Moisture Monitoring in Kiln Operations

A technical discussion of how to keep track of drying conditions in a kiln in real time. April 18, 2010

I am looking for leads on an in-kiln stack MC measurement and data logging system. I remember seeing a multi-probe system which went into the stack years ago. I can't seem to locate a company that still exists and offers such devices.

I can build my kiln control system with atmosphere data logging using Omega parts. Cheap, easy. What I can't seem to find is data logging from inside the stack. Kiln is a 2,000 bdft DH built around a Nyle 200. I am not looking for a monster softwood steam based kiln control system - which seems to be everywhere. I am drying tricky, one-off loads of odd hardwoods.

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Years ago, Wagner Electronics had a system used for softwoods. It does not really measure MC, but rather responds to the overall conductivity and impedance of the stack in the proximity of the probe. It works best with the same species, same stacking, same size pieces, same location, metal tracks. It provided one of several estimates of the MC; by itself was not highly accurate all the time.

From the original questioner:
Gene, I am honored to see a response from you. Do you think it makes any sense to bother correlating the kiln chamber atmosphere data to the electro-conductivity data from the stack? I know how to do the physical weight measurement. I am nagged by why there are no stack MC loggers out there. Do you think they are useless? That would explain the absence from the marketplace.

From Professor Gene Wengert, forum technical advisor:
Thanks for your kind words.

We need to separate the need to measure the MC of hardwoods and softwoods for interior use and the MC of softwood construction lumber. With hardwoods, we do not want to know the average MC. Rather we are concerned about the driest and wettest, which must be close to each other. The average can be rather low, but if there are a few wet pieces (or a few over-dry ones), we will have trouble.

The only way to accurately determine MC of wood is to use the weight system. The accuracy required (o.1% MC) and the high RH in the kiln and temperature effects mean that electrical methods are often not suitable.

Using a weight-based system that measures the weight of a stack, there are problems because the stickers are also wood and are changing weight. Also, determining the initial MC is difficult. There have been a few systems that weigh an individual piece automatically (in the same manner that most hardwood drying uses manual weighing). In these systems, I have not seen a system that performs accurately without a lot of attention. If you will spend a lot of time with the system, then manual weighing perhaps with a computer to record the data and make the calculations is better. Also, with some species, visual inspection of the samples is also important, so if the samples are to be inspected, manual weighing at the same time is not an excessive process.

Note that the critical part of drying is initially (which is when most defects develop and this may be during air drying for a lot of operations) and at the end (getting the correct, precise final MC). We might consider manual weighing in these critical times with automatic weighing in between. Certainly, the automatic MC systems today that also have kiln control systems are exceptional because of their ability to precisely control kiln conditions.

For softwood construction lumber, we can use the average MC somewhat. A few wetter pieces are not totally a disaster. However, the better we control final MC, the better the product.

From contributor S:
What's wrong with watching your wet bulb/dry bulb deviations and a good MC meter? 2k bf is not a very big load to monitor. In my opinion, get a good kiln schedule book, follow the recommendations and all's good.

From Professor Gene Wengert, forum technical advisor:
Although we can measure the MC in a kiln holding 100,000 BF of wood, we do have to appreciate that often the kiln conditions vary from top to bottom and along the kiln's length. Hence, an accurate system that is automatic (probably expensive too) will not provide much benefit to a company if the kiln conditions are variable. To address the variability, I developed the zone kiln back in the late 70s. We used TDAL as an estimate of MC and therefore as a control feature within a zone. Unfortunately, some kilns did not correctly measure the TDAL and gave poor results at times.