Actual Vs. Relative Humidity in Gauges?

[Guest]

I wandered over to another hermit crab board and found this post, let me know what you all think/know about it! Thanks.I noticed a number of questions regarding whether thehumidity guage they had was Relative or Absolute (sometimesreferred to as Actual) Humidity.Unless you bought your gauge from a scientific, meteorological,or industrial supplier, you will have a Relative Humidity (RH)gauge. Pet Stores ONLY sell RH guages.The statement that the "Actual humidity should read 50-60 (or50-60%)" is meaningless. It appears to be misinformationcirculating within the hermit crab owners community.Absolute humidity is usually given in units of Mass (of water vapor)per Volume (of Dry Air) or Mass (of water vapor) perMass (of Dry Air). Most commonly the units are grams/m3 orgrams/Kilogram (often used by the Heating, Ventilation, and AirConditioning community). At 75 degrees F, 70% relative humiditycorresponds to about 15grams/m3 or about 20grams/Kilogram. Ido not believe the range 50-60 would make sense in any common units.

[Guest]

Well, this is interesting. I am trying to search for the information on actual and realative humidity gauges Mjen posted a couple of weeks, ago, but alas errors.I'll ask Mjen again when whe gets back from vacation.Anywho, this raises lots of questions. T-Rex made actual gauges until two years ago, when they switched to relative. So, it is my understanding at this point there are some actual gauges on the market. In Mjen's post she had some information on the different ways humidity is read. hmmmmm. Kind of need to get to the bottom of this, because this is a little too important. If there are no actual gauges on the market, then the gauges that read 50-60 percent while relative gauges read 70-80 need to be on a do not use list compilation. ANd this is confusing, because:>Absolute humidity is usually given in units of Mass (of water vapor) per Volume (of Dry Air) or Mass (of water vapor) per Mass (of Dry Air). Most commonly the units are grams/m3 or grams/Kilogram (often used by the Heating, Ventilation, and Air Conditioning community). At 75 degrees F, 70% relative humidity corresponds to about 15grams/m3 or about 20grams/Kilogram. I do not believe the range 50-60 would make sense in any common units. Actual humidity gauges read humidity by a different method, this being per Mjen's post. SO, it would not make sense if you applied relative workings to an actual gauge idea.These are my concerns with this new information. Who is this person who posted this? Is there a way that we can ask him/her some more questions about how they know for sure about the actual/humidity thing? Also, since I can't seem to use my search function without errors, could someone find that post from Mjen from a week or so ago?And, with my actual gauges, when the humidity goes over 60, massive condensation happens. When I stick a relative gauge in, they seem to be equally proportionate, ie actual reads 65, relative 85 etc. So, how can this be?A concerned Gertie

[Guest]

Okay, I found something on AskJeeves. I *think* specific humidity is also termed actual (which, by the definition below, could be expressed as a percentage) - while absolute is something totally different. Read these definitions and tell me what you think: Absolute humidity: The mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter Relative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways. One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent.Specific humidity: The mass of water vapor in a parcel divided by the total mass of the air in the parcel (including water vapor)

[Guest]

So, relative humidity is the one we want, right? It measures humidity by figuring the percentage of water vapor in the air to the air's capacity for vapor.

[Guest]

I posted the message referred to on the Yahoo HermitCrab list.The reason I did was that on yahoo groups (and I also see onthis forum) ther were a lot of peopleasking whether they had a relative or "actual" humiditygauge. The distinction is important because theideal humidity range is usually listed as being about20 lower for actual verses relative (although there area fairly wide range of recommended relative humidityranges, see the list I collected at the end of this message).The answer in 100% of these cases was that they hada relative humidity gauge. I don't believe it ispossible to buy a "actual humidity gauge" that reads50-60 when near the ideal hermit crab humidity.Since you can't get one, you shouldn't worry that youmight have one.I say this for a couple of reasons. First, there isn'ta term "actual humidity" listed in Websters dictionaryor Encyclopedia Britannica. However, some people use the term"actual humidity" as synonymous with "absolute humidity".Also, you could have the phase in a statement like:"My relative humidity gauge said 75%, but that gaugealways reads about 5% too high so the actualhumidity was 70%". But then this would be referring toa relative humidity reading corrected for instrument error.Second, If an "actual humidity gauge" is supposed to mean"absoulte humidity gauge", then I cannot figure outwhat the range 50-60 could possibly mean in any common units.Sometimes the range is listed as 50-60% meaning that itwould be a ratio between two values... if so, what two values?The term "absolute" usually means it is not a ratio.In the most common units @ 75 degrees F 70% Relative Humiditycorresponds to about 15grams/m3 Absolute Humidity (nowhere near 50-60).Note that Relative Humidity is the ratio of "theamount of water vapor in the air" divided by "the maximumamount of water vapor that the air could hold at that temperature"expressed as %.I did notice that someone here said they had "actual humiditygauge" that read 50-60 for ideal hermit crab conditions.I would really like to know what that thing is.Where can you buy such a thing?Units and terminology do vary from place to place and fromtime to time and between different industries. Perhaps thereis some place, time, industry where the term "actual humidity"is used.Britannica.com states that "Relative humidity is so commonly usedthat a statement of humidity, without a qualifying adjective,can be assumed to be relative humidity".As far as I can tell, all of the humidity gauges in pets stores,digital ones you can buy at Radio Shack, those sold atSears, etc are all Relative Humidity % gauges. To buy anabsolute humidity gauge, you would normally have to goto a industrial, meterological or scientific instrument supplier.Some hermit crab recommened Relative Humidity % I havefound include:Vanessa's Land Hermit Care:70-80%exoticpets.about.com: ~70%petco care sheet: ~70%petplace.com: 55-65%hermit-crabs.com: ~70%BellaOnline.com: 50-70%crabmom: 70-80%laydbug15057: 65-75%

