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Author Topic: Relative Humidity Differences?  (Read 2788 times)
ace1979
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« on: December 04, 2006, 02:46:19 AM »

I just purchased a Holmes digital hygrometer to keep in our bedroom where my new Larrivee D03WL is.  I already have a Homes room humidifier in the room that has a built in hygrometer....now the interesting thing is that there is about a 10% difference in readings of the devices currently...they are only about 10-12 inches apart so I am not sure what the deal is...any ideas and any thoughts on which one to base things on?  Thanks
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poki
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« Reply #1 on: December 04, 2006, 08:37:57 AM »

i don't think 10% difference is anything unusual.  the problem is unless you have an accurate point of reference your not going to know for sure which is correct, if any.  like two clocks showing different times you won't know which is correct without calling time of day or looking at some other point of reference.  i have 3 hygrometers and there's about 10% variance between them, often times less, so i just assume the correct humidity has got to be somewhere within the 3 which is close enough for my needs.  from what i've heard the planet waves hygrometer is suppose to be among the most accurate so you should be safe using it as a point of reference.
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« Reply #2 on: December 04, 2006, 12:51:24 PM »

That's precisely why I keep three hygrometers in my guitar room and average their readings.

OG
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« Reply #3 on: December 04, 2006, 02:26:03 PM »

I have been using a digital hygrometer for many years, and always relied on them as a point of reference.  After seeing the new Planetwaves model at the local guitar shop, I asked them about it and was told that they had never seen an accurate digital unit yet.  They were testing this one against their inhouse professional unit which is calibrated twice each year, and the digital one wasn't even close.

I  took my own hygrometer in to the store in August and compared readings. Where my unit showed humidity at 50% (right in the sweet zone) their's was reading 70% - representing a 40% VARIANCE!  Holy smokes!  Let's just say that I don't put much faith in any of these "over the counter" units any more, other than to show me how the humidity levels change from day to day.  To get a true reading may just be wishful thinking. Even guys who are using 3 hygrometers are telling us that no two units are giving the same reading.

The good news is that when my hygrometer reads 38% as it did this morning, my calculations tell me that I'm probably near 50% RH.  If anyone has a line on a hyrometyer that can be relied on for accuracy, just let me know.

jimmy
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aboss
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« Reply #4 on: December 04, 2006, 03:29:16 PM »

It's a good idea not to put blind faith in your hygrometer (or your 3 hygrometers) because i think it's been made pretty clear that none of them are exact. I always just trust my common sense (and my eyeballs). There's plenty of info to be found on the net (and on this forum) that explain what to look for in the way of too high/low humitidy, so I just give my guitar the once over every day when i'm playing it to make sure nothing seems amiss. So far so good. 

Obviously the "check it over every day" method doesn't work for those who have a whole pile of guitars. But for those of us with only 1 "good" guitar, it works wonders  bigrin
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« Reply #5 on: December 05, 2006, 04:29:08 AM »

interesting and enlightening discussion.  i have my Oregon Scientific hygrometer inside an industrial incubator at work and it typically reads 41-42% RH while the wet bulb humidity gauge in the incubator typically reads 81-82% Wet Bulb which calculates to 46-48% RH.  i'll test my planet wave hygrometer tomorrow to see how it compares.
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« Reply #6 on: December 05, 2006, 07:02:06 PM »

I have three Oregon Scientific hygrometers.  If I set them all together they are always within 2 or 3 % of each other.  On the other hand my Hunter humidifier has a built in digital display, this thing is always off by over 10% compared to the others, so I never look at it.

I think the Oregon Scientific wireless models are great.  You can put one in a case and it transmits the signal back to the main unit that sits in my office.  This is very handy to keep track of humidity in the room and in the case. 
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« Reply #7 on: December 05, 2006, 08:54:54 PM »

Poki / Steve

I did a Google search and found that Oregon makes many models in all price ranges.  Can anyone suggest a model that is considered reliable?

jimmy
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« Reply #8 on: December 06, 2006, 12:27:47 AM »

Poki / Steve

I did a Google search and found that Oregon makes many models in all price ranges.  Can anyone suggest a model that is considered reliable?

jimmy

The Oregon Scientific model I have is the one in the first link below.   What you are looking for is what they call a 'weather forecastor'.  These have both inside and outside temperature and humidity sensors, the inside sensor is in the base unit and the outside sensors are small remotes that transmit back to the base.  Many of the cheaper models don't have the remote humidity sensors only temp.  The units below have the remote humidity and temp sensors.  I think they only come with one remote and you can buy more, up to 3 remotes with each base.

