What is it?
The midmar wind sentry is a small weather station located at midmar dam which constantly logs weather data (wind speed and direction, temperature, relative humidity). It also has the facility to alert subscribers if it gets windy at the dam!
How does it work?
The anemometer takes a windspeed measurement every 10 seconds of every day! Every 10 minutes these readings are averaged and recorded along with the maximum gust speed recorded during that interval. These data are stored on a logger and periodically downloaded so we can analyse the wind patterns and history at the dam.
The sentry part is a set of rules built into the logger program which triggers an SMS alert if certain conditions are met. We have set up these conditions in such a way as to alert us as soon as it gets windy at the dam, hopefully with enough time to get there and have a good session!
So, within about 10min (15 min worst case scenario) of the wind getting up, the sentry will send an SMS to a central server. The server will then send that SMS on to all subscribers, telling them that the wind is up and what the recorded windspeed was. Once this SMS has been sent out, there will be no more messages until the wind drops, in which case a second SMS will be sent out telling all subscribers that the wind has now dropped.
The idea is that the moment you get the SMS you start packing your kit, calling in sick, excusing yourself from the current meeting / social occasion and start heading up to the dam. If you receive a second message telling you the wind has now dropped, you abort your mission (provided you received it in time, of course!). We have set the "dropped" threshold lower than the "up" threshold so that small dips in average windspeed will not trigger the SMS and you should only get a message when it really is dead.
Having said that, naturally the system cannot tell you how long the wind will stay "up" and, apart from the initial 10min average, it cannot tell you how big it is. For these you will have to rely on your own knowledge, forecasts etc.
How do I subscribe?
Subscription is open to anyone at all. You are charged per message you receive in a pre-paid fashion. You simply buy credits ahead of time and each time you receive a message your credits will be decreased by 1. When you run out of credits you stop receiving alerts until you top them up again.
To purchase credits, just transfer as much cash as you like
into the following account and use your cell-phone number
as the reference. E-mail me 
to let me know you've done it
and I'll top up your credits as soon as I get a chance. I'll
send you an SMS (which won't decrease your credits)
to let you know when I've done that.
Bank: Nedbank Acc holder: JW Thorpe Acc type: Cheque Acc number: 1341026922 Branch: Scottsville Branch Code: 134125
Note:
- How many credits you buy is up to you but obviously if you buy too few you will need to keep buying more (which is a schlepp for you and me) and if you buy too many you risk me skipping the country with your cash. Start with a reasonable number like 20 and then see how often you have to top up and you can figure it out from there.
- Each credit has a specific cost (see cost of credits) and you need to do the math to figure out how many credits you are buying and make sure you deposit an amount of cash that is divisible by the cost of a credit. Partial credits will be lost to you.
- Every wind event will generate two messages so 10 windy days will mean 20 credits are required to keep you informed. Remember if the wind gets up and drops in the middle of the night you won't get a message (see rules for details).
Where is it?
The weather station is located at midmar dam, close to the office and Henley Midmar Yacht club at the top of a large mast. It is raised high enough to be able to measure winds from any direction without interference from surrounding trees or structures.We are working on calibrating the system with measured windspeed at various sailing points throughout the dam so we can get an idea how the wind channels and strengthens / lessens at different areas. We will post this information here when it is complete.
Alert rules
The rules currently built into the logger for sending out the sms alert are based on an internal windspeed flag. When the windspeed reaches its threshold, the internal windspeed flag is set on and the logger sends an SMS. The alert rules check the wind speed every 10 minutes according to the following:- Only run the SMS routine during daylight hours which is from 6am to 6pm.
- If the average windspeed over the last 10 min is greater than or equal to 12 knots (and internal windspeed flag is off) then send SMS with average windspeed (and set internal windspeed flag on).
- If the average windspeed over the last 10 min is less than or equal to 8 knots (and internal windspeed flag is on) then send SMS with average windspeed (and set internal windspeed flag off).
Cost of Credits
A credit will currently cost you 80c.Remember that each windy event will generate two SMSes so it will use up two credits. I don't know how many events happen per month at midmar (during the day) but I would guess about 6 per month on average which means 12 credits per month. Therefore, if you want about 6 months of SMS services, just buy 6 x 2 x 6 = 72 credits = R57.60
Remember that I will not bother to keep track of half credits so if you deposit R58 you will still get only 72 credits. In fact, even R58.39 will get you 72 credits but R58.40 will get you 73.
Each SMS you receive will also tell you how many credits you have left so you can top up in good time.
Who do we have to thank for the service?
Warrick Boyd of syntax solutions, the software company which developed the SMS server software and is currently hosting the service for us andAlistair Clulow for the technical expertise and efforts setting up the weather station and logger at the dam and the CSIR for making the instrumentation available.