Comment by krisoft | Hacker News Reader

Comment by krisoft | Hacker News Reader

Parent Full thread

> please, don't nitpick, I am aware of wet bulb thermometers

I don’t want to nitpick. Everything you wrote is correct as far as I know. I only want to add to it.

I think understanding what a wet bulb thermometer is crucial to understand the term. And it is a very simple thing: you take a totally normal thermometer, you put it in the shade. So far everything is normal, this is how you measure air temperature since forever. You then put a little sock on the end of the thermometer and make it wet. Super simple. On a traditional mercury thermometer the end is a little glass sphere where the mercury is stored. This glass sphere is called the “bulb”. A wet bulb thermometer is just a thermometer where the bulb is wet. Super simple.

Why would you do this? Obviously this “wet bulb” thermometer will measure a lower temperature than the dry one. This is because evaporation of the water takes away energy from the thermometer. The faster the water can evaporate bigger the difference you will see between the dry and the wet thermometers. If the air is saturated with water vapor then the two measurements will be the same. If the air is dry you will have lots of evaporation and the wet bulb will be way colder than the dry one. So if all you have is two thermometers a sock and a bucket of water you can measure the relative humidity in this indirect way by comparing the two temperatures.

An other way to think about a wet bulb thermometer is that it is a very crude modell of a sweating human. You keep the sock wet and that simulates the thermometer sweating. The temperature the termometer reaches is the limit of temperature you can cool yourself down using evaporative cooling.

Yes, this is completely correct.

I would add the wet bulb thermometer needs large evaporation area (so the sock cannot be too wet and definitely you can't put it in the bucket) and/or some airflow around it. Otherwise it will show some temperature between actual dry bulb and wet bulb temperatures.

In practice, nobody seriously uses wet bulb thermometers outside of school experiments because they are such a PITA. It is easier to just measure dry bulb temperature and humidity separately and then look up wbt in a table.

In general measuring temperatures correctly is a tricky business, what with temperature gradients in the objects you try to measure and the thermometer itself. I just recently needed to explain to a bunch of people why there is no such thing as "temperature in the sun" and that you can pick any temperature between WBT and around 5800K and it is going to be a correct temperature "in the sun" depending on properties and shape of the object you are trying to measure..

>nobody seriously uses wet bulb thermometers outside of school experiments because they are such a PITA. It is easier to just measure dry bulb temperature and humidity

How is humidity typically measured? I was under the assumption a hygrometer uses both dry and wet bulb measurements.

The traditional 5 dollar analog hygrometer uses a paper/metallic strip coil:

https://www.youtube.com/watch?v=jCUHDFYdG44

I guess I was dancing around the point a bit. I meant the definition of relative humidity uses dry/wet-bulb comparisons but we use all sorts of more pragmatic calibrated proxies to measure it in practice.

Wet bulb requires it to be... you know... wet.

Not very practical to include in your electronics.

I don't know exact method used but I can assure you, there is no water reservoir in any of my gadgets:)

It looks like I’m confusing a classic psychrometer with more modern ones. The modern ones still use absorption of water to change some physical property like capacitance or resistance but this is also why they need to be continually calibrated against a true dry/wet bulb hygrometer.

My experience is that they tend to get out of cal quite often which can lead to bad scenarios, like if using enthalpy setback for air-handling

Well, wet bulb thermometer works specifically by evaporating water and measuring temperature.

Psychometer is completely different idea.

Yes, measuring humidity reliably is a difficult problem as basically any other measurement that requires direct exposure of eroding/corroding components and fragile electronics to elements.

I believe a psychrometer operates on the same principle.

"A psychrometer, or a wet and dry-bulb thermometer, consists of two calibrated thermometers, one that is dry and one that is kept moist with distilled water on a sock or wick. At temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, such that the wet-bulb thermometer will be at a lower temperature than that of the dry-bulb thermometer. "

https://en.wikipedia.org/wiki/Hygrometer

The psychrometer just compares the wet and dry bulb temperatures to arrive at a relative-humidity value. I.e., it still operates on the principle of a wet-bulb thermometer.

Reply on news.ycombinator.com