http://www2.oregonscientific.com/shop/product.asp?cid=2&scid=4&pid=224
http://www2.oregonscientific.com/shop/product.asp?cid=2&scid=4&pid=668
http://www2.oregonscientific.com/shop/product.asp?cid=2&scid=4&pid=79


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ace1979
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« Reply #9 on: December 06, 2006, 11:05:57 AM »

thanks fro the replies...ok well now the two are "settling" and they are within 5% of eachother..but the weird thing is that consistently now the reading is between 60-70% which seems high...and the humidifier is not on....how long should I let this go (guitar in case currently) prior to intervening somehow..and I really don't want to get a dehumidifier and then be going back and forth constantly....also it is in a one bedroom apartment so there will be variances day to day....so would it be more wise to keep inthe case or keep out in the room (from a humidity standpoint) not any other standpoint such as safety, etc....thanks!

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« Reply #10 on: December 06, 2006, 10:13:45 PM »

Like the look and functionality of these Oregon models. Good tip!
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« Reply #11 on: December 07, 2006, 04:37:52 AM »

i finished testing my Humicase, Planet Waves and Oregon Scientific hygrometers against a wet bulb gauge inside an incubator.  all the units were allowed to sit next to eachother close to the wet bulb gauge inside the incubator for several hours before checking them.   made two checks a few hours appart.  results as shown in RH.

the wet bulb gauge indicated 54% 1st check/ 53% 2nd check

Planet Waves  indicated         49% 1st check / 47% 2nd check

Humicase indicated               46% 1st check / 44% 2nd check

Oregon Scientific indicated    44% 1st check / 42% 2nd check

all the units readings including the wet bulb gauge varied a little during the day but the Planet Waves always had the closest reading to the wet bulb gauge but always somewhat lower.  this wasn't a scientific test by any means, just an informal comparison.





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« Reply #12 on: December 07, 2006, 02:28:33 PM »

Very interesting.  Based on my observations at the local shop where they were comparing the Planetwaves against their wet bulb unit, I did not expect the Planetwaves model to outperform the others.

My personal hygrometer reads 35% at 7 AM, which I believe would be closer to the mid 40s adjusting for it's (lack of) accuracy.

A conundrum!

jimmy
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hatofthecat
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« Reply #13 on: December 07, 2006, 04:27:54 PM »

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i finished testing my Humicase, Planet Waves and Oregon Scientific hygrometers against a wet bulb

Interesting results poki which at least show each unit is consistent with itself i.e. no random swings in the readings. 

Whilst its the old standby, wet bulb (psychrometer) is not really a great tool for measuring instantaneous relative humidity due to its lag time time in producing a reading and systematic errors due to airflow and radiant heat, best tool for that is "cold mirror" which uses a mirror surface chilled until dew point is reached  (used for industrial process monitoring etc.) I don't know what the little electronic hygometers use but I assume its a capacitance sensor     which relies on variation of conductivity of air with moisture content.

Remember you are only looking for relative humidity to be roughly in the 40-60% range..... a few percentage points either way are neither here nor there... 
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poki
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« Reply #14 on: December 07, 2006, 07:08:01 PM »

it's true the wet bulb is not suitable for reading quick changes in humidity due to it's inherent method of operation.  on that note both the OS and HC were quick to respond to rapid humidity changes when i opened the door to the incubator.  their readings would plunge downward about 10 seconds after opening the door while the planet waves was much slower to respond like it has more of a delay built in.  the slowest to respond was the wet bulb.   on a good note all the units temp gauges were within a degree of variance with the incubators except the planet waves which may have been very slightly more than a degree lower but maybe not.  it could be the planet waves is just more sensitive to slight temp variations.
 
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« Reply #15 on: December 08, 2006, 01:20:51 AM »

Whilst its the old standby, wet bulb (psychrometer) is not really a great tool for measuring instantaneous relative humidity due to its lag time time in producing a reading and systematic errors due to airflow and radiant heat, best tool for that is "cold mirror" which uses a mirror surface chilled until dew point is reached  (used for industrial process monitoring etc.) I don't know what the little electronic hygometers use but I assume its a capacitance sensor     which relies on variation of conductivity of air with moisture content.


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hatofthecat
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« Reply #16 on: December 08, 2006, 04:35:53 PM »

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while the planet waves was much slower to respond like it has more of a delay built in

Same here, I have the Planet Waves case one myself.  I assume it only actually takes a reading every few minutes in order to maximise battery life.  A bit of a delay is a good thing for a case hygrometer as that way you get to see what the reading was before you popped open the lid.

Pete
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