ProfessorV Also, RTDs are not usually wirewound; in my experience they tend to be thin-film. Making a standard PT100 RTD with wirewound construction would require significantly more expensive platinum metal than making it with a thin vapor-deposited film. Or however it is they get the film onto the substrate for these; I don't know exactly. \$\endgroup\$– HearthCommented Oct 10 at 0:00
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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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1\$\begingroup\$ That's a ϑ, a theta, not an omega (Ω or ω) at all. \$\endgroup\$– HearthCommented Aug 14, 2021 at 20:11
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1\$\begingroup\$ @Hearth I don't agree. It looks exactly like the omega symbol in your comment. \$\endgroup\$– BarryCommented Aug 14, 2021 at 23:01
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3\$\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\$– HearthCommented Aug 15, 2021 at 0:03