<p>Instead of «whirling thermometer» you probably refer to «<a href="https://en.wikipedia.org/wiki/Hygrometer#Sling_psychrometer" rel="nofollow noreferrer">sling psychrometer</a>» consisting of a dry-bulb thermometer and a wet-bulb thermometer.  Yet, different to a static wet-bulb thermometer shown in the question, the constant rotation of the two thermometers permits evaporation from the wet cloth around the web bulb thermometer to «pristine parts» of surrounding air.</p>
<p><a href="https://i.sstatic.net/euwN4.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/euwN4.jpg" alt="enter image description here" /></a></p>
<p>(<a href="https://automationforum.co/what-is-a-sling-psychrometer-limitations-application-working/" rel="nofollow noreferrer">credit</a>)</p>
<p>If the air around the wet-bulb thermometer is less than water saturated, the temperature of shown on the wet-bulb thermometer will be lower than the one read on the dry-bulb thermometer.  The swirling motion aims to expose the wet-bulb thermometer to air which was not yet in contact with the former.  Thus, both recordings allow you to report both (dry) temperature, dew temperature, and humidity of the air.</p>
<p>A beginner's introduction is equally given <a href="https://www.youtube.com/watch?v=OB0_yM8g0nM" rel="nofollow noreferrer">here</a>, a computation chart relating the temperature difference read (one for the Celsius, an other for the Fahrenheit scale) to the relative humidity <a href="https://www.tqcsheen.com/en/product/sling-psychrometer-en/" rel="nofollow noreferrer">here</a>.</p>
<hr />
<p>Suggested by user <code>@uhoh</code>, it is worth to set the quality of the readings obtained with this instrument into a context.
The 1993 edition of <a href="https://www.nps.gov/museum/publications/conserveogram/03-01.pdf" rel="nofollow noreferrer">Conserve O Gram 3/1</a> (accessed from the series' program <a href="https://www.nps.gov/museum/publications/conserveogram/cons_toc.html" rel="nofollow noreferrer">overview</a>) mentions «These instruments are accurate to <span class="math-container">$\pm \pu{2\% RH}$</span> when properly maintained and operated by trained and practiced users.»  Equally, the guide reminds to the <a href="https://en.wikipedia.org/wiki/Clausius%E2%80%93Clapeyron_relation" rel="nofollow noreferrer">Clausius-Clapeyron relation</a> when stating:</p>
<blockquote>
<p>«Psychrometers and psychrometric charts are intended for use within a certain range of atmospheric pressure. High altitude, above 900 m (approx. 3,000 ft), will directly affect the accuracy of the RH reading unless a pressure correction formula is applied or a psychrometric chart or slide rule for the appropriate pressure is used. For further information see Ann Hitchcock and Gordon C. Jacoby, “Measurement of Relative Humidity in Museums at High Altitude,” Studies in Conservation 25 (1980): pp. 78-86. (Copies are available to NPS sites from Curatorial Services Division, Harpers Ferry office.»</p>
</blockquote>
<p>A study by <a href="https://doi.org/10.1007/s10765-007-0293-9" rel="nofollow noreferrer">Wang and Tang</a> /  Singapore's metrology centre published by 2007 mentions:</p>
<blockquote>
<p>«... that although the psychrometer coefficient of a particular psychrometer can be quite consistent with long-term stability typically within the range of  ± 1% rh equivalent, there are larger variations (up to 4% rh equivalent) among different psychrometers even of the same type.»</p>
</blockquote>

