Your Rain Gauge Under-Reads After a Storm: The Drip Test and Adjusting Screws

The storm passed at dawn, and by mid-morning the weather station's rain total read 38 millimetres. The neighbouring station, three fields away, read 52. The farm's irrigation schedule ran off the station's numbers, so a 14-millimetre gap was not an academic argument — it was the difference between watering and not watering, and between a crop that got what it needed and one that did not. The gauge was clean, the funnel was clear, and the logger showed a steady stream of tips through the night. Nothing looked broken. But the gauge had been quietly under-reading for months, and the storm was simply the first event big enough to make the error visible.

A tipping bucket rain gauge that under-reads after a storm is one of the most common field faults in weather monitoring, and it is also one of the most fixable. The gauge is a simple mechanical device, and its accuracy depends on three things that all drift with time: the level of the instrument, the cleanliness of the mechanism, and the calibration of the buckets themselves. This article explains how a tipping bucket works, how to prove whether yours is accurate with a drip test, and how to adjust it — plus the installation mistakes that make a good gauge read badly.

How a Tipping Bucket Measures Rain — and Where the Error Creeps In

A tipping bucket gauge catches rain in a funnel of known area and channels it into a small two-sided bucket mounted on a pivot. Each side of the bucket holds a fixed volume of water; when one side fills, it tips, empties, and brings the other side under the funnel. Every tip closes a switch, and the logger counts tips. Because the funnel area and the bucket volume are both fixed, each tip corresponds to a fixed depth of rain — the gauge's resolution — and the total rainfall is simply the number of tips multiplied by that depth. The design is elegant, cheap, and reliable, which is why it is the standard gauge on most weather stations.

Its weakness is that the measurement depends on the bucket tipping at exactly the right volume, every time, for years. The bucket is balanced on a pivot and held level by two small adjustment screws. If the gauge is not level, one side of the bucket fills slightly more than the other before tipping, so alternate tips deliver different amounts of water and the total drifts. If dust, insects, or mineral deposits build up inside a bucket, its effective volume changes. If the pivot wears or corrodes, the bucket tips sluggishly, and in heavy rain some water can spill past the bucket while it is mid-tip — the classic cause of under-reading in intense downpours, when the gauge is losing a little water on every tip. And in very heavy rain, the funnel itself can overflow if the drain rate cannot keep up, though that is rarer in a well-designed gauge.

Wind adds a second class of error that has nothing to do with the mechanism. Rain does not fall vertically in wind; it falls at an angle, and a gauge's funnel catches only the rain that passes through its horizontal opening. In strong wind, a significant share of the rain blows across the funnel mouth instead of into it, and the gauge under-reads. This is a siting error, not a calibration error, and no adjustment screw will fix it. It is also why a rain gauge next to a building or a tall hedge will read differently from one in open ground, even when both are working perfectly.

The Drip Test: Proving the Gauge with a Measured Volume

The drip test is the standard field calibration for a tipping bucket, and it needs nothing more than a measuring jug, a slow drip, and patience. The idea is simple: pour a known volume of water through the funnel at a controlled rate, count the tips, and compare the result with what the gauge should have recorded. If the funnel area is known, a measured volume corresponds to a known depth of rain — depth equals volume divided by funnel area — so the test gives you a direct check of the gauge's calibration.

Start by levelling the gauge. Put a spirit level across the funnel rim in two directions and adjust the mounting until it is level; a gauge that is out of level cannot be calibrated, because the two buckets will tip at different volumes. Then clean the funnel and the buckets, and check that the bucket pivots freely and the adjustment screws are not corroded. Now pour water slowly — a slow, steady drip or a thin stream, not a fast pour. The rate matters: pouring too fast floods the bucket and causes the spill-over loss you are trying to measure, so the test must mimic real rain rather than a tap running at full pressure. A practical approach is to drip a measured volume, such as half a litre, through the funnel over several minutes and count the tips as they happen.

Compare the tip count with the expected count. If the gauge recorded fewer tips than the volume should have produced, it is under-reading, and the adjustment screws are the fix. Each screw sets the rest position of one side of the bucket: turning a screw raises or lowers that side, changing the volume it holds before tipping. Adjust in small steps — a quarter turn at a time — and repeat the drip test after each adjustment until the tip count matches the expected value. Test both sides by watching the tips alternate; if one side consistently tips early or late, that side's screw needs the adjustment. When the count matches, run the test a second time to confirm, and record the result with the date. A gauge that needs a large adjustment, or whose screws are seized, may need a new bucket assembly rather than a tweak.

