Zero rainfall being registered (may be only intermittent)
If this is a new installation, ensure that the plastic tie that prevents the buckets from tipping in transit has been removed. Ensure that the rain-gauge funnel is correctly installed such that:
- The funnel is correctly located in its groove and is locked into place with the central hole correctly positioned over the bucket mechanism;
- The funnel is level (check with a short spirit level across the top of the funnel)
- The debris screen and (recent VP2 models) bird spikes are in place;
For existing installation the first check is always to ensure that the central hole has not become blocked. If it is blocked then obviously remove the funnel and clean it thoroughly.
And, as with all rain gauges, keeping the gauge and mechanism clean is important. It is perfectly possible for a spider web to be spun around the buckets and so prevent them from tipping and hence registering rainfall. So ensure that the bucket mechanism and the area around it is kept clean.
Checking operation manually
If none of the above helps to resolve the problem then remove the funnel and try tipping the buckets slowly by hand. As the buckets pass the midway position then those with good hearing should be able to hear a faint click as the reed switch closes. Leave a few seconds between each tip. Check visually also that the buckets are swinging in the expected vertical plane; it’s not unknown for the spindle to become slightly unseated at one end and if this happens then the magnet at the bottom of the bucket assembly may be passing over the reed switch at an angle rather than horizontally and therefore not registering correctly. If necessary, the two vertical arms that hold the spindle can be prised very carefully apart until just enough room is created to allow the spindle end to slip back into place.
The total rainfall that you have generated manually should then register on the console display. For example, five tips of the 0.2mm metric gauge should cause 1mm total rainfall to register. Be aware that there will be a slight delay between the manual tips and updating of the display – wait for 30-60 seconds before looking for the total to appear.
This test is obviously best done on an otherwise dry day when the daily rainfall total starts off at zero. This will make it easier to clear the artificial rainfall that the tipping test causes (see console manual for details of how to do this). Obviously, this test is only possible when you’re sure that there is good reception of other weather parameters like wind and temperature at the console. Also the test is only possible when using the standard display console – with Envoy consoles, checking for the test rainfall will clearly need to be done via software readings.
The principle of the VP2 comments above all apply but obviously the Vue rain gauge mechanism is mechanically rather different, being a tipping spoon in an accessory part that can easily be removed for checking and cleaning.
So, if it’s a new Vue system then check that you did actually install the tipping spoon part as per the instructions. And tipping the spoon by hand is achieved by poking a long thin stick (or eg thin kebab skewer) down through the central hole of the funnel above.
Excessive rainfall readings
‘Excessive’ in this context means over-reading by e.g. 50% or a still larger proportion, which will usually indicate some fault with the rain gauge. Over-reading by a smaller proportion will more often be caused by a calibration error – see the main ‘Rain gauge faults’ topic.
There are typically two possible causes of serious over-reading with VP2 stations – moisture in the ISS SIM socket or a faulty reed switch. The cause will rarely be in the SIM circuit board or console or software – none of these other possibilities can be ruled out absolutely, but troubleshooting should always centre on the rain gauge itself and its connection to the SIM board in the first instance.
ISS SIM socket: If moisture has somehow seeped into the socket where the short cable from the rain gauge plugs in then this can simulate the effect of frequent closures of the reed switch that normally registers a tip of the rain gauge buckets and hence cause a large rainfall total to be recorded. The check required is the obvious one – remove the SIM board cover (preferably when it’s not raining of course), unplug the rain gauge cable and check visually for moisture droplets. Then allow the moisture to dry out – usually quicker if the plug remains out temporarily.
The commonest cause of this problem is obviously if the front cover to the SIM compartment has not been replaced correctly and is allowing the rubber seal to leak somewhere – the cover should of course be on squarely and latched fully home – but there can be other causes such as a twisted front cover or a missing or degraded foam plug that normally closes the cable access port. Unsurprisingly, this sort of fault is most likely to be seen under conditions of very heavy rain, especially if combined with strong winds to force errant raindrops through a gap in the seal.
Faulty reed switch: Once the reed switch under the tipping bucket mechanism starts to wear seriously then the contacts can bounce and so each tip of the buckets can register as multiple tips so considerably increasing the rainfall registered. This isn’t an easy fault to diagnose and is relatively uncommon, but can sometimes be picked by slow manual tips of the buckets where a larger rainfall total than that expected from a specific number of tips may be seen. (Remember that the final total does take some seconds to register on the console – it is not instantaneous). The fix is to replace the reed switch – this is one repair that can be performed by the user if they have competent electronic soldering skills. But otherwise the rain gauge base, complete with tipping bucket mechanism, may need replacing.
To be added;