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Dew point temperature related to wet mushroom caps.

A glass of ice water sweats in the summer but not in the winter. The explanation for sweating or condensation on the glass relates to the dew point temperature of the air around the glass. Mushrooms can "sweat" if the dew point temperature of the air next to the cap is higher than the cap temperature. Wet mushroom caps are good surfaces for growing the bacteria that cause blotch. A problem with blotch in spring or fall might be related to dew point temperature. This article describes dew point temperature, how to measure it, how it relates to relative humidity, and what to do to control it.


Air is considered to be a mixture of water vapor and dry air. An understanding of the mixture is necessary to prevent sweating on your mushrooms. Four properties of air as a mixture are important: dew point, relative humidity, dry bulb and wet bulb. Dew point temperature (d.p.) has units of degrees, either Fahrenheit or Celsius--d.p. is one measure of the amount of water vapor in the air. In our daily lives, the property of relative humidity (RH) is the most common indicator of moisture in the air. However, relative humidity by itself does not address the problem of wet mushrooms as well as does d.p. Another property is dry bulb temperature (d.b.), which is commonly called just "air temperature." The term dry bulb comes from the appearance of the thermometer bulge as compared to the "wet bulb (w.b.)." When the enlarged end at the bottom of a glass thermometer is covered with a small wet sock it is considered a "wet bulb." These four properties, d.b., w.b., d.p. and RH, are described in more detail in this article.

The properties of air interrelated one with another and these relationships can be shown graphically in what is called a psychrometric chart or abbreviated--the chart. Because of the interrelationships, any two of the properties can identify air in a room as a point on the chart and thus the other properties need not be measured directly, but can be read from the chart or from a table. The most reliable measurements are dry bulb and wet bulb temperatures. Table 1 can be used to illustrate the relationships between dry bulb, wet bulb, and relative humidity. If the w.b. increases and the d.b. stays the same (A > B > C), then the RH will increase. In the second sequence, as the d. b. increases and the w. b. stays the same (D > E > F), then RH decreases. These air points can be shown on a simplified chart (Figure 1). Let us look at a new point on the chart (G). Can you determine d.b., w.b. and RH for point G? You should read: d.b. = 67[degrees]F, w.b. = 63[degrees] F and RH = 80 percent. Notice that d.b. lines are vertical, w.b. lines are slanted and straight, and RH lines are curved. Now, consider dew point as a measure of moisture in the air. The definition of dew point is the temperature at which the water vapor in the air will just start to condense or sweat on a cold glass. Referring to Figure 2, d.p. is found on the saturation line where RH is 100 percent. From any point on the chart, follow horizontally with your eye to the left to the saturation line and read the saturation temperature. That temperature is the d.p. What are the d.p. values for air points B and C? Figure 2 shows that for B--d.p. is 56.5 F and for C, d.p. is almost 62 F. Notice that for the same d.b. temperature, a higher wet bulb means a higher dew point. If the air in a growing room has a d.b. temperature of 68[degrees]F and w.b. of 66[degrees]F, what is the d.p.? On Figure 2, point H represents the air in the room. Follow across horizontally, the d.p. temperature for H is 65[degrees]F.


Dew point temperature is a concern during cropping particularly when the dry bulb temperature is changing. In order to know what the d.p. temperature is in a room, you need to measure it directly or determine it indirectly from the chart. To measure dew point directly, an expensive cooled-mirror instrument is required and would not be justified for mushroom growing. To determine the d.p. from the chart or a table, the most common measurements are d.b. and w.b. temperatures. The instrument recommended for these two measurements is called a psychrometer and is readily available at a reasonable cost. Both a sling psychrometer and a battery-powered psychrometer provide enough air movement over the wet sock to allow the w.b. temperature to reach a stable reading. The other temperature measurement needed to address a problem with sweaty mushroom caps is the cap temperature. It is not required that you measure the temperature of the mushroom cap, but it could help you understand the situation in your house. Take a sharp-pointed temperature probe for sticking into compost and insert it in a mushroom to check the temperature of the cap. It is reasonable to assume that the mushroom cap surface is the same temperature as the inside of the mushroom, or slightly lower temperature. If the cap temperature is lower than the dew point temperature of the air then water vapor will condense on the surface of the cap. Think of the cold glass for comparison: if the surface is warmer than the d.p., then there is no condensation.



As was mentioned earlier, relative humidity, wet bulb, and dew point are all related. If the relative humidity in the room is below 90 percent, then dew point will not be a problem. Measuring relative humidity above 90 percent is best with a psychrometer using d.b. and w.b. temperatures, rather than with an electronic humidity meter. Typical electronic meters are reliable to about 95 percent RH. If the difference between d.b. and w.b. is 2[degrees]F or more, then the RH will be less than 90 percent. In Table 2, the approximate RH is given for a narrow range of air temperature that is typical during cropping. Notice that the closer the two temperatures are to the same reading, the wetter the air is in the room. It is important to understand both dew point and relative humidity and to use what ever is most comfortable for you. For example, the d.p. of air in a room will always be less than the w.b. If the w.b. is 2[degrees]F lower than d.b., then the d.p. will be 3[degrees]F lower than d.b. A 3[degrees]F difference between d.b. and d.p. is good for keeping mushroom caps dry.


