Hi
We're using the SR50A to measure snow depth. Some of our sites are quite exposed to wind and rain. One sensor failed just 2.5 months after installation and it was full of water when I opened it (picture here: http://yfrog.com/3zimg5089j )
I opened and replaced the desiccant before installation and it's just incredible how much water was in there. I think the problem is the extreme change in temperature (-10deg at night and probably about +30deg during sun shine) which results in condensation.
I wonder if anybody experienced similar problems and if there are better solutions than replacing the desiccant every week (this involves using a helicopter...).
Currently I have the following ideas:
Mounting the sensors in a big radiation shield. This would reduce the temperature during the day.
Drilling a hole into the transducer head, attaching a dry-air-bag (might be nitrogen filled) and sealing the venting holes.
Any better ideas or experience with one of the above?
Cheers from NZ
Thomas
The achilles heal of the SR50/SR50A is that the sensor has to be vented otherwise it will not work and can be damaged if then subjected to large pressure changes. This is not normally a problem in alpine conditions but in temperate, damp climates like Scotland and NZ the length of time the desiccant lasts can be an issue.
The SR50A has more desiccant in the vulnerable parts of the sensor cf the SR50, but the amount of desicacant is still limited by the size of the sensor body.
Adding a shield to limit the temperature changes of the body may help but you will only reduce not stop temperature cycling plus you will not stop the venting due to atmospheric pressue changes.
First though I would advise you check the 'o'-ring seals of the sensor to make sure these have seated correctly and sealed the body. The next step is to try and organise more frequent changes of the desiccant.
If you do not have limits on power there is one other official option you could consider. This is a special sensor front that has a small internal heater (search on SR50AH on the net). This is primarily designed to reduce issues with frost and ice forming on the front of the sensor but will also act to drive moisture out of the sensor and prevent condensation.
If power is limited and you are willing to invalidate the warranty then you could tap into the plastic sensor head and attach a large external desiccant bottle. What you must not do though is block the vent tubes as the sensor must be able to breathe.
* Last updated by: aps on 9/9/2009 @ 11:30 AM *
Hi aps
Thanks for your message. I was just wondering if there is a user group experiencing similar problems.
Re your message:
- more frequent desiccant changes: to replace these $0.50 parts we need two people (H&S), one working day and a few hours of helicopter time. All sensors are installed in alpine regions and visiting them more than once a year is not an option.
- shield: I'll give this a go. It might reduce problems with icicles as well.
- external desiccant bottle: this is another option but I think creating a sealed and dry amount of air is a better idea. For other sensors we have so called dry-air-bags, basically a flexible plastic bag with a tube attached. The whole thing comes in a plastic/metal box to make it Kea-proof. The sensor breathes air from/into this bag.
- power: is very limited (solar powered) and we need most of it for sat-comm.
Cheers
Thomas
Using some form of dry-air-bladder-bag is a trick I have never heard anyone use before with an SR50, I have only heard of its use by some users of differential pressures sensors. It will be interesting to hear if anyone else has tried it.
You will need to be very careful to ensure the bag and tube do not get blocked or restricted in some way (snow and ice etc) as it would not take much over pressure in the sensor to rupture the diaphragm.
* Last updated by: aps on 9/10/2009 @ 3:46 PM *
Hi Thomas,
Did you ever try any of these? I'm having similar problems with the SR50A in southeast Alaska - I made the first spring maintenance trip to what's actually our driest alpine site, and after five months the snow sensor was about as soaked as the one in your picture. I think I'm willing to void the warranty, and wondered if you had any luck with an external bottle. I'm imagining a kind of dessicant...donut that could wrap around the narrower top of the sensor and feed into to the plastic.
Thanks for the ideas
Jessica