Topic: Dewpoint Setting

[wmcatty]

Most of you know the data transmission problem I had during the month of Sept., therefore, I will not go into any of that other than to say that the new console, router and modem are all in sync and the data is transmitting perfectly.  The new equipment has been in place for 11 days now and has not skipped a beat!  The problem I am now having is with the CWOP analysis.  I have been getting flagged every day indicating that my dewpoint reading is too low and they want me to increase it 2.7 to 3+ degrees.  Since the dewpoint is a component of temp. and humidity, obviously the dewpoint reading cannot be calibrated by itself on the console.  I checked with other stations in my area and found that I was pretty consistantly 2 degrees low (humidity) on average.  So, I increased the humidity calibration in the console by 2 degrees a week ago.  It is now pretty close to the other stations in my area (45 mile radius), but CWOP keeps posting the following message relative to the dewpoint analysis:

"There is a significant mean error in your dewpoint readings. You may need to calibrate your dewpoint sensor. Adjust your sensor to read 3.5 °F higher. Your readings are not within an acceptable error range"

I live at the south end (base) of a fairly shallow valley so my nighttime temps are always a couple of degrees cooler than alot of the areas here in Central Texas.  In fact, most of the other reporting stations are located on a flat plains type terrain, while I am not.  Additionally, the only two large airport stations are sited directly adjacent to large lakes, so I do not think their humidity readings are what I should compare to ( being Lake Waco and Lake Belton). I do not know what to do about this.  Or is this something that I need to pay any further attention to?  My temp. readings are on the mark (hot, damned hot, and too hot to do anything outside) and coincide with most other stations in my area as well during the daytime.  I also do not know what to do about the humidity readings other than to take my entire unit to an airport for tech. analysis...which is not possible for me.  I compared my humidity readings with 5-6 other stations early this morning and we were all detecting 20% humidity with a dewpoint of 42 degrees, but have not compared this afternoon's readings.  Your input is always appreciated.

[SlowModem]

Since you don't know who you're being compared to, it's kind of a subjective, relative thing.  I look at the QA page occasionally, but not too much.  It is useful if you have a dead or nearly dead sensor.  Other than that, it could be the other guy that's supplying bad information and you could be spot-on.  If you're in the ballpark, I'd say go with it.

[d_l]

You could get a sling psychrometer to calibrate both your inner and outer humidity readings.  That way you would know that yours are accurate or at least accurate at that humidity percentage as there is some slight non-linearity across the full humidity range on VP2 sensors.

[wmcatty]

Do you all think there are any other options other than purchasing a psychrometer at a cost of $90-150.00?

[DanS]

Not the best idea but you could bring your ISS inside and set it next to your console for a day or two comparing the inside to 'outside' humidity/temps. There's less chance that they're both off and by the same amount/direction.

[Old Tele man]

...how does $105.13 (before tax, s&h) sound?

...if you're interested, SENSIRION™ sells a USB-interfaced "Evaluation Kit" (EK-H5) for engineers to use to 'test & experiment' with one of their combined TEMP/RH sensors (SHT75). The SHT75 combined TEMP/RH sensor is very accurate: 1.8%RH!




...you can buy the pair from NEWARK Electronics ( http://www.newark.com ) for $68.37 for EK-H5 and $36.76 for SH75

...here are links to the SENSIRION™ data sheets:

http://www.sensirion.com.cn/down/downimg/Datasheet_EK-H5_V1.0_C1.pdf
http://www.sensirion.com/en/pdf/product_information/Datasheet-humidity-sensor-SHT7x.pdf

[dalecoy]

Do you all think there are any other options other than purchasing a psychrometer at a cost of $90-150.00?

Sure:

1.  If there is a high school or college near you, they would probably have such a thing, and would be willing to either loan it to you, or have some students come over and take the measurement.

2.  Cheap temperature/humidity meters are available at Radio Shack, Wal Mart, etc.  They aren't very accurate - but would give you a comparison, anyhow.

And I advise not worrying overmuch about CWOP's QA.  Your location is rather unique and somewhat isolated, so how would they know?

[Bushman]

Make your own psycrometer:  http://www.sciencebuddies.org/science-fair-projects/project_ideas/Weather_p011.shtml

[xykotik]

Make your own psycrometer:  http://www.sciencebuddies.org/science-fair-projects/project_ideas/Weather_p011.shtml

  I just lernt sumthin!

At first I thought it was a "baffle 'em with BS" moment, like when the Star Trek writers would put the tag " [tech] " in the script so the consultants could insert some impressive sounding jargon into the script.  Or the Dr. Fox Lecture.

