# Homework Help: Heat-related problem

1. Mar 8, 2005

### mengshuen

Hi all, this is my 1st time posting here, so please bear with me.

I have this odd physics question which my teacher claimed it was correct. In fact, the entire physics department agreed with her. I may be wrong, but I need confirmation.

For a resistance thermometer, the resistance DEcreases when the temperature of a object INcreases. It is thus inversely proportionate. Is this correct?

For different thermometers, specifically the mercury and the resistance thermometer, when the reading of the mercury thermometer reads 40 degree Celsius, the resistance thermometer should read much higher than 40, in fact, in the 75 range. My teacher told me that this is because different thermometric properties will produce different temperature scales but they will agree at the ice and steam point. Other pupils who put the reason as the resistance of the platinum wire is not based on a linear scale got the mark. I mean, if it has a non-linear scale, how can it be used as a thermometer?

2. Mar 8, 2005

### ctw

Question: "For a resistance thermometer, the resistance DEcreases when the temperature of a object increases. It is thus inversely proportionate.Is this correct?"

For a resistance thermometer, the resistance decreases when the temperature of a object increases if temperatures are above curie points.

For a resistance thermometer, the resistance increases when the temperature of a object increases if the temperature is lower then the curie points.

Inversely proportionate is not true in both cases as there is no linear graph. (Please see below)

It is only applicatble for linear graphs.

Statement:
"For different thermometers, specifically the mercury and the resistance thermometer, when the reading of the mercury thermometer reads 40 degree Celsius, the resistance thermometer should read much higher than 40, in fact, in the 75 range. My teacher told me that this is because different thermometric properties will produce different temperature scales but they will agree at the ice and steam point. "

Reason: Your teachers are quite correct. There is no such thing as a linear graph if x be the temperature, the y be the property of the constant being changed. It would result in a sort of curve. Please do a search for the Steinhart Hart equation.

Statement:
"Other pupils who put the reason as the resistance of the platinum wire is not based on a linear scale got the mark."

Reason: They should... The scale is not straight, its a curve.

Statement:
"I mean, if it has a non-linear scale, how can it be used as a thermometer?"

Reason: Even if it has a non-linear scale, it can be used as a thermometer. No one knows for sure if there is a linear scale to start with. That is because you only have 2 points: the boiling and melting point.

Hope I was of help...

Last edited: Mar 9, 2005
3. Mar 8, 2005

### xanthym

There are actually 3 types of RESISTANCE based temperature sensors in general use. The first 2 types are called "THERMISTORS", with one type of Thermistor called "NTC" ("Negative Temperature Coefficient") because its Resistance DECREASES with increasing temperature, and the 2nd type of Thermistor called "PTC" ("Positive Temperature Coefficient") because its Resistance INCREASES with increasing temperature. The last 3rd type of Resistance-Based temperature sensor is called "RTD" ("Resistance Temperature Detector") whose Resistance generally INCREASES with increasing temperature.

Most RESISTANCE temperature sensors today are either the NTC Thermistors or the RTD Platinum sensors. Neither one has a perfectly linear relationship between its Resistance and the Temperature. However, the RTD Platinum sensor is much more linear than the NTC Thermistor. However, the NTC Thermistor is usually much more sensitive than the RTD Platinum.

Web Site #1 listed below compares the NTC Thermistor with the RTD Platinum sensors in the 2nd graph. Note that the NTC Thermistor rapidly decreases in resistance whereas the RTD Platinum slowly increase resistance with increasing temperature. Neither one is linear.

Even though Resistance is not linear with Temperature, formulas are provided to convert measured Resistance to the correct Temperature. Web Site #1 listed below illustrates some formulas towards the bottom of the page.

Web Site #2 listed below compares the NTC Thermistor and RTD Platinum sensors in more detail.

Web Site #1:
http://wuntronic.de/sensors/therm_cal.htm [Broken]
Web Site #2:
http://www.enercorp.com/temp/Thermistors_comparision.html

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Last edited by a moderator: May 1, 2017