"Tips for Using the Wet-bulb Thermometer in the Curing of Flue-Cured Tobacco"Mitch Smith, County Extension DirectorPitt County Extension Center
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Introduction
The wet-bulb thermometer is an underutilized tobacco curing device. Without it, a tobacco grower could waste as much as 24% of his fuel due to over- ventilization. The correct use of this device could save a grower approximately $100 in reduced fuel costs.
Background
The wet-bulb thermometer (WB) is the same as a dry-bulb thermometer (DB) except that it is connected to a water reservoir through a cloth wick. As the moisture from the wick evaporates, a lower temperature from the WB results due to evaporation. The DB measures the temperature of the air, the WB indicates its moisture content because evaporation from the wick is greater when the air is drier and lesser when the air is more humid.
The DB temperature is controlled by the amount of heat introduced into the barn while the WB is controlled by the amount of drier, outside air allowed to enter the barn through the ventilators. The difference between the readings of both thermometers indicates the drying potential of air in the barn. This concept is often misunderstood by some tobacco growers who think heat is the primary factor affecting the leaf drying rate during yellowing and the early stages of leaf drying.
Leaf temperature can be critical during the yellowing and leaf drying stages. Since evaporation from the leaves and the wick of the WB occur at equal rates, the WB temperature is also indicative of the leaf temperature. A "browning" reaction (scalding) occurs when the leaf temperature reaches about 112-113 F, especially for thinner-bodied, lower stalk tobacco. This reaction is enhanced when leaves contain excessive moisture when they reach this temperature. This is why it is recommended that the WB temperature not exceed 105 F before leaf drying has been completed. This condition is normally when the DB temperature is raised above 135 F at the beginning of the stem drying stage and when ventilation is reduced in order to raise the WB temperature to 110 F.
How to Use the Wet-bulb Thermometer
The DB temperature is controlled by the amount of heat being introduced into the barn. The WB reading is also influenced by heat but is controlled primarily by opening or closing the ventilators. The WB reading can be used as a guide in knowing when it is safe to start closing the ventilators during the latter part of the leaf and stem drying periods.
If the WB exceeds the desired level, the ventilators should be opened more so that the drying rate will be increased and the WB reading lowered. For example, if both temperatures are reading 100 degrees and one wants to get a 2 or 3 degree spread to get more drying, the ventilators will need to be opened in order to bring in dryer air. To bring the readings closer together, the ventilator openings will need to be reduced. It is generally believed that the rate of curing and quality of the leaf will not be damaged by closing the ventilators as long as the WB temperature does not exceed 105 degrees during the leaf drying period and 110 degrees during the stem drying period.
Note: The recommendations for using the WB does not change with the installation of heat exchangers. However, heat exchangers produce a "drier" heat because the moisture produced during combustion is expelled from the barn rather than being allowed to enter the curing chamber. Therefore, growers who have retrofitted but don't use the WB and traditionally provided ventilation by "personal experience" are likely to overventilate until they become more accustomed to the drier curing air.
Wet-bulb Location
Proper placement of the WB and the DB is essential in a curing barn. For up-draft barns, the DB should be located under tobacco, preferably at an observation port where it can be read without opening the doors. It is suggested that the WB be placed between racks or on top of the curing boxes. The reverse would be true for down draft barns. A general rule of thumb is to place the WB in the area where strong air flow passes over the wick. If it is placed on top of the curing boxes near a ventilator, it should be as far back as possible to the side of the vent opening so that the readings are not influenced by outside air.
Listed below is information concerning the relationship of the WB and DB during the three stages of tobacco curing.
Yellowing Stage
Yellowing and leaf drying (sometimes called "wilting" and/or "color setting") are the most important stages in curing. Failure to proper temperature recommendations during this stage can result in poor cures. During the yellowing stage, three basic processes occur: 1) chlorophyll is degraded and yellow pigments are unmasked, 2) starch compounds which are relatively high in green leaves are converted to sugars, (This process occurs at about the same rate as yellowing), and 3) about 20-25% of the moisture in the green leaves is removed from the barn. If too much moisture is lost too fast, processes 1 and 2 are not completed, resulting in green tobacco high in starch. This tobacco has harsh smoking properties and is subject to severe price discrimination or possibly rejection by buyers.
