Background
The standard VP2 ISS obviously combines the rain gauge, T/H sensor in its radiation shield and transmitter board into a single assembly for easy mounting. This arrangement works well for most users but we do occasionally get asked if it’s possible to separate the ISS into its individual components.The answer is yes, but it does involve a couple of spare parts and a little work.
There are usually two reasons cited for separating the ISS:
- Users want to mount the rain gauge elsewhere. Sometime it’s simply that there’s a preference for measuring rainfall in a slightly different location from temp/hum readings. More often it’s a wish to mount the rain gauge closer to or right at ground level, where it will read more accurately because wind effects on rain collection will be minimised. (NB In more temperate climates such as lowland UK where deep snow is rare, ground level is the official height for the base of the gauge. In snowier climates such as the northern US the recommendation is to to have the gauge at 4-5ft (ie as per the VP2 design) presumably to avoid the potentially deep snow at ground level, even if this results in lower rain catch.)
- Users are concerned about the black mass of the rain cone and mounting bracket affecting temperature readings. In practice, this is not a significant issue – there is an air gap between rain gauge and radiation shield, plus a heat shield at the top of the screen to minimise heat conduction and radiation. And, if anything, one might expect the black cone to slightly increase any convective UPdraught past the screen. Moreover, careful measurements have failed to reveal any significant temperature over-reads vs a standard screen in full sun (see Stephen Burt’s review ). However, some users will still prefer the extra confidence in readings from separating the ISS.
Practical options
Alternatives
Before detailing how to separate the ISS, remember that there are at least three simpler alternatives to operating on the ISS that will address some of the perceived ISS concerns:
- The new Davis aerocone for the rain gauge is designed to minimise wind-induced under-catch. The aerocone is available on its own as a spare part and is suitable for VP2 rain gauges of any age;
- Any Davis standalone rain gauge (eg 7852/7857/6463/6465) can be used as an alternative to the built-in gauge. The 12m cable supplied with these gauges allows the unit to be located some distance from the ISS and its cable simply plugs in in place of the built-in rain gauge. This is often the simplest option for separating the rain gauge, albeit with a somewhat higher price tag.
• A 7747 daytime-only fan (DFARS) is available as an add-on kit for the standard VP2 radiation shield. The fan assistance helps move air through the shield and hence improves T/H accuracy;
Separation
If you still wish to separate the ISS then here’s what you need:
- A new rain gauge base/mounting bracket;
- A U-bolt for the new base (if you wish to attach it to a pole);
- An extension cable; often a short 2.4m cable will suffice, but longer (12m etc) extensions are also available;
These items are all available as spares. Instructions for performing the separation (which is really best done indoors on a table/bench top) as follows:
- The main operation is to move the radiation shield to the new mounting bracket;
- Unplug at least the T/H sensor from its socket on the SIM board and pass the cable through the cable port at the back of the baseplate;
- Remove the rain gauge cone and then (carefully!) the three long bolts that hold the shield elements in place. If done carefully on a table top then all the loose shield elements should stay together.
- Reassemble the shield on the new mounting bracket;
- (Not essential but recommended): Move the transmitter and backplate to the new bracket. This is for two reasons: first the shield is likely to be higher from the ground than the rain gauge and this will help wireless range; second, it’s recommended that the T/H sensor should have only a short cable length to the SIM board – cable length, within reason, on the rain collector is much less important;
- Fit the extension cable, being sure to fully weather-proof any cable joint;
NB If you have solar and/or UV sensors then these will also need unplugging to start with and (probably – depending on circumstances) moving to the new baseplate. In practice, it is possible with commonsense to combine the steps above in different ways, but the steps detailed above provide broad guidance.
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