Humidity 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 humidity data logger will be discussed at this place.

The logger unit

The 8-bit Gemini temperature data logger (Tiny Tag Plus) measures over the range 0 to 100% relative humidity. The logger itself has an operational range from –20^oC to +85^oC. The logger measures 34 x 59 x 80 mm and weights 110 g. The logger is waterproof to 15 m (50ft).

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 16k (EPROM; non-volatile) and will store about 16,000 readings. If the logger is programmed to take hourly readings this corresponds to almost two years of service. If running at 15 minute intervals about 5 months of data may be obtained. 

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.

Sensor

The sensor (capacitive) is mounted on the side of the logger, see photo above. The measurement range is from 0 to 100% RH, with an accuracy of ±3% at 25^oC. The resolution is better than 0.5% RH and the response time is about 10 seconds at 90%.

Price

The current price (December 2000) is about US $ 200 for the logger unit with sensor. 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 logger in a very humid environment care should be taken to ensure that the connector cap (for offloading) is securely fitted. Silicone may with considerable success be applied at this point.

The sensor apparently has satisfactorily long-time performance. It can be wetted without damage, although the accuracy then will be impaired temporarily and it may need about 30 minutes to recover. In order to avoid direct wetting of the sensor when used in the field, the unit may be placed in a small, ventilated housing or otherwise protected towards direct wetting.

The unit should not be operated in a very salty environment, as salt solutions may cause permanent damage to the sensor. Salt crystals forming within the porous surface layer of the sensor will affect the moisture level there.

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! 

Latest update: 3. July 2001.