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Temperature and Humidity Measurement Accuracy and Drying Kiln Control

Instruments have limited accuracy. So to fine-tune a kiln's performance, you have to monitor the drying progress by weighing wood samples. July 29, 2012

Measuring Kiln and Predryer Temperature and RH (or EMC or WB depression)
From Professor Gene Wengert, forum technical advisor:

The drying rate is controlled by temperature... hotter is faster. However, it takes quite a few degrees of extra heat to result in a small increase in drying speed. Hence, accurate temperature measurement (air temperature or dry-bulb) is not a critical factor in drying. One or two degrees off from the absolutely accurate value is not an issue.

Drying rate is dominated by the relative humidity (and air flow at high MCs, but that is another posting). Although I will use RH here, appreciate that EMC and depression are the same effect with different measurement terms; that is, if you know one value, you know the other two.

When drying a wood like oak or hickory (and even other species like maple), at high MCs, especially above 40% MC, it is critical to have the correct RH. A variation of 3% RH with oak can easily result in 20% faster or slower drying; that difference can be very serious in terms of checking and staining.

This is a measure of how many digits are present in the reading. Or how close can you read the temperature.

This is a measure of how close the reading is to the perfectly accurate temperature.

In years past, temperature in a dryer was measured with a 1 degree F sensor. That is, the closest you could read the temperature was 1 degree. If you had a reading of 105 F, the temperature could be 104.5 to 105.5 and you could not read any difference, where you were using a thermometer or a chart recorder. This is the resolution of the reading. In the old circular charts, the ink trace was often 1 or 3 degrees wide, so should you use the middle of this wide line or one of the edges? All in all, the uncertainty was very high. Sometimes, in the old days, we had 2 degree thermometers, so the situation was worse.

On a thermometer with a liquid (mercury in the old days), the etching of the lines and the continuity of the column were also factors. It would take several hundred dollars at the time to get a thermometer that was also accurate to better than 1 degree F; in other words, most thermometers were not that accurate. That is, it was common to find thermometers that were off by a degree. Comparing the readings of three of them together and using the two closest is certainly a good idea, but the two chosen can still be off from the true value by a degree, easily; the one not chosen could actually be more correct.

This problem with thermometers still exists today. It is virtually impossible to find a temperature device that will measure closer than 1 F unless one spends over $1000. For example, the Fluke 971, which is a commonly used meter in drying, costing $225, the resolution is 0.1 degree F (the screen shows 105.2 F), but this value is subject to an absolute error of plus or minus 2 degrees F. So, at 105.2 F, the true temperature is 103.2 to 107.2 F. Granted, most of the instruments will not be at the fringe, but closer to the middle. This sounds bad, but it is not much of an issue on drying lumber, even 8/4 oak.

Bottom line: A handheld device cannot be used to calibrate a dryer control system real accurately, but it can get it close enough for all practical drying purposes.

We really have to know the humidity accurately (maybe 1% RH) in order to get the correct dryer conditions (i.e., follow a kiln or predryer schedule).

When using a wet-bulb, the same discussion about resolution and accuracy applies to the wet-bulb as mentioned for the dry-bulb. However, when using a wet-bulb, the important value is the difference between DB and WB. In other words, the potential problem is twice as large... The DB can be 1/2 F too high and the WB 1/2 F too low and so our error in depression is 1 F which is 3% RH or 20% drying variation. And this discussion does not include errors introduced by having air flow under 600 fpm (commonly found in pre-dryers when the WB is behind a beam and the air flow is very low anyway). We can try to minimize the problem by choosing two thermometers, old liquid type or electronic, that read the same when they are at the same temperature... In other words, for this test, we make the WB into a DB and then put the two sensors next to each other. But even this has problems, as the temperature we read in a thermometer will be one value if we approach from the cooler side (the sensor is initially cooler than the air) and a different value if we approach from the warmer side (sensor is initially warmer than the air).

The Fluke instrument mentioned has a humidity accuracy of plus or minus 2.5% RH (10% RH to 90% RH). This does not consider age related drift in calibration. This span in accuracy at $250 is potentially plus or minus 20% in drying speed (translate into checking or staining risk).

Our best opportunity for accurate measurements is to use a system that has been calibrated in closely controlled factory conditions (such as the controls from a reputable manufacturer). This calibration will last for decades and will be closer than anything else unless we spend more than $1000 for a handheld.

For this large uncertainty, we find that the best procedure is to measure the drying rate of the samples. In other words, we find out how the wood is reacting to the dryer抯 conditions. This obviously requires properly prepared samples and properly located samples. Further, the stacking of the packs and the loading of the dryer must be correct. Stated another way, we used the standard kiln schedules (temperature and RH) and fine tune them with the drying rate info.

Forum Responses
(Sawing and Drying Forum)
From contributor B:
Thank you for your information; like the last one, it is very particular and tends to make a person think that it is splitting hairs to get the art of drying wood correct, but as I have a tendency to screw things up, a little extra info is always welcome.

I downloaded a paper by Richard D. Bergman, dated October 2008, titled "Operation and cost of a small dh dry kiln" (your name was mentioned as a reference in it), and am planning on putting one together. I have no experience, and will be working from schedules I hope to find in the database. I will be drying mostly big leaf maple and local (NW Oregon) black walnut. The only modification I plan to make will be a heater I will use to sterilize the wood; I will use a hot water heater and a recirculation system through an automotive radiator and fan to get the room up to temp. Any suggestions will be welcomed.

From Professor Gene Wengert, forum technical advisor:
To maintain color in maple, make sure the initial RH is under 60%. You may have to reduce the amount of wood in the kiln to achieve that RH. With walnut, use a good end coating and make it thick enough.