Davis Pro2 Single Spoon Tipping Bucket

An easy way to measure rain for an electronic station is a system called tipping bucket. When a certain amount rain fills the bucket the bucket will tip and generate a pulse which the station counts as 0.1mm or 0.2mm depending on the resolution of the device. The nature of the tipping bucket over the traditional way of measuring rainfall does have several issues. Rain can splash out as the bucket tips so rainfall is lost in the count. This normally happens with very heavy rain. The bucket tips too soon giving too much rainfall which can be due to poor calibration. Debris collects in the bucket so it tips too soon.

Davis recently introduced a new design for the Davis vantage pro2 rain gauge is a single tipping bucket design. You can see this shown in the left hand picture. This replaced the older dual tipping bucket which you can see on the right. The idea is that new design is more accurate and less prone to issues of the older design. Well we see later. The older design does allow for adjustment by turning to two screws which was something you had to do as the accuracy straight from the factory was a long way off. The new gauge has no calibration which as you'll see later on is an issue. The part number for the new design is Davis 7345.147 while the older unit is 7345.444

Installation of the new unit.

This is actually fairly simple to do you need a screw driver and a little bit of patience and gentle persuasion. Davis do have a good video on how to do this see here. The video does make it look rather simple and you'll likely find that the plastic is a little more stubborn to give than shown. Once installed you'll need to check the gauge is level using a spirit level. Once that is done you'll need to plug the cable into rainfall socket of the ISS. You are then ready to test the new unit to make sure that it registers when the bucket is tipped. You can do this either by tipping the bucket once and seeing if the rainfall is shown or by pouring a little water down the funnel.

Real life testing.

The real life testing revealed how poorly the new gauge performs. When the rainfall is very light it is more accurate than the other older gauge it's replaced. However when the rain is moderate or heavy the gauge under reads by a fairly large amount. Below you can see three different days results.

New Davis Tipping Bucket accuracy
Recorded rainfall	Actual rainfall	Percentage difference
10.8 mm	10.3 mm	4.85% too high
7.8 mm	7 mm	11.42% too high
32.4 mm	27.5 mm	10.55% too high

Final conclusion.

The tests are very disappointing for the new unit. I double checked that everything was level and that there wasn't any debris blocking the funnel or present in the bucket. Considering the price of the unit £78.00 from Weather spares it should be more accurate. The device needs an adjustment designed into it so this is a very serious design flaw of the unit. When I queried this with Davis and was told that the unit is calibrated precisely at the factory. This doesn't allow for drift during shipping or for the inherit design flaws of the tipping bucket. Calibration at least allows you to reduce these flaws somewhat. If anybody buys one of the new Vantage pro systems the units which will ship with newer design. I would advise to try and find the older twin tipping bucket which does have calibration. Ebay is probably a good bet.

Update 14/07/2024.

One way to calibrate the unit is simply adding a little Blue tac to the arm of the tipping bucket. The amount you'll need will differ from device to device but does work fairly well once you've got it right. This doesn't distract from Davis failing to provide any calibration unfortunately so still marked down for that.

Davis Weather Station Overall
Pros	Cons
More accurate for light rainfall than the unit it replaced	Cannot measure moderate to heavy rain accurately and the lack of calibration is a major flaw
Easy to fit	Expensive
	Lack of calibration for the end user is a major issue.