Wet bulb wicks for corrosive atmospheres

Disclosed is a wick for a wet bulb, wherein the bulb is elongated with a free end and a supported end. An absorbent fabric has a portion configured as a folded over sleeve to at least partially closely surround the bulb. The sleeve has two ends, one of which is open and the other of which is closed to permit the open end of the sleeve to be slipped over the free end of the bulb and passed to the supported end until the closed end of the sleeve approaches the free end of the bulb. A plastic film lining inside the sleeve is resistant to corrosive materials to protect the bulb from corrosive materials and has a lower coefficient of friction than the fabric to ease installation over the bulb. A tail contacts a water supply to pick up water and transport it to the sleeve to wet the bulb.

BACKGROUND OF THE INVENTION 
The present invention relates to improvements in wicks for wet bulb boxes 
of humidity sensing devices such as hygrometers, RTD sensors and vapor 
tension sensors, particularly for use in corrosive environments. 
In the art of drying lumber, the lumber is dried in a kiln to remove water 
vapor. The process is monitored using a wet bulb and dry bulb temperature 
sensors to indicate the conditions of the atmosphere in the kiln. 
Typically, the humidity and temperature are carefully monitored and 
controlled to dry the lumber at a selected rate to avoid damaging the 
lumber. For example, too-rapid drying can cause the lumber to crack or 
split, but drying too slow is, of course, inefficient. 
Wet bulb sensors are well-known. One type is a hygrometer consisting of two 
temperature sensors for measuring the temperature of an ambient 
atmosphere. One directly measures the temperature, while the other is kept 
wet by a wick, so that the cooling effects of evaporating water will cause 
a temperature difference between the wet and dry sensors. The lower the 
humidity of the atmosphere, the greater the drying rate and temperature 
difference. A particularly common device for controlling the rate of 
lumber drying is a vapor tension controller. In such a controller the wet 
bulb is provided in the form of a chamber filled with a preselected, pure 
gas. The chamber is connected via a capillary to controller, so that as 
the gas in the chamber contrasts or expands with temperature fluctuations, 
this is communicated to the controller through the capillary. One known 
controller of this type is made by The Coe Manufacturing Company, 
Paynesville, Ohio. 
However, in drying lumber, the vapors emanating from the lumber include not 
only water vapor, but also acids. The acids dissolve in the water of the 
wick of the wet bulb sensor. This problem has been known a long time. 
Attempts to solve it have taken the form of providing protective coatings 
on the temperature sensors, but these have not proven to provide 
sufficiently long-lasting protection. When the corrosion eats through the 
wet bulb, changes in the gas pressure are no longer accurately 
communicated to the controller. Repair requires not only replacing the 
bulb, but also fully evacuating the chamber and some controller components 
and replacing the pure gas, all of which is quite expensive. 
The wicks which are placed on the wet bulb sensors are replaced rather 
frequently so they can adequately maintain the wet bulb condition. If the 
wicks become crusty, slimy, torn or otherwise perforated, they lose their 
effectiveness and transmit a false signal to the wet bulb. The rate of 
replacement of wicks varies, depending on a number of conditions. For 
example, in a setting where the water supply is hard, mineral deposits 
build up quickly on the wick, requiring rapid replacement. Similarly, it 
is believed that the acids in the atmosphere attack the wick itself, 
causing its early failure. Because of this and the fact that wicks are 
inexpensive, they are considered to be short-term, disposable commodities. 
Replacement of the worn-out wick in a hot kiln is not a pleasant job. This 
is particularly so if the temperature sensor has a bit of age, so that 
corrosion pits and roughens its surface, snagging the wicks. 
As a result, those of ordinary skill in the art have a need for an improved 
wet bulb wick to reduce corrosion of the wet bulb sensor, permit easy 
installation of a new wick and lengthen the lifetime of wicks. 
SUMMARY OF THE INVENTION 
The present invention fulfills this need in the art by providing a wick for 
a wet bulb including an absorbent fabric having a portion configured to at 
least partially surround the wet bulb. A lining inside this portion is 
resistant to corrosive materials to protect the bulb from the corrosive 
materials. 
In a preferred embodiment the portion is a sleeve. A preferred fabric is a 
woven fabric including absorptive cotton fibers bound by polyester yarns. 
The lining material can be any suitable material, such as a plastic or 
metal film. The portion of the wick surrounding the bulb can be formed as 
a folded over and stitched sleeve. If a thermoplastic film is used as the 
liner, the sleeve can be conveniently made as a folded over and heat 
sealed sleeve. 
Preferably, the lining has a lower coefficient of friction than the fabric, 
so the wick can be easily slipped on and off the sensor. 
Also preferably, the surrounding portion is configured to closely surround 
the wet bulb, so that accurate temperatures are reflected. 
In a commonly used wet bulb sensor, the wet bulb is elongated with a free 
end and a supported end. In such a case the wick portion is preferably a 
sleeve having two ends, one of which is open and the other of which is 
closed. This permits the open end of the sleeve to be slipped over the 
free end of the bulb and passes to the supported end until the closed end 
of the wick approaches the free end of the wet bulb, to protect the free 
end of the bulb. 
The wick also preferably includes a tail for contacting a water supply to 
pick up water and transport it by capillary action to the surrounding 
portion to transmit the temperature of evaporation to the wet bulb. 
The invention also provides a method of transmitting the temperature of 
evaporation to a wet bulb temperature sensor. The method includes 
providing a wick made of an absorbent fabric having a tail and a portion 
configured to at least partially surround the wet bulb temperature sensor 
and a lining inside the portion, the lining being impervious to corrosive 
materials to protect the bulb from corrosive materials. Also provided is a 
water supply. The method proceeds by positioning the portion of the wick 
in surrounding relation to the temperature sensor and positioning the tail 
of the wick in contact with the water supply. 
