Fine Offset sensors - Printable Version

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RE: Fine Offset sensors - JT118 - 10-04-2016

That ebay rain gauge looks looks relatively cheap then.
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RE: Fine Offset sensors - Wizza - 10-04-2016

Ok, turns out, it was the guy selling the rain gauge on Amazon that had really high shipping costs, and this inflated the total cost. So deleted the rain sensor from the order. The Davis wind sensors were only about $19 for shipping, so I hit the button and got them on the way. Every item is getting me that little bit closer to getting online. Thanks guys...

Mark R

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RE: Fine Offset sensors - Wizza - 11-04-2016

JT, I bought that ebay rain gauge off eBay, that you pointed out yesterday. Many thanks...

Mark R

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RE: Fine Offset sensors - JT118 - 11-04-2016

You will have a really good set of instrumentation, just add a decent radiation shield such as http://www.metspec.net/unaspirated-radiation-shields and add an SHT25, and / or a SHT15 , and that makes up a complete research grade station.
See here for calibration,
http://www.meteocercal.info/forum/Thread-Rain-Bucket-Software-Settings?pid=2635#pid2635

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RE: Fine Offset sensors - Wizza - 11-04-2016

JT, The radiation shield is the last piece of the puzzle that I have been searching for. Didn't really want to buy good sensors, and then put up a cheap plastic radiation shield that will only go brittle from the Sun's UV and not last long. Once again, haven't found much in Aust yet. I'll keep searching. Lets know if you have any leads on a good radiation shield. Cheers for now...

Mark R

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RE: Fine Offset sensors - JT118 - 11-04-2016

http://www.davisinstruments.com.au/#!extras/c1ldy

7714 and 7747

Seem good value, Radiation shields both aspirated 7747 and non aspirated 7717. The TX module can be hooked up to the aspirated version I think.

Alternatively buy a cheap FO one and wait for something on ebay.com.au

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RE: Fine Offset sensors - Wizza - 13-04-2016

If you had two options for the solar radiation shield and the temp /hum sensor position, would you put it up on the mast below the wind sensors, in the open weather and sun, or on the southern side (opposite side to the equator - Southern Hemisphere / Aust) of the house, on the wall, in the shade most of the time (approx 11 or 12m cable)..?

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RE: Fine Offset sensors - JT118 - 13-04-2016

(13-04-2016, 12:47)Wizza Wrote:  If you had two options for the solar radiation shield and the temp /hum sensor position, would you put it up on the mast below the wind sensors, in the open weather and sun, or on the southern side (opposite side to the equator - Southern Hemisphere / Aust) of the house, on the wall, in the shade most of the time (approx 11 or 12m cable)..?

In theory in the most open part of the garden or area you are going to put the rest of the kit. Presumably this is also where the anemometer mast will go. One is trying to measure ambient conditions. Therefore if you put it on the wall you are effectively measuring a micro-climate created by the wall. The radiation shield should do the work of reflecting the suns direct energy which you also do not want to measure and the design of the shield will allow a free flow of ambient air (which you are trying to measure ) and stop pockets of hot or cold air stalling next to your sensor.

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RE: Fine Offset sensors - werk_ag - 13-04-2016

(13-04-2016, 12:47)Wizza Wrote:  If you had two options for the solar radiation shield and the temp /hum sensor position, would you put it up on the mast below the wind sensors, in the open weather and sun, or on the southern side (opposite side to the equator - Southern Hemisphere / Aust) of the house, on the wall, in the shade most of the time (approx 11 or 12m cable)..?

Whenever is possible the Rain Gauge and the Temperature / Humidity sensors should be placed at 1 to 2 meters above the ground level, and the Wind sensors around to 10 meters.
Avoid having the Temperature / Humidity sensor close to walls, or just above roofs. They should always have a good exposition to air flow from all directions.

The images below are from official weather stations, here in Portugal
I'm putting them just to illustrate the idea







As you can see the radiation shield and the Rain Gauge are always placed around 1 meter above the ground level.


If you are planning to use I2C sensors (SHT2x) with 10 to 11 meters cables it's better start to planning the use of I2C bus extender circuit. The TX boards already have a chip for this, but another chip should be used on the other end of the cable. Also the pull up resistors of the I2C extended bus should be carefully calculated, according to length and cable type (capacitance).
Preferably try to place the TX unit in a way that the longest cables are those for the anemometer / wind vane.

In some cases is more easy use one TX unit for the high level instruments (Wind speed and direction) and another TX unit for ground level instruments (Temp / Hum , Rain Gauge).

