# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Artificial horizons and mercury**

**From:**Peter Ifland

**Date:**2003 Jul 20, 12:50 -0400

What follows is a lift from http://acc6.its.brooklyn.cuny.edu/~scintech/mercury/WhatBigDeal.htm . [m^3 means cubic meter] "VC = S-Q [I think this equation is a little fuzzy] Where V is the room volume in cubic meters (m^3), C is the concentration in micrograms per hour, and Q is the air flow rate from the room in m^3. S, the mercury evaporation rate, is the rate of mercury vaporization per unit area of mercury. This is a constant value of 7 ug/cm^2/hour at 20*C." The Agency for Toxic Substances and Disease Registry (ASTDR)'s minimal risk level (MRL) for mercury vapor inhalation is .3 ug/m^3. This is an estimate of the daily human exposure that will most likely not result in risk. The occupational exposure limit set by the U.S. National Institute for Occupational Safety and Health is 50 ug/m^3." Here's a simple example, use a 6 cm X 10 cm mercury horizon for one hour in a 5m X 5m X 4m room at 20 degrees C with a 2 m^3 per hour ventilation. The mercury lost from the horizon equals (7 ug/cm^2/hr) X (6cm X 10cm) =420 ug/hr into a (5 X 5 X 4m) room with a 2 m^3/hour turnover of air = 420 ug/hr /(100-2) m^3 = 4.2ug/m^3/hr. If left for 12 hours, the concentration builds to 50.4 ug/m^3 , i.e.. the occupational exposure limit. Conclusion: Keep the mercury container in a sealed flask indoors, fill and use the horizon outdoors. If you inhaled ALL of the evaporation of mercury from a 6cm X 10 cm horizon for six minutes out of every hour while taking sights, you would still not exceed the occupational limits. Thus, personal exposure will be well within safe limits. Did I get it right? Peter Ifland P.S. Thanks to Paul Hirose for pointing us to a very informative site.