The Wika gauges are very common in industry and everywhere due to their low cost. If freeze protection is required a diaphram sealed unit will protect the gauge but the cost will make you think twice about how much trouble it is to remove the gauge...and you still have to replace the diaphram if it bursts. One thing I have noticed with most sand filters is the housing being made of plastic makes it possible to overtighten the gauge which could damage the filter housing, I have put a 1/4" NPT nylon plastic street elbow in the housing with a SS wetted parts bottom ported gauge on the filter with SS snubber inline to provide pulsation protection to the gauge. The plastic elbow will crack and break without damaging the filter housing and is cheap to replace and the bottom ported gauges are cheaper to buy, having plastic and metal components threaded together means different expansion values for the components which could crack the plastic. To aid in sealing and prevent overtightening I use 10 wraps of teflon tape on the gauges and snubbers on plastic NPT threaded interfaces.
This is the gauge I have mounted on my filter, it is the most common process gauge I stock in 0-30psi range and looks similar but filled with glycerine.
http://www.wika.ca/23X_30_en_us.WIKA?ActiveID=23772
Here is the SS snubber I use to protect the gauge from pressure pulsations.
http://www.coleparmer.com/catalog/product_view.asp?sku=6880082
Here is the nylon street elbow to make the conversion to bottom mount, Cole Parmer can ship anywhere in the world and is a great supplier of plastic fittings and gauges etc.
http://www.coleparmer.com/catalog/product_view.asp?sku=0634941
Cole Parmer also sells Ashcroft gauges which are excellent too, this model is all SS and has a built in snubber which reduces the number of parts to buy/ship.
http://www.coleparmer.com/catalog/product_view.asp?sku=6802202
It may be more expensive up front to purchase these quality gauges but snubbed and taken inside every winter it will likely be the last gauge you ever buy, not to mention remove a copper/brass component in the water stream that can corrode.
BTW: back ported or bottom ported gauges are the same and water will collect in the bourdon tube no matter what you do, the only way to prevent water collecting in the gauge is to invert a back mounted gauge so it self drains or buy an expensive process series gauge (back or bottom mount) which is basically the same gauge with the bourdon tube at the top making it self draining...but the gauge reads right side up. Now you know why some engineers invert back mounted gauges on process equipment, its cheaper than buying a process gauge which is the same internally anyway.