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RE: Fine Offset sensors - JT118 - 15-04-2016

The Gauge that Mark has bought is a Hydrological Services TB2 Syphoning Tipping Bucket Rain Gauge.
Sighting of rain Gauges is always a problem. Though many government stations that are put up for local monitoring have the gauge off the ground, true scientific gauges are at either 305mm or ground level with an anti splash grid around them. You will see that the gauges at 1000mm above ground have a different (and lower ) quality code for their data than those at 305mm or Ground level. See these attachments. (by the way the anemometer and vane at the CEH are the same as mine see avatar pic) Mark; your Gauge is mentioned in attached doc. Your model (TB2) has been superseded by the TB3. None the less the TB2 was one of the best gauges and many are still employed in sites around the world. Syphoning systems are a way of evening out rainfall events and reducing error in the tipping bucket, adding a syphon makes the
gauge intensity “flatter” at low rainfall rates. I think the TB2 has a diameter of 200mm, if not, you will have to alter the arithmetic below accordingly.



This picture from Wallingford Site ( http://www.ceh.ac.uk/our-science/monitoring-site/wallingford-meteorological-station )

   

cut and paste from my word document. I cannot find my post it is there somewhere:

Correct way to calibrate and check Rain Gauge:

The tipping-bucket mechanism is a simple and highly reliable device.

The rain gauge must be calibrated with a controlled rate of flow of water through the tipping-bucket mechanism. The maximum rainfall rate that most tipping bucket rain gauges can accurately measure is 25mm of rain per hour (approx. 36 seconds between bucket tips). Therefore, the rain gauge is calibrated using a water flow rate equivalent to, or less than, 25mm of rain per hour (more than 36 seconds between bucket tips). If the flow rate is increased beyond 25mm / hr, a properly calibrated instrument will read low. Decreasing the rate of flow will not materially affect the calibration. The reason for this is obvious. Watch the tipping bucket assembly in operation. With water falling into one side of the tipping bucket, there comes a point when the mass of the water starts to tip the bucket. Some time is required for the bucket to tip (a few milliseconds). During the first 50% of this tipping time water continues to flow into the filled bucket; the last 50% of this tipping time water flows into the empty bucket. The amount of water flowing during the first 50% of time is error, the faster the flow rate the greater the error. At flow rates of 25 mm/hr or less, the water actually drips into the buckets rather than flowing. Under this condition, the bucket tips between drips, and no error water is added to a full moving bucket.

To Calibrate:
NB: Use a vernier caliper accurate to 0.02mm.

1) First work out the cross sectional area of your rain gauge. Measure the internal diameter of your rain gauge receiver if it is round use: A=πr^2. Assume a for arguments sake our rain gauge has a diameter of 200mm.

i) 200 / 2 = 100

ii) 100 x 100 x π = 31428.6 mm2

iii) 314.29 x 2 = 629 ml of water will represent 20mm of rain falling on your gauge.

2) Obtain a plastic or metal container of at least one litre capacity. Make a very small hole (a pinhole) in the bottom of the container.

3) Place the container in the top funnel of the Rain Gauge. The pinhole should be positioned so that the water does not drip directly down the funnel orifice but rather run down the funnel cone.

4) Pour exactly 629ml ( or the amount you have calculated for your rain gauge) of water into the container. Assume each tip of the bucket represents 0.2 mm of rainfall. Then your gauge should have tipped 100 times. If it has not tipped 100 times after the 629mm of water has passed through then you need to adjust until you get to 629 mm creating 100 tips. Of course one of the advantages of weatherduino is that you can put any figure into your rainfall calibration on the rx config file. However assuming your gauge is 200mm diameter and correctly set you should get this result and be able to put 0.200 into the RX config.

5) If it takes less than one hour for this water to run out, then the hole (from step 1) is too large. Repeat the test with a smaller hole.

(Adjusting screws on Davis and the better quality gauges are located on the base of the Rain Gauge housing underneath the buckets. Turning the screws clockwise increases the number of tips per measured amount of water. Turning the screws counterclockwise decreases the number of tips per measured amount of water. A 1/4 turn on both screws either clockwise or counter-clockwise increases or decreases the number of tips by approximately one tip. Adjust both screws equally; if you turn one a half turn, then turn the other a half turn.)

6) Repeat Steps 3–6 as necessary until the Rain Gauge has been successfully calibrated.

Some interesting numbers; 1mm of rain falling over 1 hectare is 10,000 litres of water or 10 tonnes of water.

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