A float tank, also known as a sensory deprivation tank or R.E.S.T. (Restricted Environmental Stimulation Therapy) environment, is a light and sound-proof container filled with shallow water. The water is saturated with hundreds of pounds of Epsom salt (Magnesium Sulfate), allowing a person to float effortlessly. Despite the high salt content, which often raises concerns about dehydration, float tanks do not typically cause water loss from the body; the skin acts as an effective barrier against the salty solution.
The Unique Chemistry of Float Tank Water
The extremely high salt concentration is the defining characteristic of float tank water. Approximately 800 to 1,000 pounds of Magnesium Sulfate are dissolved into the water, creating a saturated solution.
The high concentration of Epsom salt, which can be around 25% to 30% by mass, drastically increases the solution’s density. This density is measured by its specific gravity, which typically ranges from 1.23 to 1.30. For comparison, regular water has a specific gravity of 1.0.
This significant difference in specific gravity generates extreme buoyancy, allowing the body to float without effort or muscle tension. The water is also maintained at a specific temperature, usually between 92°F and 96°F, which is intended to be skin-temperature neutral. This thermal neutrality helps minimize sensory input, a core goal of the experience.
Osmosis and the Body’s Hydration Balance
The question of dehydration is rooted in osmosis, the movement of water across a semipermeable membrane to equalize solute concentration. The highly saline float solution is classified as a hypertonic environment, meaning it has a much higher concentration of dissolved particles than the fluids inside the human body.
If the skin were a simple, fully permeable membrane, this hypertonic environment would draw water out of the body, leading to rapid dehydration. However, the skin is a complex, multi-layered organ that forms a highly effective barrier. This barrier successfully prevents massive osmotic water loss.
The high concentration of salt does not pull water out of the body’s cells, which is why the skin does not wrinkle or prune up. While the skin prevents water loss, it can facilitate the absorption of beneficial magnesium from the Epsom salt.
Any feeling of dryness after a float is not systemic dehydration but rather a residue of salt left on the skin’s surface. Rinsing the body after the session removes this salt and alleviates the residual dry sensation.
Practical Steps for Pre- and Post-Float Hydration
Although the float tank itself does not dehydrate you, maintaining normal hydration is important for comfort. It is recommended to drink water regularly in the hours leading up to the session, ensuring your body’s fluid balance is optimized.
Avoid consuming large amounts of liquid immediately before entering the tank, as this can lead to a full bladder and interrupt the relaxation session. A light meal 90 minutes to two hours prior is suggested to prevent hunger pangs or noisy digestion from becoming a distraction.
Stimulating substances, such as caffeine and alcohol, should be avoided for several hours before a float. These compounds can increase heart rate and interfere with the deep state of relaxation the environment promotes.
After the session, a return to normal drinking habits is all that is necessary. Rehydrating with water or herbal tea helps the body transition back to normal activity. There is no need for aggressive fluid replacement, only a continuation of healthy, regular hydration.