Introduction
The investigation of speleothem growth is of great interest in the field of cave science. While the interior of a stalagmite contains a wealth of isotopic information, the dynamics of stalagmite formation and growth must be properly understood. One of the driving parameters is the rate at which carbonate-laden water falls onto the growing stalagmite: this requires an instrument that can be taken to the cave site, that is robust enough to be knocked about, submerged or dropped, and that can be left unattended to record over many months.
In 2005 PiTech Research Ltd was approached by Prof. Dave Mattey of Royal Holloway College, University of London, with a request to design and build an instrument that would fit the requirements above. Subsequent presentation of the data at an international conference generated such interest that it was decided that the device should be marketed more widely, and that it needed a name. Hence the STALAGMATE was born.
In 2005 PiTech Research Ltd was approached by Prof. Dave Mattey of Royal Holloway College, University of London, with a request to design and build an instrument that would fit the requirements above. Subsequent presentation of the data at an international conference generated such interest that it was decided that the device should be marketed more widely, and that it needed a name. Hence the STALAGMATE was born.
What is it and what does it do?
The Stalagmate is a self-contained battery-powered box slightly larger than a 5cm cube that records drips, typically from stalactites, for up to 4 years. It is fully protected against water spray and complete immersion for short intervals (IP67), and robust enough to be stood on, dropped, and generally knocked about.
How it works
The principle of operation is simple: the lid is effectively a microphone which is “pinged” each time it receives an impulse from a falling drop. The resulting damped oscillatory response is processed using ultra-low power electronics, and converted into a clean rectangular pulse that drives a logger/counter on a separate PCB. Each time a pulse is received, this is signalled by a brief flash from one of the red LEDs on the side of the box.
Sensitivity
The Stalagmate in standard form will record 0.1 ml drops from a height of 20 cm up to a few metres. At heights of several metres, water drops tend to flatten out and become aerodynamically unstable, sometimes breaking up into a number of smaller drops scattered over the intended target area. For sites where the fall height is very low, e.g. where the stalactite or roof straw tip is only about 10 cm above the Stalagmate, a special high sensitivity version is available.
Logger
The datalogger is a proprietary product of Gemini Loggers Ltd, and is programmable using Gemini Explorer for Windows XP, Win 7 and Win 10. A time-limited version of the software is available free from the Gemini website. Power for the detector board comes from the logger which provides current only when the logger is activated after “Launch”. Since the logger draws only a few μA when in “sleep mode”, the life of the lithium thiosulphate battery is, at least in theory, about 10 years. Note that if the Stalagmate is activated simply for testing purposes, it should then be deactivated by pressing the “Stop” button in Gemini Explorer. Logging is indicated by a flash from the green LED at roughly 4 second intervals. Each drop is indicated by a flash from one of the two red LEDs. When in standby mode, this interval is about 8 seconds. A logger failure is indicated by the second red LED flashing - this is extremely rare.
Programming
The logger can be set up to start counting before setting off into the field, but this does mean that any “data” collected will be due to knocks and bumps in transit, and will have to be edited out later. Or, the logger can be set to start after a programmed time delay of one minute to several months, or to some absolute time in the future.
Communication
Communication with a PC is via a special USB cable. The Stalagmate is rated to IP67 only when the supplied blanking cap is fitted to the data socket. Make sure that the threads on the socket and/or blanking plug are clean, dry and free of grit. The sealing element in the cap is a rubber washer, and a good seal can be made with quite a small finger-tight torque. Warning! Screwing the protective cap too tightly may make it difficult to remove later and may even damage the socket.
Positioning
While stalagmite tips are rarely ideal for the careful positioning of rigid boxes, it has been found that a very good method for doing this is to make a small sandbag using a nylon sock, for example, although a large handkerchief (or other square of material) tied off with string or a nylon TyRap is OK (don't use rubber bands, which will perish). Preferably choose a sock containing a significant fraction of synthetic material, which is less prone to rot than natural fibres. The surface tension between the wet grains of sand holds them in place without moving, and the result is a stable platform which can be easily positioned as required. There are many other solutions to this particular problem: a sand-filled plastic bag will work as well, and also effective is a ball of aluminium kitchen foil scrunched up to form a stable interface between Stalagmate and stalagmite; plasticine or its equivalents can probably also work quite well, though don’t use modelling clay since it dissolves in water!