[George Richardson]

wmcatty,

You might try: http://www.ebay.com/itm/270828321174?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

For what its worth

George

[SLOweather]

For a one time deal, you could roll your own with a single liquid thermometer that you trust. Take a reading with the bare thermometer, slip a sock cut from a shoelace or something over the bulb, wet it, fan it, read the lowest temperature, and then enter the values (along with BP) here: http://www.srh.noaa.gov/epz/?n=wxcalc_wetbulb

[kcidwx]

I never compare my readings with anyone else. What I do is calibrate my instruments across the range then I know it's accurate regardless of what the station 2 miles away says.

I'm not very familiar with the VP sensor so you may not even be able to do this but if you can get it out of the FARS and into a gallon size zip lock bag then you can calibrate it. This is how I calibrate all my humidity sensors. I typically use the ones in bold in the list below. You can get a bottle of Magnesium Chloride for around $10/bottle. The Sodium Chloride is just pure table salt with no additives. Distilled water is preferred. I use the cap off a one gallon milk jug. Fill the milk jug cap with the salt. Add just enough water to make a sludge but not dilute it. Place the mixture in the zip lock bag with the humidity sensor. Seal it and allow it to settle overnight.

SALT BATH PUBLISHED RELATIVE HUMIDITY AT 77°F

LITHIUM BROMIDE 6.37%
LITHIUM CHLORIDE 11.30%
POTASSIUM ACETATE 22.51%
MAGNESIUM CHLORIDE 32.78%
POTASSIUM CARBONATE 43.16%
MAGNESIUM NITRATE 52.89%
SODIUM BROMIDE 57.57%
POTASSIUM IODIDE 68.86%
SODIUM CHLORIDE 75.29%
POTASSIUM CHLORIDE 84.34%
POTASSIUM SULFATE 97.30%

It is convenient for us that a solution of ordinary salt mixed with water (preferably distilled water) produces a predictable humidity over a wide range of temperatures. The humidity created, with ordinary salt (Sodium Chloride) and water, is 75.29% at an ideal temperature of 77 degrees Fahrenheit. The temperature of the room is not critical for our purposes. For example, the RH is quite stable even with large variations: Salt solution at 59 degrees Fahrenheit will produce 75.61% RH and at 86 degrees Fahrenheit the RH is 75.09%.

To calibrate the lower end, 33% humidity, Magnesium Chloride (a salt) and water is used again. At the ideal temperature of 77 degrees Fahrenheit, this solution will produce an RH of 32.78%. At 59 degrees Fahrenheit it will produce an RH of 33.30% and at 86 degrees Fahrenheit it will produce 32.44% RH. Once again, "room temperature" is not critical.

[Beaudog]

The CWOP comparison uses the last 24 days unless you tell it otherwise. Click the last 3 days or last 7 days option and see if you are not closer to correct.

[wmcatty]

Thanks everyone.  I am going to stop at a Radio Shack and get a unit there.  Time constraints require I do that before delving into the "self-help" area before initiating home built units.  I appreciate all the input, thought and posts.

[LFWX]

The CWOP comparison uses the last 24 days unless you tell it otherwise. Click the last 3 days or last 7 days option and see if you are not closer to correct.

As beaudog suggested, make sure you are looking at the most recent data!

You look fine to me (you, plus "analysis", plus a station 5.5 miles away):
http://weather.gladstonefamily.net/qchart/D8373?date=20111006&addnl=D4371&Add+to+charts=Add+to+charts&.cgifields=addnl

100% on Dew Point using a 3 day span:
http://weather.gladstonefamily.net/site/D8373?tile=10;days=3#Data

[wmcatty]

Thanks WFWX...you just saved me from spending $69.00 on Friday at Radio Shack.  I think I will sit as status quo for a while and see what the results are.  Many thanks for all the responses...it is no wonder you guys are referred to as a weather guru.

[C5250]

Do you all think there are any other options other than purchasing a psychrometer at a cost of $90-150.00?

A sling psychrometer doesn't cost anywhere near that much, it's just two glass bulb thermometers (one with a wet bulb) that has a handle with a pivot point.

http://www.google.com/search?hl=en&biw=1050&bih=552&q=sling+psychrometer&oq=sling+p&aq=2&aqi=g10&aql=&gs_sm=c&gs_upl=12107979l12111735l0l12115078l7l7l0l0l0l0l233l701l5.1.1l7l0

I got mine from Forestry Suppliers (who is pricey), this one and it didn't cost anywhere near that much.