CC BY-SA 4.0
2021-05-01T23:00:36.633Z
Excellent! There seems to be a small difference at least from the chart on my thermometer to the one in your link. It's not huge but it's in the right direction to match your explanation. Mine (I think, I can't read Chinese) says a 5 degree difference at dry=30 C indicates 65% relative humidity https://i.sstatic.net/gESbF.png whereas the conversion chart you link to says 67%. This suggests that a 5 degree difference would have to result from dryer air for mine than for yours, consistent with a bit higher rate of evaporation,
uhoh
CC BY-SA 4.0
2021-05-02T01:25:11.980Z
@uhoh Depending on the source consulted, at least for non-calibrated devices, one observes $\pm2\dots\pu{4\% RH}$ difference to the true value of RH.  See equally the edit of the answer, too.
Buttonwood
CC BY-SA 4.0
2021-05-02T01:55:39.430Z
Sure, but I'm talking about a *systematic difference*, not a +/- random variation, and in *tabulated values* used for the RH determination, not in measurements.
uhoh
CC BY-SA 4.0
2021-05-01T22:35:33.833Z
2021-05-02T01:21:09.537Z
2021-05-02T01:21:09.537Z
Buttonwood
<p>Answers to my previous question <a href="https://earthscience.stackexchange.com/q/19151/6031">How is relative humidity determined from a wet and dry bulb readings?</a> provide equations for converting the two temperature measurements to a relative humidity.</p>
<p>In comments under <a href="https://earthscience.stackexchange.com/q/21197/6031">this question</a> I have heard of a <a href="https://www.preservationequipment.com/Catalogue/Instruments/Hygrometers/Whirling-Hygrometer" rel="nofollow noreferrer">Whirling Hygrometer</a> which seems to be the same basic beast (two thermometers, one with a wet bulb from a cotton sock and small reservoir) but like the name says, this one you have to &quot;whirl&quot; around for most of a minute; presumably the evaporation is more efficient in the blowing air.</p>
<p>But I don't understand if that leads to a lower cold temperature and therefore requires a different equation, nor if the speed that it is whirled makes a difference. And I'm not sure <em>why</em> whirling is used if the static non-whirled kind also can be used, without moving parts.</p>
<p><strong>Question:</strong> How does a whirling hygrometer work, how does it differ from static dry/wet bulb thermometer, and how to obtain humidity from its readings?</p>
<hr />
<p>Related:</p>
<ul>
<li><a href="https://earthscience.stackexchange.com/q/19152/6031">How do the pros measure humidity?</a></li>
<li><a href="https://earthscience.stackexchange.com/q/19145/6031">What is the second thermometer in the image from the Esperanza Antarctic temperature record?</a></li>
<li><a href="https://earthscience.stackexchange.com/q/15349/6031">How is the specific heat of a wet bulb determined?</a></li>
</ul>
<p><a href="https://i.sstatic.net/Yu941.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Yu941m.jpg" alt="wet/dry bulb humidity measurement" /></a> click for larger</p>

CC BY-SA 4.0
2021-04-30T18:09:57.630Z
"and how to obtain humidity from its readings" I think is covered by your previous https://earthscience.stackexchange.com/questions/19151/how-is-relative-humidity-determined-from-a-wet-and-dry-bulb-readings question?  Not easy, and while I'm no expert on the topic, maybe I can offer some additional input.  But hopefully that can help you limit this to one question?  If you want some other measure of humidity, perhaps you could ask a separate question, though it's quite easy to get from RH to like dew point temperature or vapor pressure, and there's probably already questions on such.
JeopardyTempest
CC BY-SA 4.0
2021-04-30T18:13:47.967Z
@JeopardyTempest I'm not sure that's true yet. As I've just explained in this question the whirling affects the rate of evaporation. Isn't there a chance then that the temperature difference could be larger than for a static pair? If it's the same, then I'd like to see that in an answer, along with an explanation of why it isn't different. I've asked over 3,000 SE questions, and I have found that when there are questions so tightly coupled as this, the answerer to one will almost certainly be able to answer the other. I think this will be just fine!
uhoh
CC BY-SA 4.0
2021-04-30T18:18:21.077Z
@JeopardyTempest [The wet bulb temperature is always read first because it will start to warm as soon as the hygrometer stops whirling.](https://earthscience.stackexchange.com/a/19153/6031) proves that it can't be the same equations.
uhoh
CC BY-SA 4.0
2021-04-30T18:23:28.927Z
I never used a static wet bulb, dry bulb thermometer. I always used a whirling hygrometer. The one catch about using a whirling hygrometer is the wet bulb temperature must be read first & it must be read immediately the hygrometer stops rotating. The reason for this is once the wet bulb thermometer stops rotating it will slowly start to warm up due to the warmer temperature of the atmosphere.
Fred
CC BY-SA 4.0
2021-04-30T18:32:24.677Z
@Fred and when that happens, it will *still be colder*, and function just like my static pair, right?
uhoh
CC BY-SA 4.0
2021-04-30T19:04:21.503Z
The answer will be no to your question "isn't there then a chance that the temperature difference could be larger for a static pair", any properly done wet bulb measurement will result in the same value for the same air (and any improper measurement has error that won't be accountable by a simple equation).  Give me some time and I'll update answers to both this and the other question from what I've found\know.
JeopardyTempest
CC BY-SA 4.0
2021-04-30T19:24:29.793Z
@JeopardyTempest okay thanks for the update!
uhoh
<meteorology><measurements><humidity><instrumentation><field-measurements>
How does a whirling hygrometer work, how does it differ from static dry/wet bulb thermometer, and how to obtain humidity from its readings?
uhoh
# How does a whirling hygrometer work, how does it differ from static dry/wet bulb thermometer, and how to obtain humidity from its readings?

