Picture a small weather station that has two jobs: alert you the moment rain starts, and show you in the evening how much water fell during the day. That is exactly where the rain gauge vs rain sensor question splits in two. A rain sensor answers “Is it raining right now?” with yes or no. A rain gauge answers “How much did it rain?” with a number in millimeters. The two devices can look surprisingly alike, but the data they deliver is completely different.
Mix them up and you build an alert that waits for values the device never sends. A rain sensor has no idea what the daily total is, and a rain gauge cannot reliably tell you when a surface has dried off again. Below you will learn how both methods work, how to convert and interpret rainfall readings, and what matters when you place, evaluate and maintain the hardware.
How does a rain gauge work?
The principle is simple: an opening with a known area catches rain, and the volume of water collected gives you the rainfall depth. The classic Hellmann gauge, used for decades by Germany’s national weather service (DWD), has a collecting area of 200 cm². With a simple garden gauge, you read the depth straight off a scale on the measuring cylinder and then empty it by hand.
An electronic rain gauge does the reading for you. The most common design is the tipping bucket: a funnel guides the water into a small seesaw with two buckets. When one bucket fills, it tips, empties, and the other bucket moves under the funnel. Each tip briefly closes a magnetic reed switch. Every one of these pulses stands for a fixed amount of water, typically 0.1 or 0.2 mm of rainfall, depending on the funnel size and bucket volume.
The third design is the optical rain gauge. It has no funnel or seesaw, just a transparent dome with infrared light reflecting off its inner surface. When drops land on the dome, they change that reflection, and the electronics estimate the rainfall amount from the signal. Many optical gauges still provide a pulse output, so they connect to a counter input exactly like a tipping bucket.
How does a rain sensor detect rain?
You have probably used the best-known rain sensor already: it sits behind the windshield of many cars and switches on the wipers. It works optically, too. Infrared light reflects off the outside of the glass, and water drops deflect part of it. The electronics detect wet glass, not a rainfall amount in millimeters.
In weather stations and on buildings, you more often find rain sensors with an electrode grid on a circuit board. When water lands on it, it bridges the traces and the electrical resistance drops. Many of these sensors include a heater that actively dries the surface after a shower. Without a heater, the surface often stays wet long after the rain has stopped, and the sensor keeps reporting “rain” even though none is falling.
The output is correspondingly basic: a switch contact, a status bit, or a yes/no value over an interface such as RS485. You cannot derive rainfall depth from how long a rain sensor reports “wet.” Ten minutes of drizzle and ten minutes of downpour look the same to it. If you need an amount, a rain sensor is the wrong tool, even if it seems faster and cheaper.
What does 1 mm of rainfall mean?
Rainfall is expressed as a depth: how high would the water stand on a flat surface if none of it ran off, soaked in or evaporated? That leads to a very practical conversion. One square meter times one millimeter equals one liter. So 1 mm of rainfall means 1 liter per square meter.
That makes quick calculations easy. If 5 mm falls in a day, an area of 20 m² receives 100 liters of water on paper. For scale: Germany receives around 800 mm of precipitation per year on long-term average, and the DWD issues heavy-rain warnings when roughly 15 to 25 liters per square meter fall within one hour.
The conversion also shows how small the volumes inside the instrument really are. With a collecting area of 200 cm², 1 mm of rainfall equals exactly 20 milliliters. A tipping bucket set to 0.2 mm per pulse therefore tips every time 4 milliliters have collected. You will need that number when you check a gauge with a measured amount of water.
Optical vs tipping bucket rain gauge: which is more accurate?
There is no blanket answer, because both methods have typical sources of error. A tipping bucket measures water it actually caught, so you can check it with a measuring cup. In very heavy rain, however, it loses part of the volume, because water keeps flowing in while the bucket is tipping. Leaves, pollen and insects in the funnel can slow the flow, and a seesaw mounted off-level tips too early or too late.
The optical rain gauge has no moving parts and no funnel to clog. On the other hand, its sensing area is small, and it does not measure water directly but estimates the amount from the drop signal. How well that works depends on drop size. Fine drizzle and very large drops can cause deviations, and dirt or scratches on the dome change the optical signal.
The table below puts the differences side by side:
| Feature | Tipping bucket | Optical rain gauge |
|---|---|---|
| Measuring principle | Collected water tips a seesaw | Drops change the light reflection in the dome |
| Moving parts | Yes, seesaw on a pivot | No |
| Typical errors | Losses in heavy rain, clogged funnel, tilted mounting | Dependence on drop size, dirty dome |
| Verification | Possible with a measured amount of water | Only by comparison with a reference gauge |
| Maintenance | Clean funnel and screen regularly | Keep the dome clean and scratch-free |
The table shows that each design makes different trade-offs. If you want to compare daily totals with an official weather network, a properly sited collecting gauge is the benchmark. If a low-maintenance device without mechanics matters more to you, one that reliably registers rain and delivers plausible amounts, the optical rain gauge plays to its strengths. Deviations of a few percent between two gauges at the same site are normal with either method.
How does a digital rain gauge send rainfall data?
A pulse on its own is not yet a rainfall amount. There is always a chain between the sensor and your dashboard: the rain gauge generates pulses, a controller counts them, transmits the count over the air, and software converts it into millimeters. In a LoRaWAN setup, a sensor node with a counter input handles counting and transmitting, and a gateway forwards the message to the network. That is what turns a plain gauge into a wireless rain gauge.
