Thermocouple and mercury style thermometers never agree. Why?

[Shadow_Storm56]

Why do the mercury style thermometers never agree with electronic style thermocouple sensors? I think it's alcohol now that replaced mercury...but still. Which one should be more accurate? The non electronic read alot lower and the electronic read higher but agree with each other.....so wtf lol. Idk what it means

[Guest]

Electric are more accurate. The pressure of the mercury is calibrated at the bulb, and sealed with a constant pressure, so as the mercury heats up, it expands, so the temperature gauge rises. The thermometer uses the relationship that at constant pressure, temperature and volume have a direct relationship. It’s calibrated using fixed points. 

 

An electric thermometer is much more accurate. It uses a metal probe with an electric current running through it. At a different temperature, the metal probe will have a different resistance to the current, as the atoms vibrate more (Remember temperature is the average amount of kinetic energy in a system - the atoms vibration) it is harder for the electrons to flow, thus more resistance. This is a more subtle change than the mercury thermometer (which we use alcohol now because oopsie), so it is more precise.

[Shadow_Storm56]

44 minutes ago, floofer said:

Electric are more accurate. The pressure of the mercury is calibrated at the bulb, and sealed with a constant pressure, so as the mercury heats up, it expands, so the temperature gauge rises. The thermometer uses the relationship that at constant pressure, temperature and volume have a direct relationship. It’s calibrated using fixed points. 

 

An electric thermometer is much more accurate. It uses a metal probe with an electric current running through it. At a different temperature, the metal probe will have a different resistance to the current, as the atoms vibrate more (Remember temperature is the average amount of kinetic energy in a system - the atoms vibration) it is harder for the electrons to flow, thus more resistance. This is a more subtle change than the mercury thermometer (which we use alcohol now because oopsie), so it is more precise.

Thanks! I was assuming that but in some cases the simple stuff works better. Then again measuring resistance is pretty simple as well so neither is horribly complex. 

[Guest]

2 hours ago, Shadow_Storm56 said:

Thanks! I was assuming that but in some cases the simple stuff works better. Then again measuring resistance is pretty simple as well so neither is horribly complex. 

It’s not really complex. In smaller increments the standard error be smaller, because the amount of variability will also be smaller. Typically, the amount of error in a measuring device is half the smallest increment. 

[Mr.Dingle]

10 hours ago, floofer said:

 

An electric thermometer is much more accurate. It uses a metal probe with an electric current running through it. At a different temperature, the metal probe will have a different resistance to the current, as the atoms vibrate more (Remember temperature is the average amount of kinetic energy in a system - the atoms vibration) it is harder for the electrons to flow, thus more resistance. This is a more subtle change than the mercury thermometer (which we use alcohol now because oopsie), so it is more precise.

Thats not what a thermocouple does . 

 

11 hours ago, Shadow_Storm56 said:

Why do the mercury style thermometers never agree with electronic style thermocouple sensors? I think it's alcohol now that replaced mercury...but still. Which one should be more accurate? The non electronic read alot lower and the electronic read higher but agree with each other.....so wtf lol. Idk what it means

One relies on physical properties of the material used ( mercury or alcohol ) to accurately represent the change in temperature via change of volume which you then read off a scale , while the other uses the thermo-electric effect which you can then read off of a multimeter / voltmeter or do an analog to digital conversion to use in displays and what not ... From inexpensive K-types which have a very wide range of measurement but are less precise to something like Type-B which have a narrower range of measurement but have +/- .5% error.