(Asked by: username_0 on 2021-04-30)

Answers to my previous question [How is relative humidity determined from a wet and dry bulb readings?](https://earthscience.stackexchange.com/q/19151/6031) provide equations for converting the two temperature measurements to a relative humidity.

In comments under [this question](https://earthscience.stackexchange.com/q/21197/6031) I have heard of a [Whirling Hygrometer](https://www.preservationequipment.com/Catalogue/Instruments/Hygrometers/Whirling-Hygrometer) which seems to be the same basic beast (two thermometers, one with a wet bulb from a cotton sock and small reservoir) but like the name says, this one you have to "whirl" around for most of a minute; presumably the evaporation is more efficient in the blowing air.

But I don't understand if that leads to a lower cold temperature and therefore requires a different equation, nor if the speed that it is whirled makes a difference. And I'm not sure *why* whirling is used if the static non-whirled kind also can be used, without moving parts.

**Question:** How does a whirling hygrometer work, how does it differ from static dry/wet bulb thermometer, and how to obtain humidity from its readings?

---

Related:

* [How do the pros measure humidity?](https://earthscience.stackexchange.com/q/19152/6031)
* [What is the second thermometer in the image from the Esperanza Antarctic temperature record?](https://earthscience.stackexchange.com/q/19145/6031)
* [How is the specific heat of a wet bulb determined?](https://earthscience.stackexchange.com/q/15349/6031)

[![wet/dry bulb humidity measurement](https://i.sstatic.net/Yu941m.jpg)](https://i.sstatic.net/Yu941.jpg) click for larger

**Tags:** meteorology, measurements, humidity, instrumentation, field-measurements

> **Comments:**
> **username_1**: "and how to obtain humidity from its readings" I think is covered by your previous https://earthscience.stackexchange.com/questions/19151/how-is-relative-humidity-determined-from-a-wet-and-dry-bulb-readings question?  Not easy, and while I'm no expert on the topic, maybe I can offer some additional input.  But hopefully that can help you limit this to one question?  If you want some other measure of humidity, perhaps you could ask a separate question, though it's quite easy to get from RH to like dew point temperature or vapor pressure, and there's probably already questions on such.  
> **username_0**: @username_1 I'm not sure that's true yet. As I've just explained in this question the whirling affects the rate of evaporation. Isn't there a chance then that the temperature difference could be larger than for a static pair? If it's the same, then I'd like to see that in an answer, along with an explanation of why it isn't different. I've asked over 3,000 SE questions, and I have found that when there are questions so tightly coupled as this, the answerer to one will almost certainly be able to answer the other. I think this will be just fine!  
> **username_0**: @username_1 [The wet bulb temperature is always read first because it will start to warm as soon as the hygrometer stops whirling.](https://earthscience.stackexchange.com/a/19153/6031) proves that it can't be the same equations.  
> **username_2**: I never used a static wet bulb, dry bulb thermometer. I always used a whirling hygrometer. The one catch about using a whirling hygrometer is the wet bulb temperature must be read first & it must be read immediately the hygrometer stops rotating. The reason for this is once the wet bulb thermometer stops rotating it will slowly start to warm up due to the warmer temperature of the atmosphere.  
> **username_0**: @username_2 and when that happens, it will *still be colder*, and function just like my static pair, right?  
> **username_1**: The answer will be no to your question "isn't there then a chance that the temperature difference could be larger for a static pair", any properly done wet bulb measurement will result in the same value for the same air (and any improper measurement has error that won't be accountable by a simple equation).  Give me some time and I'll update answers to both this and the other question from what I've found\know.  
> **username_0**: @username_1 okay thanks for the update!  

