Precipitation Measurements using Gemini Miniature Data Loggers

Ole Humlum, The University Courses on Svalbard (UNIS), Norway.

General

Gemini data loggers comes in several varieties, precipitation, humidity, movement, shock and temperature. Only the precipitation data logger with external rain gauge will be discussed at this place.

The logger unit

The 12-bit Gemini precipitation data logger (Tiny Tag Plus High Resolution) should not be operated outside a temperature range from –40^oC to +85^oC. The logger may not  work properly outside this temperature range. The logger is waterproof to 15 m (50 ft), it measures 34 x 59 x 80 mm and weights 110 g.

The resolution on the individual readings is 12 bit. The logging interval is user-controlled and may be set from 1 sec to 10 days. There are several start options: now, delayed (up to 45 days), event, magnetic impulse. Stop options: when full, after N readings, never (wrap around). Reading type: actual, min or max. The logger may be offloaded while running when logging in minute multiples.

All programming and offloading of the logger is done by means of easy-to-use Windows-based software.

Memory

The logger has an internal memory of 32k (EPROM; non-volatile), storing about 16,000 readings. If the logger is programmed to take hourly readings this corresponds to almost two years of service, or you may obtain 5 month data at 15 minute intervals. 

Battery

The logger uses Saft 3.7V 1/2AA Lithium cells, having a battery life up to 5 years. However, battery replacement is recommended every 2 years. The logger should be stopped before replacement. A complete Tinytag Plus Service Kit including battery, seal and silica gel pack may be obtained from the firm.

External rain sensor

The external rain gauge is a drop-contact sensor with a diameter of 110 mm and a height of 90 mm. It is connected to the data logger by a 2 m cable (see illustration above). The measurement range is 0-25 mm per interval. The resolution is 0.2 mm, the accuracy is ±0.1 mm per interval. The maximum amount of precipitation which can be recorded is 25.5 mm per interval, or 0.03 mm per second. During very heavy rainfalls or during strong winds, additional errors may be encountered due to raindrops splashing over the side of the sensor.

Price

The current price (December 2000) is about US $ 290 for the complete unit (data logger and rain gauge). The Gemini Group are based in UK, but you may obtain the logger and probe from local distributors in several countries. Please check the Gemini homepage for further details.

Special notes

When using the logger at low temperatures, condensation may form inside the chase. Therefore, before opening the chase for any reason (e.g., battery replacement), allow the unit to reach the room temperature if the operation is carried out indoor. You need no open the chase for offloading of data.

When using the unit in very humid environments take take care to ensure that the connector cap (for offloading) is securely fitted. Silicone may with considerable success be applied at this location.

This type of data logger have been used with satisfactorily results for several years in Greenland, the Faroe Islands, Scotland, Denmark and on Svalbard. The EPROM memory ensures that you often will be able to recover data from a logger which accidentally has been water filled. After proper drying and battery replacement you may even succeed in reviving the logger after such an event !

As seen in the illustration below, the unit may easily be installed flush with the ground surface, which is of considerable relevance within a geomorphological context, compared to the standard 2 m measuring height used for many normal meteorological stations.

A drawback of installation at the ground surface is the risk of getting the rain gauge blocked by debris, especially if operated in a windy environment. Therefore the rain gauge should be inspected and cleaned whenever possible.

During installation, due to the drop-contact principle, care should be taken to ensure that the upper surface of the sensor is horizontal. If not, some drops may not make proper contact in the lower part of the device and will not be recorded.

The unit can be left in the field during winter as no liquid water is stored in the gauge and this has no moving parts. No freezing damage will therefore be induced. If the unit is left in the field during the winter season, no precipitation will of cause be registered as long as temperatures stay below freezing. However, if properly installed, the unit will record the subsequent melting of the snow cover present at the measuring site when melt commences. This is a very useful information in relation to cold-climate geomorphology.

Rain gauge (lower part of picture) installed flush with the ground surface.

Latest update: 3. July 2001.