The most important step is the pulse factor. If every pulse represents 0.1 mm, 37 pulses equal exactly 3.7 mm. If the application mistakenly assumes 1 mm per pulse, it shows 37 mm. The sensor works perfectly, yet the dashboard is off by a factor of ten. After setup, check with a known number of pulses that the displayed amount is correct.
Just as important is what the radio node actually sends. Some devices transmit a running total counter, others only the pulses since the last message. With a running total, you have to calculate the difference from the previous message. If you add up every counter value instead, the rainfall total grows absurdly with each message. After a restart, a running counter often jumps back to zero, and your evaluation must not record that reset as negative rain.
A missing message caused by poor reception is also not the same as “0 mm.” A running counter has an advantage here: the next message lets you recover the missed amount from the difference; only the timing becomes less precise.
Where should you place a rain gauge?
Placement often has a bigger effect on data quality than the instrument itself. Trees, house walls and roof edges either shield the gauge from rain or add drips that never came straight from the sky. The US National Weather Service gives a simple rule of thumb: an obstruction should be no taller than twice its distance from the gauge. If a hedge is 2 m tall, the gauge belongs at least 1 m away from it, and preferably much farther.
Wind is the second big source of error. The windier the site, the more drops get carried past the opening, and the less the gauge records. A flat roof or the top of a tall mast is therefore usually a poor spot, even if it would be ideal for a wind sensor. A sheltered clearing that offers some wind protection on all sides without blocking the rain works better.
Mount the device level and solid. A tipping bucket that hangs a few degrees off level skews every reading, and an optical dome should also point straight up. Keep clear of surfaces that throw up splash water. Record what surrounds the gauge, because growing trees slowly degrade a good site.
How do you calculate rainfall amount and intensity?
Rainfall amount and rainfall intensity are two different quantities. The amount is a total over a defined period, such as an hour, a day or a month. Intensity is a rate, given in millimeters per hour. You calculate it by dividing the amount for an interval by that interval’s duration.
If 2 mm falls in ten minutes, for example, that corresponds to an average intensity of 12 mm/h. It does not mean the rain kept going like that for an hour, nor that 12 mm fell in total. The longer the interval, the more short peaks get smoothed out. If your radio node only transmits every 20 minutes, a violent five-minute shower shows up as a moderate average.
Resolution sets limits, too. At 0.1 mm per pulse, the smallest non-zero intensity in a 20-minute interval is 0.3 mm/h. Light drizzle therefore often appears as single pulses with gaps between them.
For daily totals, you need to decide when a day starts. A midnight-to-midnight total in local time is not the same as the last 24 hours as a rolling window. Both are useful as long as they are clearly labeled. Store the daily total, event total and intensity as separate values instead of one field called “rain.”
How do you clean and maintain a rain gauge?
On a tipping bucket, the funnel is the weak point. Leaves, pollen, bird droppings and insects collect in the screen and slow the flow until the gauge under-reads. Check the funnel and screen regularly, and more often in fall. While cleaning, also make sure the seesaw tips freely and the gauge is still level.
To check the measurement, slowly pour a measured amount of water into the funnel. With a 200 cm² collecting area, 100 ml should produce exactly 5 mm. Pour really slowly, or you will cause exactly the losses that occur in heavy rain.
On an optical rain gauge, the dome is the measuring window. Wipe it with a soft cloth and clean water, without abrasives or solvents, because scratches and streaks permanently change the optical signal. Remove spider webs, algae and limescale spots as well.
One crucial point: maintenance creates pulses. Moving a tipping bucket while cleaning it, or letting water run over an optical dome, produces “rain” that never fell. Note the date and time of every service visit so you can spot and remove those values later. Snow and ice are a separate topic. Without a heater, no ordinary rain gauge measures snow reliably as water equivalent.
How does the Dragino WSS-08 optical rain gauge work?
The Dragino WSS-08 is an optical rain gauge with a transparent dome and a sensing area 6 cm in diameter. According to the manufacturer, it provides one pulse per 0.1 mm of rainfall and handles rain up to 24 mm per minute. It needs a supply voltage of 9 to 30 V DC and is specified for temperatures from −40 to 60 °C and humidity from 0 to 99 % without condensation.
One point matters for planning: the WSS-08 is a sensor head, not a complete wireless station. It has no LoRaWAN radio of its own and is designed for Dragino’s LoRaWAN weather station, whose controller counts and transmits the pulses. That controller sends every 20 minutes by default. According to the documentation, the rain gauge input works in trigger mode out of the box. In that mode, rain does trigger a message, but the message contains no rainfall amount. Only with counting mode enabled are the pulses accumulated. The factor defaults to 0.1 mm per pulse and can be changed to 0.01 mm if needed.
The controller’s counter input accepts only one pulse device. You cannot connect a second rain gauge or another pulse sensor alongside it. The manufacturer does not state an accuracy figure, and no heater for snow is described. Plan the WSS-08 as a low-maintenance gauge for rain, not as a snow gauge.
Where can you buy a rain gauge?
Start by deciding which question your device has to answer. If all you need is a fast “it’s raining” signal, a rain sensor is enough. If you need millimeters, daily totals or rainfall intensity in mm/h, there is no way around a rain gauge. Then look at the measuring principle: a tipping bucket can be checked with a measuring cup, while an optical gauge works without a funnel or mechanics.
Also check the pulse output and its factor, the supply voltage, and whether the device transmits on its own or needs a controller with a counter input. Plan the site with enough distance from trees, walls and windy rooftops. Budget for maintenance as well, because a clogged funnel or a dirty dome costs more accuracy than any datasheet figure.
Once you have settled the rain gauge vs rain sensor question and are looking for a suitable rain gauge, you can find it in our precipitation sensors category.