[Guest]

I considered if by "actual humidity", they really meant"specific humidity", but the numbers don't work out.A cubic meter of air at 75 degrees and 70% Relative Humidityholds about 15 grams of water. But a cubic meter of room temperature airhas a mass of about 1.3 Kilograms. Thus, typical valuesfor ideal hermit crab Specific Humidity would be around.011 or about 1.1%... No where near the 50-60 value quotedfor the "actual humidity gauge".

[Guest]

I had to slow down and read carefully, and I had to read some paragraphs twice, but it does make sense....I am not saying that you can't have other gauges (non-relative), but the explanation of why it would be measured in terms of relative humidity (as a percentage of the full capacity -- how much the air can hold) does make a lot of sense.With 100% humidity, the air is holding ALL (100%) of the vapors it can hold (at that particular temperature). When it only holds half of what it can hold, it is 50% humidity, and so on.... So, that is why they say that humidity is measured as relative humidity. The percentage is relative to the other factor (what the air can hold at a given temp).And how the use of the term "actual" was justified as being used when an adjustment is being made in an instrument's reading makes a lot of sense, too. I've heard the term "actual" used in the manner described.Thanks so much for sharing with us...

[Guest]

Interesting. I am wondering if there is a misnomer in terminology. There are two different kind of gauges in the market. Even if one is not actual, than it is something else, like specific. This definitely needs some more investigation before deciding one way or another. This doesn't explain why T-Rex company claims they made actual gauges until two years ago. I don't see what they would have to lose by coming up with a lie.I think for the time, I am gonna stick with my relative gauges. ZooMed, Flukers and Exo-Terra read relative. The other presumed actual gauges I own will not be used until I find out more. A concerned Gertie

[Guest]

I used to breed birds and incubate eggs. During artificial incubation humidity is critical so I did a lot of research at that time. Since my memory is a little rusty I've spent the last couple days reviewing information on humidity and I've also consulted with a scientist from a government research facility (forestry management--they do weather, plants, & insects mostly).Relative humidity (RH) is measured as a percentage--absolute and specific humidity are NOT. If it reads in % it's a relative gauge. RH gauges are fairly inexpensive and commonly available. Gauges measuring absolute or specific humidity are hard to find and expensive; they also read in either g/kg or g/cubic meter (or the non-metric equivalent) not %.Dial RH gauges are rarely calibrated properly by the time they reach your home. This is NOT the fault of the manufacturer. It is simply the way it is with this type of hygrometer whether it's for your tank or for your home. This probably accounts for the wide variation of humidity levels that have been recommended. You can do something about this though Calibrating your hygrometer:First check to see if your hygrometer can be adjusted. Look at the back to see if the small center post has a slot for a small screwdriver. If it does you'll need to have a screwdriver ready to make any adjustments as soon as you open the jar. If it doesn't, don't worry, this procedure will still allow you to use your hygrometer more effectively. Half-fill a shot-glass or other small dish with table salt and add enough water to wet the salt completely but not dissolve it--it should be like wet sand. Put the shot-glass and hygrometer in a wide-mouth jar, and seal it airtight. After six hours (or more), the humidity in the jar is 75%. The hygrometer should read somewhere around 75%. If it does not, take the screwdriver and turn the screw on the back of the hygrometer until the meter reads 75%.If your hygrometer is non-adjustable and reads other than 75% in the jar, you must correct all subsequent readings by this difference. Example: if the hygrometer reads 67% in the jar, it is reading 8% low. Thus, it will always read 8% less than the actual RH. Using this example if you wanted the humidity to be 60-70% then the reading on your hygrometer would have to be 52-62%.Dial RH gauges can be off by 20% or more! Please remember that it is simply the nature of the beast and NOT necessarily poor quality or manufacturing. The spring mechanism is delicate and goes through a lot (shipping, etc) before reaching your home.T-Rex Hygrometers:I contacted T-Rex's Research & Development department regarding their hygrometers since I've seen mentions in several places that they used to make gauges that weren't relative. I received this reply:"Thanks for your enquiry to T-Rex concerning our humidity gauges. I'm pleased to confirm that our humidity gauges have always been relative humidity gauges, rather than actual humidity gauges."I hope this helps clear up some of the confusion regarding humidity and hygrometers.Thanks,Rai