A Quick Question About Thermistors

I am currently in a discussion concerning temperature controllers. The controllers use hermetically sealed NTC 10kΩ temperature probes. An individual opined that a single digital temperature controller using a NTC 10kΩ probe can be used as a hygrometer without modification. Further, any probe can be used as a hygrometer. His exact words:

"Any probe especially water proof can be used as a hygrometer regardless of the resistance if you know how to use it. And that's our secret which I will not share."

I have several issues with that statement but I am trying to do my due diligence.

Question: can a hermetically sealed NTC 10kΩ thermistor "be used as a hygrometer," and if so, how? I am just not seeing it.

You could make a "wet bulb" thermometer using one.

By using another dry temperature sensor in close proximity, with either another complete temperature indicator or by switching between the wet one and a matched dry thermistor on the same instrument, the two readings could be used to work out air humidity.

That is one class of hygrometer.

Thank you for your reply. I brought up the possibility of using two temperature controllers, one as a dry bulb thermometer and one as a wet bulb thermometer to create a simple psychrometer. My comment was:

"A thermistor or temperature sensitive resistor, has a small amount of current running through it (bias current). With an NTC (Negative Temperature Coefficient) thermistor, when the temperature increases, resistance decreases, and more current is passed. When the temperature decreases, resistance increases, and less current passes. The controller reads the change in current.

That particular hard-wired temperature probe (thermistor) is hermetically sealed (waterproofed) so the probe cannot react to changes in humidity. You might be able to replace the thermistor with a resistive or capacitive sensor (used in hygrometers) but capacitive sensors aren't very accurate, I think there would be calibration issues, and it would be easier to buy a controller which already has a resistive or capacitive sensor designed for hygrometers.

It might be possible to use two controllers, one as a dry bulb thermometer and one as a wet bulb thermometer, to create a simple psychrometer but why reinvent the wheel?"

His response I relayed in my original post above. He is adamant that "any probe" can be used as a hygrometer without modification. Personally, I think the guy is nuts and that he is flat-out wrong but I thought to check myself before I wreck myself.

naa60512 said:

He is adamant that "any probe" can be used as a hygrometer without modification.

Click to expand...

Ask him how a Hall effect magnetic probe could be used as a hygrometer.

alec_t said:

Ask him how a Hall effect magnetic probe could be used as a hygrometer.

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Lol, diolch am eich ateb. I am trying to be extremely polite. There are cultural differences to be considered (I am an American, he is a Filipino). I'd hate to realize after the fact that I missed something incredibly obvious. Wouldn't be the first time, tho.

If you are missing anything then I am also missing the same thing.

naa60512 said:

"Any probe especially water proof can be used as a hygrometer regardless of the resistance if you know how to use it. And that's our secret which I will not share."

Click to expand...

The statement is nothing short of childish. Years ago I used wet bulb/dry bulb quite a bit and with thermocouples. Additionally did quite a bit of hygrometer calibration using saturated salts. Just maintain your cool as you have been doing and politely tell the person if he can't share how it is done you will merely assume he is fabricating or simply lacks any understanding of the subject matter. I am having trouble conveying this because polite correctness was never my strong point. How to tell someone they are full of crap or an idiot was something I was always direct about. Read up on saturated salts and how they apply to hygrometer calibration and then ask this humidity guru how do we calibrate hygrometers accurately.

On the subject of relative humidity calibration this is a good read. Interesting also is the avalability today of sensors to accurately measure a wide range of things at relatively inexpensive prices.

Ron

Assuming you don't need quick readings, is the following possible? Read the temperature with the thermistor then measure what power is required to keep the thermistor 5°C above the measured temperature. Would higher humididy mean it needs more power to maintain the temperature? Also, assuming still air.

Mike.

Thank you for your reply and for the link to the .pdf!

I occasionally use the saturated salt method to check my hygrometers.

Pommie said:

Assuming you don't need quick readings, is the following possible? Read the temperature with the thermistor then measure what power is required to keep the thermistor 5°C above the measured temperature. Would higher humididy mean it needs more power to maintain the temperature? Also, assuming still air.

Mike.

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Interesting idea, thank you!

I think that moist air (higher humidity) holds heat more efficiently than dry air, so wouldn't it require less power to maintain the temperature? To be honest I rarely worry about the RH but the guy kinda set me off. I am not overfond of people posting bad info. Thanks!

I assumed the heated air would rise and new air would need heating. I also assumed that moist air requires more energy to heat it.

Mike.

Pommie said:

I assumed the heated air would rise and new air would need heating. I also assumed that moist air requires more energy to heat it.

Mike.

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This is why I ask experts, I am just a hobbyist. This gives me something to look in to, thanks!

Whilst there are some expert on here, I don't class myself as one of them.

Mike.

This is a brief read on the subject and I can't attest to its accuracy. The consensus is that it will take more energy to heat humid air but the difference is negligible. In the case of the power dissipated by a thermistor I really don't see much difference happening. Then too this stuff was not my forte, thankfully.

Ron

Bear in mind that a hermetically sealed thermistor is not or at least shouldnt be thermally sealed, the temp of the resistive element is very close to that of the surroundings.

I would tell the guy that you are interested in learning how we can measure humidity with only one thermistor and, once you and the rest of the team understand and believe. The robustness of the technology, you'll be happy to use the idea. Also, The team buy-in is important because a sustainable project needs to be documented and team members trained on how to repair the device or make future modifications - especially if this brilliant guy finds more interesting things at some point in the future and leaves the team.

the first people I replace on new teams are people who behave like they are irreplaceable.