The providing step preferably includes providing a piece of absorbent 
fabric and applying a piece of lining material adjacent the piece of 
absorbent fabric. Then, the pieces of absorbent fabric and lining material 
are folded to form the portion having the lining material interiorly of 
the absorbent fabric. This is followed by securing the folded pieces of 
fabric and lining material together along a line of securement so that the 
portion is on one side of the line and the tail is on the other side of 
the line. The securing step may include stitching the fabric along the 
line. Alternatively, the securing step may include heat sealing the lining 
along the line. The applying step may include fusing the lining material 
to the absorbent fabric.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A wet bulb box is depicted in FIG. 1 as being mounted on a wall 7 of a 
lumber kiln. As will be appreciated, the invention is applicable to wet 
bulbs in numerous environments in which the advantages of the invention 
may be obtained. The wet bulb box 14 is provided with inlet piping 10 and 
outlet piping 12 to maintain a desired level of water 18 in the box 14, as 
more fully described in my prior U.S. Pat. No. 4,651,780, the entire 
disclosure of which is incorporated herein by reference. As shown in FIG. 
1, the box 14 supports one end of a temperature sensor 8. Since the sensor 
8 is of conventional configuration, it will not be described herein in 
detail. However, the embodiments contemplated for use with the present 
invention include thermocouples, RTD sensors and vapor tension sensors, 
particularly the latter, since some are made of copper and susceptible to 
corrosion. The temperature sensed by the sensor 8 is communicated along 
line 6 to a suitable, known control apparatus. 
The temperature sensor 8 is sheathed by a wick 16 in accordance with the 
present invention. As seen in the sectional view in FIG. 2, the sheath 16 
includes an outer layer of fabric 20 and an inner lining 19. The wick 
fabric includes a tail 26 extending down well into the water 18 in the wet 
bulb box 14. The fabric 20 of the wick 16 can be of any suitable absorbent 
configuration. However, a particularly preferred fabric is that used in 
conventional continuous rental toweling, provided any sizing on the fabric 
has been washed off. It is thin, but quite absorbent. The fabric structure 
includes a polyester net base for strength, with cotton fiber interlaced 
therewith for absorbency. 
Interiorly of the fabric 20 is the lining 19. The lining 19 can be any 
material which permits intimate thermal contact between the fabric 20 and 
the heat sensor 8, yet prevents the corrosive elements of the kiln 
atmosphere from reaching the temperature sensor 8. Specifically 
contemplated are metallic foils and plastic films having melting points 
above the highest kiln temperature expected. If desired the plastic film 
can be provided with a pressure sensitive adhesive to facilitate adhering 
the film to the fabric. 
If the lining 19 is made of a thermoplastic material, manufacture of the 
wick can proceed quite easily, since the plastic can be bonded to the 
fabric. Then the fabric can be folded and heat-sealed along a securement 
line to make a sleeve or pocket suitable for at least partially 
surrounding the sensor. 
While various coatings may be suitable for the practice of the invention, 
any coating on the inside of the wick must not prevent proper wicking 
action. 
Two embodiments of the wick can be seen in FIGS. 3 and 4. In FIG. 3, the 
folded over fabric is stitched along a line 22 to form a sleeve which can 
fit over the free end of the sensor 8. This construction is suitable for 
any desired lining material. If the lining material is a thermoplastic, 
the sleeve can be formed by a seal 24, as shown in FIG. 4, to form a 
pocket or sleeve which can fit over the temperature sensor 8. As can be 
appreciated, the sleeve has an open end and a closed end. 
The lining 19 serves as a barrier to prevent corrosive materials from 
coming into contact with and therefore corroding the temperature sensor 8. 
In addition, since the lining material typically has a lower coefficient 
of friction than the fabric 20, the wick provided with the lining material 
more readily slips over the temperature sensor 8, even if the temperature 
sensor 8 has become pitted by earlier corrosion. 
And, surprisingly, it has been found that the wicks themselves last longer 
when provided with the lining material 19 than prior, unlined wicks. It is 
not known precisely why this is the case, but it may be that the contact 
of the corrosive material of the atmosphere, as dissolved in the water in 
the wick, acts on the metal of the temperature sensor 8 to, not only 
corrode it, but also the wick. With the lining shielding the fabric from 
the metal of the sensor, anodic and/or cathodic reactions are reduced and 
therefore less damaging to the wick itself. 
Typically, the kiln is operated at a temperature range of from about 
95.degree. F. to about 240.degree. F. for lumber drying. The operators of 
the kilns who, in the past, were asked to replace the wicks much 
appreciate this invention. The low coefficient of friction of the lining 
material makes the old wicks slide off easily and new wicks slide on 
easily, considerably reducing the effort required and shortening the 
period of time in which they must remain in the heated kiln. 
Another advantage of providing the protective lining on the wick is that, 
as the wick is replaced, the protection of the sensor provided by the 
lining is renewed. Thus, any corrosive attack of the lining will not 
ultimately lead to corrosion of the temperature sensor. This should be 
contrasted with the prior attempts to protect sensors by coating them. 
Once that coating is corroded, the sensor is open to attack. 
The wick with the lining closely contacts the temperature sensor so that 
the difference in temperature of the bulb caused by the evaporation from 
the wick is accurately transmitted to the temperature sensor. 
As will be appreciated, various modifications to the invention as 
specifically disclosed herein can be carried out by those of ordinary 
skill in the art, and such modifications are deemed to be within the scope 
of this invention.