---

## 1 answers

### Answer by username_3 on 2021-05-01. Score: 3

Instead of «whirling thermometer» you probably refer to «[sling psychrometer](https://en.wikipedia.org/wiki/Hygrometer#Sling_psychrometer)» consisting of a dry-bulb thermometer and a wet-bulb thermometer. Yet, different to a static wet-bulb thermometer shown in the question, the constant rotation of the two thermometers permits evaporation from the wet cloth around the web bulb thermometer to «pristine parts» of surrounding air.

[![enter image description here](https://i.sstatic.net/euwN4.jpg)](https://i.sstatic.net/euwN4.jpg)

([credit](https://automationforum.co/what-is-a-sling-psychrometer-limitations-application-working/))

If the air around the wet-bulb thermometer is less than water saturated, the temperature of shown on the wet-bulb thermometer will be lower than the one read on the dry-bulb thermometer. The swirling motion aims to expose the wet-bulb thermometer to air which was not yet in contact with the former. Thus, both recordings allow you to report both (dry) temperature, dew temperature, and humidity of the air.

A beginner's introduction is equally given [here](https://www.youtube.com/watch?v=OB0_yM8g0nM), a computation chart relating the temperature difference read (one for the Celsius, an other for the Fahrenheit scale) to the relative humidity [here](https://www.tqcsheen.com/en/product/sling-psychrometer-en/).

---

Suggested by user `@username_0`, it is worth to set the quality of the readings obtained with this instrument into a context.
The 1993 edition of [Conserve O Gram 3/1](https://www.nps.gov/museum/publications/conserveogram/03-01.pdf) (accessed from the series' program [overview](https://www.nps.gov/museum/publications/conserveogram/cons_toc.html)) mentions «These instruments are accurate to $\pm \pu{2\% RH}$ when properly maintained and operated by trained and practiced users.» Equally, the guide reminds to the [Clausius-Clapeyron relation](https://en.wikipedia.org/wiki/Clausius%E2%80%93Clapeyron_relation) when stating:

> «Psychrometers and psychrometric charts are intended for use within a certain range of atmospheric pressure. High altitude, above 900 m (approx. 3,000 ft), will directly affect the accuracy of the RH reading unless a pressure correction formula is applied or a psychrometric chart or slide rule for the appropriate pressure is used. For further information see Ann Hitchcock and Gordon C. Jacoby, “Measurement of Relative Humidity in Museums at High Altitude,” Studies in Conservation 25 (1980): pp. 78-86. (Copies are available to NPS sites from Curatorial Services Division, Harpers Ferry office.»

A study by [Wang and Tang](https://doi.org/10.1007/s10765-007-0293-9) / Singapore's metrology centre published by 2007 mentions:

> «... that although the psychrometer coefficient of a particular psychrometer can be quite consistent with long-term stability typically within the range of ± 1% rh equivalent, there are larger variations (up to 4% rh equivalent) among different psychrometers even of the same type.»

> **Comments:**
> **username_0**: Excellent! There seems to be a small difference at least from the chart on my thermometer to the one in your link. It's not huge but it's in the right direction to match your explanation. Mine (I think, I can't read Chinese) says a 5 degree difference at dry=30 C indicates 65% relative humidity https://i.sstatic.net/gESbF.png whereas the conversion chart you link to says 67%. This suggests that a 5 degree difference would have to result from dryer air for mine than for yours, consistent with a bit higher rate of evaporation,  
> **username_3**: @username_0 Depending on the source consulted, at least for non-calibrated devices, one observes $\pm2\dots\pu{4\% RH}$ difference to the true value of RH.  See equally the edit of the answer, too.  
> **username_0**: Sure, but I'm talking about a *systematic difference*, not a +/- random variation, and in *tabulated values* used for the RH determination, not in measurements.  

---