Failure to remove sufficient moisture during yellowing and early leaf drying increases the risk of scalding during the latter part of leaf drying. For normal, ripe tobacco, yellowing at about 100 F DB and maintaining the WB temperature at about 97 F until most of the green has disappeared (except for some green veins in the butts in the upper third of the barn) will accomplish all three items above. For dry weather tobacco, it may be necessary to run the WB at 98 F - 99 F in order to keep the tobacco from drying too fast (i.e. less ventilation than normal), while for wet weather tobacco, it may be necessary to run the WB at 96 F - 95 F to cause faster drying (i.e., more ventilation than normal). Some wilting and crisping of the leaf tips should also be noticeable at the end of yellowing, particularly in the lower third of the box (or the lowest tier of a rack barn). When air flow is inadequate, it may be necessary to enhance drying and wilting in the upper part of the barn with heat to improve air flow during leaf drying. In this instance, moving the DB up to 105 F during the later stages of yellowing but keeping the WB between 98 F and 100 F will help wilting to occur faster. However, initiating this process too soon may set some green color.
WB temperatures for yellowing range from 95 to 105 degrees F. To maintain high humidity and prevent color setting before it is desired, outside air dampers should be almost closed. "Sappy" or high-moisture tobacco requires considerably more moisture removal before color-setting than does droughty or low-moisture tobacco.
Leaf Drying Stage
Most leaf wilting occurs during the first half of the leaf drying stage (i.e. DB 110-120 F), and another 20% of the leaf moisture should be removed from the barn by the late leaf drying stage (DB 130-135 F). During early leaf drying, enough wilting should occur so that the tobacco loosens in the containers and air can move through the tobacco more freely. Therefore, do not advance the WB higher than 100 F during early leaf drying and do not advance the DB more than 120 F until the leaves are completely wilted. When the leaves have reached the desired yellow color and are thoroughly wilted, advance the DB from 120 to 130-135 F at 2 F per hour and allow the WB to rise to about 105 F soon after leaving 120 F DB temperature. This is the color-setting stage and is critical because the leaves are sensitive to temperature change and both temperature and airflow must be controlled in order to reduce undesirable colors (hence the need for proper wilting as discussed above as well as uniform loading of curing containers). Impatience to capture a good color by advancing the temperature too fast often produces a browning reaction or scald as mentioned earlier, particularly in "tight" spots that have not been properly dried to this point (hence the reason for not allowing the WB to climb above 105 F during the second half of leaf drying). On the other hand, drying too slowly may produce a less severe but more general browning called "sponging", particularly for thin- bodied lower stalk tobacco.
The WB temperature for leaf-drying ranges from 100 F to 105 F. During leaf drying, when the DB temperature is raised above 110 F, the dampers should be open enough to hold the WB temperature down to 100 F. The WB temperature should never be allowed to increase above 100 F until the mid-vein is dry. For diseased or extra-thin tobacco, a lower WB thermometer temperature (90 F to 100 F) may produce a brighter cure.
Stem Drying Stage
If the cure has progressed properly to this stage, one is almost "home-free". After the leaf lamina is essentially dry, advance the DB from 135 F to 165 F at 2-3 F per hour, gradually reducing ventilation to maintain the WB at 110 F until the DB reaches about 160 F. Dampers are usually closed at the time the leaf is completely dry and the temperature has reached 160 F. Once tobacco is dry enough to take the DB readings above 135 F, maintaining a WB temperature of 110 F or higher can conserve fuel. Keeping the WB at 110 F and not advancing the DB more that 2 degrees per hours during the first 12-18 hours of stem drying gives added protection against scalding. Stem drying should be completed at 160-165 F but higher DB temperatures could also caramelize sugars which causes the tobacco to turn red in color.
Refer to a curing chart to determine when to adjust fresh air dampers to control wet bulb temperatures in relation to dry bulb temperatures.
MORE FLUE-CURED TOBACCO CURING INFORMATION! - This link provides information related to the curing of flue-cured tobacco.
TO REQUEST A TOBACCO CURING CHART, CONTACT MITCH SMITH, COUNTY EXTENSION DIRECTOR AT 757-2801 EXT. 24.
The North Carolina Cooperative Extension Service in Pitt County wishes to express gratitude to Dr. Gerald Peedin, Professor Emertus for his contribution to this fact sheet.
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