Drop Aiming
If the stalactite is producing frequent drops, it is a simple matter to locate the box under the drips. With practice, the sound of the drop falling onto the lid is a good indicator, but the red led, of course, flashes with each drop. However, if the stalactite is "dry", i.e. if the cave is not currently very active, a good method is to use a garden pistol spray in jet-mode. These have a range of several metres, and are very useful for wetting the stalactite to produce the drops you will need to position the box. Another trick is to use a small piece of coloured paper on top of the box: the contrast between the wet and dry areas is much greater than just the box by itself.
Pooling
When new, the lid of the box may be slightly hydrophobic, causing the water to pool on the surface. It is just possible, especially where the drop height is not great, that the consequent “cushioning” effect may prevent drops from being recorded reliably, but one solution is to tilt the box slightly so that the pooled water drains away. However, Stalagmates are currently shipped with a small strip of fabric overlapping the lid to wick the water away. This does not affect the performance in any way and it is advisable not to remove the wick unless the drops fall from a sufficient height that pooling cannot occur.
Sealing
The plastic cases are designed to IP 67, and we have gone to great lengths to avoid water ingress, including small sachets of superabsorbent polymer placed in each box. However, especially under some conditions (e.g. where the temperature and/or pressure varies significantly over time), the boxes can "breathe", allowing moisture to condense inside. This is rare, but has happened. This means it is VERY IMPORTANT to take GREAT CARE in reassembling the case after replacing the battery, and silicone grease alone is sometimes not sufficient to maintain a watertight seal. Silicone rubber is good, but it does make it harder to remove the lid later. If you use acetoxy-based (bathroom) silicone sealant, use it very sparingly since the acetic acid it gives off is corrosive to the components inside. If you're not too worried about appearance, put extra sealant around the outside after the lid has been tightened down (even when the Stalagmate is new: to be sure to be sure!). Carefully inspect the outside of the box to check for hair-line cracks. If you find one, repair it with low-viscosity cyanoacrylate (superglue), or ask us to replace the unit. As an extra line of defence, you can wrap the whole box in kitchen "cling-film" (after having programmed it, of course!). Wet the top label on the box (or use oil or grease) before you do this to ensure good mechanical/acoustic coupling between the plastic film and the lid (which is the "microphone").
Battery
The battery is a 1/2 AA Lithium Thionyl Chloride type, which produces a nearly constant 3.6V up to the point of exhaustion. The 1200mA hour capacity is sufficient to power the logger continuously for 4 years, with a shelf life of up to 10. NB LiSOCl2 technology is NOT the same as Lithium Manganese (also available in 1/2 AA sizes), which generates only 3.0 V and the former is not rechargeable.
Farnell Part Nos. 118-7252 (Tadiran); 773-992 (Saft)
RS Part Nos. 596-589 (RS); 201-9422 (Saft); 526-8425 (Tadiran)
Also available from Amazon and many other online stores
Farnell Part Nos. 118-7252 (Tadiran); 773-992 (Saft)
RS Part Nos. 596-589 (RS); 201-9422 (Saft); 526-8425 (Tadiran)
Also available from Amazon and many other online stores
FOR BATTERY CHANGING INSTRUCTIONS, click here
Technical Data
Dimensions: 60 x 65 x 40 mm
Battery type 1/2 AA Lithium thionyl chloride: 1200 mAh @ 3.6 V
Battery life (logger off) ≈ 10 years
Battery life (logger on) ≈ 4 years
No. of data buckets 32 k (32768)
Counts per bucket 16 k (16384)
Logger software Gemini Explorer
Communication Special USB cable
Sealing IP67 (only when terminal cap is fitted)
Battery type 1/2 AA Lithium thionyl chloride: 1200 mAh @ 3.6 V
Battery life (logger off) ≈ 10 years
Battery life (logger on) ≈ 4 years
No. of data buckets 32 k (32768)
Counts per bucket 16 k (16384)
Logger software Gemini Explorer
Communication Special USB cable
Sealing IP67 (only when terminal cap is fitted)