As described in more detail elsewhere on this site, the MMA project aims to take advantage of the recent emergence of low cost electronic components and innovations in micro manufacturing technologies to develop a range of affordable meteorological instruments for manufacture and use in developing countries.
The automatic rain gauge that is currently under development aims to improve the performance of the traditional tipping bucket gauge (the most commonly used automatic precipitation gauge) by adding the capability to weigh the rainwater as it is collected in the bucket using the components of inexpensive (but very precise) weighing scales that are now commonly available. Using this approach, the tipping bucket itself no longer plays a part in measuring rainfall but simply provides a means of emptying the collection vessel in a controlled way. By measuring the weight of water in the bucket at regular intervals and subtracting the previous reading, the volume of water collected per second or minute can be determined and translated into mm/inches of rainfall.
We’re calling it the hybrid gauge because it combines some of the features of the tipping bucket with some of the features of an automatic weighing gauge, which usually takes the form of a large container (that does not automatically empty) and an elaborate spring balance/drum recording device. As far as we know, this is a new idea – the following section lists some of the advantages that we see with this approach.
Advantages of the Hybrid Tipping Bucket Design
Section 18.104.22.168 of WMO’s CIMO Guide Part 1: Measurement of Meteorological Variables lists six main sources of error specific to tipping bucket rain gauges:
(a) The bucket takes a small but finite time to tip and, during the first half of its motion, additional rain may enter the compartment that already contains the calculated amount of rainfall;
(b) With the usual bucket design, the exposed water surface is large in relation to its volume, meaning that appreciable evaporation losses can occur, especially in hot regions. This error may be significant in light rain;
(c) The discontinuous nature of the record may not provide satisfactory data during light drizzle or very light rain. In particular, the time of onset and cessation of precipitation cannot be accurately determined;
(d) Water may adhere to both the walls and the lip of the bucket, resulting in rain residue in the bucket and additional weight to be overcome by the tipping action. Tests on waxed buckets produced a 4 per cent reduction in the volume required to tip the balance compared with non-waxed buckets. Volumetric calibration can change, without adjustment of the calibration screws, by variation of bucket wettability through surface oxidation or contamination by impurities and variations in surface tension;
(e) The stream of water falling from the funnel onto the exposed bucket may cause over-reading, depending on the size, shape and position of the nozzle;
(f) The instrument is particularly prone to bearing friction and to having an improperly balanced bucket because the gauge is not level.
We believe that the hybrid Tipping Bucket/Weighing Gauge that the IEPAS project is currently developing has the potential to improve on all of these 6 sources of error, when compared to a standard tipping bucket; here’s our logic:
(a) To be sensitive to light rainfall, the standard tipping bucket rain gauge tips at 0.1 – 0.2 mm of rain (per the CIMO guide, “This amount of rain should not exceed 0.2 mm if detailed records are required”). The problem with this is that during heavy rainfall, the bucket tips back and forth like a Ramones’ metronome – by our calculation, 250mm/hr rainfall would require 1250 tips per hour, which works out at one tip every 2.9 seconds. The hybrid gauge does not use the number of tips to measure the rate of rainfall; instead, it continuously weighs the rainwater accumulating in the bucket and uses the tipping action simply for emptying the bucket. This means that the gauge can be designed to tip far less regularly – the largest version of the hybrid gauge should (conservatively) tip after around 2mm of rain (every 29 seconds at 250mm/hr), so without attempting to address the cause of the error, the hybrid gauge reduces the frequency of occurrence by around 90%. The increase in rainfall required per tip is achieved by using a larger bucket and by reducing the size of the collection orifice. According to the CIMO Guide, “The size of the collector orifice is not critical for liquid precipitation, but an area of at least 200 cm2 is required if solid forms of precipitation are expected in significant quantity”, so as long as we avoid installations such as the top of Mount Kilimanjaro, the hybrid gauge with a smaller orifice (75cm2) should be fine for the tropical/sub-tropical locations for which it is intended.
(b) The hybrid gauge calculates the amount of rainfall by measuring the increased weight of water that accumulates in the bucket over a set period of time. Because the weighing scale is sensitive to one tenth of a gram, this time period can be very short (for 250mm/hr rate of rainfall, 0.1 grams will accumulate in around 0.2 seconds) so evaporation should not have a significant effect on readings, even in very light rainfall. (2.5mm/hr rate of rainfall should be detected by the hybrid gauge in 20 seconds, as opposed to almost 5 minutes for a standard gauge).
(c) The method of measurement and sensitivity of the hybrid gauge allows it to keep a continuous record even in very light rainfall, so onset and cessation of rain will register quite precisely in all but the very lightest and shortest events – the “no tip” situation that occurs when a small amount of rain falls that is not sufficient to trigger the standard gauge is not a problem with the hybrid.
(d) ,(e) and (f) These issues are related to the need for the standard gauge to tip with exactly the same volume of water every time, as the number of tips is used to calculate rainfall totals. The hybrid gauge measures the difference in water mass collected in the bucket from second to second, so it doesn’t really matter if the tipping action is inconsistent, as it is not used to calculate rainfall. The added advantage of this is that dimensional tolerances and precision in manufacturing, assembly and installation are less critical in the hybrid gauge, making it much more suited to local production.
Section 6.4 of the CIMO guide lists the major causes of error that are common to all types of precipitation gauges, namely:
(a) Error due to systematic wind field deformation above the gauge orifice: typically 2 to 10 per cent for rain and 10 to 50 per cent for snow;
(b) Error due to the wetting loss on the internal walls of the collector;
(c) Error due to the wetting loss in the container when it is emptied: typically 2 to 15 per cent in summer and 1 to 8 per cent in winter, for (b) and (c) together;
(d) Error due to evaporation from the container (most important in hot climates): 0 to 4 per cent;
(e) Error due to blowing and drifting snow;
(f) Error due to the in- and out-splashing of water: 1 to 2 per cent;
We believe that these potential errors relate to the hybrid gauge in the following way:
(a) and (f) are independent of the hybrid’s measurement approach and so are indeed common with standard gauges (we are attempting to minimize them in the design of the collecting funnel).
(b) By using a smaller orifice (and hence smaller funnel) than the standard gauge, the surface area of the internal walls of the collector are significantly reduced, which should reduce the wetting losses. This advantage is countered to some extent by the rough surface finish that results from the method of manufacture; it’s possible that we may use some treatment (e.g wax) on the wetted surfaces to improve this.
(c) As we are measuring the change in mass in the bucket from second to second, this is not relevant.
(d) As described above, this should be minimal for the hybrid gauge
(e) We don’t measure no stinkin’snow.
Sources of Error Unique to the Hybrid Gauge
Right now only three potential sources of error come to mind that are related to the use of load cells to weigh the accumulated rainfall.
(a) Inconsistent output/load response between different load cells
(b) Non-linear output from the load cell under very light load
(c) Floating “empty bucket” output
(a) Initial testing (and literature review) suggests that even the inexpensive load cells being used are extremely consistent in their response to load, to the extent that calibration of individual gauges is probably unnecessary
(b) The design of the hybrid gauge is such that the load cell is pre-loaded by the weight of the bucket and it’s support structure, so it’s always operating in its linear range
(c) It is possible that the “empty bucket” output might drift very slightly from day to day, which might give a false indication of a minute amount of rainfall (can probably be discounted during quality assurance of the data?)