An example of a dew point problem was observed in a typical Pennsylvania double that had perimeter hot-pipe heating and exhaust fans. After watering, the grower wanted to dry the mushroom caps as quickly as possible to reduce the potential for blotch. Quick drying is a recommended practice. The exhaust fans were turned on full in the late morning for about five hours. Then after this period of ventilation with cold, dry outside air, the exhaust fans were turned off and the house heating system warmed the air to about 60[degrees]F. Later that evening the grower changed the thermostat setting to about 68[degrees]F.


As a result of heating the air and consequently the beds, the w.b. and d.p. temperatures also increased. As shown in Figure 3, the d.p. temperature rose faster than the mushroom cap temperature thus causing the caps to sweat. Though the caps may have dried after watering, they were re-wetted by condensation by the dew point effect. As the air temperature declined the next morning, the caps may have dried again, but the bacteria had time to grow and cause the brownish spots typical of blotch.


To prevent a condensation problem, the grower needs to monitor air temperature (d.b.) and humidity both inside and outside the room. During stable conditions, mushroom cap temperatures are about 1[degrees]F below the d.b. temperature. If the d.p. is at least 2[degrees]F below the d.b. temperature and there is reasonable air flow, then the caps will not sweat. With the dew point at least 2[degrees]F below dry bulb, the relative humidity will generally be about 90 percent. These stable conditions should not cause a dew point problem. If conditions inside of the house are changing quickly, then there is greater potential for the cap temperature to drop below the d.p. temperature. As described above under "observations," the rapid rise in air temperature in a wet room caused a rapid rise in d.p. temperature. The cap temperature increased more slowly than the d.p., causing sweating. If the air temperature is increased slowly, the cap temperature will follow and the situation will appear almost like the stable conditions above. For manual temperature control, you can generally change the thermostat setting by 1[degrees]F once each hour. If the d.b., w.b. and d.p. all have the same value then the air is saturated and the relative humidity is 100 percent. When this happens, there are basically two ways to reduce the dew point: 1) remove water vapor from the air by a cold coil such as an air conditioner which condenses the vapor, or 2) to mix drier outside air with the wet room air by ventilation. The first method for dew point control happens during hot humid summer weather as a result of air conditioning. The second method is more dependable for cold winter weather using fresh air that is dry--has a low d.p. temperature.


Mixing outside air using exhaust fans or the fresh air damper is not as dependable when the weather conditions are close to the desired room air properties during spring and fall. There may be days when the outside air has a dew point above the desired value for the growing room. You may need to measure outside air properties to be sure that outside d.p. is lower than inside d.p. For these conditions, the air conditioner or cooling coil should be used to remove moisture even though it is not needed for d.b. temperature control. The weather shown in Figure 4 would be a potential concern because the outside temperature could be close to the desired inside temperature and thus the thermostat would not call for heating or cooling. The d.p. temperature is close to the d.b. and thus the wet air could provide for condensation.

Both heating and cooling are used at the same time to remove water vapor and also maintain air temperature. The psychrometer can be used outside to assure that dew point of outside air is at least 3[degrees]F below the desired room d.b. If condensation on mushroom caps is a problem, then raising the d.b. temperature by heating the air may not help the problem. You should understand how air properties change. In an empty room, heating the air (increasing the d.b.) does not change the dew point although heating does reduce the relative humidity. In a room with growing mushrooms, raising the d.b. may cause more biological activity which in turn creates more water vapor from the mushroom and compost. Particularly on a day before a large harvest, the water vapor production by the mushrooms could be sufficient to raise the dew point above the cap temperature. Dew point control during cropping is important to maintain mushrooms that are dry but not too dry. If you have a blotch problem, may I suggest that you measure the wet bulb and dry bulb temperature during cropping. Yes, it will take time and you may need to purchase a psychrometer, but the cost to quality of the harvest could be significant. It may be necessary to check as often as every four hours during the spring and fall days like the situation shown in Figure 4. If there is manual control for the air conditioner or the cold coils in the air handler, then turn on the cooling but leave the thermostat setting the same to call for heat. If the equipment can do both heating and cooling and maintain the d.b. temperature, then aim for a w.b. that is 2[degrees]F lower than d.b.. Adjust the cooling to achieve the 2[degrees]F differential.


Dew point temperature is a measure of water vapor in the air. Measuring and controlling dew point temperature during cropping can reduce moisture on the mushroom caps. The dew point temperature can be reduced by air conditioning but not by heating.

Ken M. Lomax, Ph.D.,

Department of Bioresources Engineering

University of Delaware, Newark, Delaware

Table 1: Example Points for Air Properties

Point d.b. [degrees]F w.b. [degrees]F RH, %

A 65 57 62
B 65 60 76
C 65 63 90
D 60 55 73
E 65 55 53
F 70 55 37

Table 2: Approximate Relative Humidity for Air 58-65[degrees]F

Temp diff d.b.-w.b. [degrees]F RH, %

0.0 100
0.5 98
1.0 95
1.5 92
2.0 90
2.5 87
3.0 85
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Author:Lomax, Ken M.
Publication:Mushroom News
Geographic Code:1USA
Date:Jan 1, 2007
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