Choosing a Rain Gauge That Holds Its Calibration

When buying a tipping bucket gauge, the funnel material and the bucket material matter more than the headline resolution. A stainless steel funnel and bucket resist corrosion and keep their shape and volume over years outdoors; a plastic bucket can warp in heat and change its tipping volume, and a painted steel funnel can rust where the coating fails. A gauge with accessible adjustment screws and a removable funnel is worth having, because calibration is a maintenance task, not a one-time factory setting — if you cannot reach the screws without dismantling the gauge, the calibration will not get done.

Check the output options against your logger. A gauge with both a pulse output and an RS485 or 4–20 mA interface gives you a choice: the pulse count for a simple logger, or a digital reading for a networked station. And consider the environment honestly: in a site with snow, a heated gauge keeps measuring through winter, while an unheated gauge will sit silent under a snow cap and then report a flood of meltwater on the first warm day.

Installation Mistakes That Make a Good Gauge Read Badly

The most common mistake is mounting the gauge out of level. A gauge bolted to a post that leans, or to a bracket that was levelled by eye, will read low from the first day, and the error is invisible until someone compares it with a neighbour. Level the gauge with a spirit level on the funnel rim, in two directions, and recheck the level after the first storm and every year after — posts settle, brackets shift, and frost heave moves things.

The second mistake is siting the gauge where wind steals the rain. A gauge tucked against a building, under a tree, or behind a tall hedge is sheltered from the wind, but it is also sheltered from the rain that the wind carries. The standard guidance is to site a rain gauge in open ground, with no obstruction closer than about twice the obstruction's height, and with the funnel rim at a height that keeps splash from the ground out — typically around waist height on a dedicated post. A gauge on a rooftop is the worst of both worlds: the wind is stronger up there, and the building's own turbulence throws rain across the funnel.

Verification: Keeping the Gauge Honest

Verification for a rain gauge is a routine, not an event. Run the drip test at least once a year, and always after any work on the gauge — after cleaning, after re-levelling, after a storm strong enough to move the post. Log the result each time, so a slow drift in the calibration shows up as a trend rather than a surprise. Between tests, compare storm totals with a neighbouring gauge after every significant event. Small differences are normal — rain is patchy — but a consistent shortfall on one gauge, storm after storm, is a calibration problem, not weather.

Also watch the data for the signature of a mechanical fault. A gauge that reports rain in neat, regular bursts long after the rain has stopped is draining a blocked funnel. A gauge that misses the heaviest part of a storm but catches the light rain before and after is losing water to spill-over at high rates. A gauge whose daily totals are always a little low, in all weathers, is probably out of level or out of calibration.

Troubleshooting: When the Gauge Under-Reads

Symptom	Likely cause	What to check
Gauge reads low after every heavy storm, fine in light rain	Spill-over loss while the bucket tips, or a worn pivot	Run a slow drip test, then a faster one; check the pivot for wear and the bucket for damage
Gauge always reads a little low, in all weathers	Out of level, or buckets out of calibration	Re-level with a spirit level, then run the drip test and adjust the screws
Rain reported hours after it fell, in bunched tips	Blocked funnel outlet	Clean the funnel and debris screen; check the outlet is clear
One side of the bucket tips early, the other late	Adjustment screws uneven	Adjust the early side's screw in quarter turns, retesting after each step
Gauge silent under snow, then reports a flood on warming	Unheated gauge in a snowy site	Consider a heated gauge or funnel; do not treat meltwater as rainfall
Gauge reads low only in strong wind	Wind-driven rain missing the funnel	Resite to open ground, clear of obstructions, with the rim at the standard height

Checklist: A Gauge You Can Believe

• Level the gauge with a spirit level on the funnel rim, in two directions, and recheck after storms and annually.
• Site it in open ground, clear of buildings and trees, with the funnel rim at about waist height.
• Clean the funnel and debris screen monthly, more often in autumn.
• Run the drip test once a year and after any work on the gauge; adjust the screws in quarter turns and retest.
• Compare storm totals with a neighbouring gauge after every significant event and investigate a consistent shortfall.
• Log every test and adjustment with the date, so drift shows up as a trend.

A rain gauge that under-reads after a storm is rarely broken — it is usually unlevel, dirty, or out of calibration, and all three are fixable with a spirit level, a jug of water, and a quarter turn of a screw. The drip test is the proof: a measured volume through the funnel, a counted number of tips, and a result you can write down. For a gauge built to hold that calibration outdoors, the RS485/4-20mA stainless steel tipping bucket rain gauge sensor gives you a stainless funnel and bucket that keep their shape and volume season after season — and when the next storm passes, the number it reports will be one you can act on.