Maple Sirup Producers Manual. Research Service, Washington. When an evaporator is operating properly, a depth gradient develops with the deepest liquid at the point of sap intake and the shallowest liquid at the point of syrup drawoff. Many syrup makers find that a liquid depth of one inch at the point of syrup drawoff is close to ideal. Check the manufacturers recommendations for your particular evaporator.
Several factors can create problems in maintaining the sap gradient in the evaporator.
Uneven firing of the evaporator weakens or destroys the gradient. During periods of low heat the sap depth tends to level and there is intermixing of sap of different sugar concentrations along the gradient. Sap intermixing, together with an increase in the average depth of sap in the evaporator, results in longer evaporation time and the production of darker and often lower-quality syrup. To maintain the desired gradient, depth of liquid, and evaporation time, it is important that the depth of liquid at the sap intake be adjusted when the sugar content of the sap changes. Keep a refractometer handy.