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Wet bulb accuracy9/9
The last charge of 5/4 RO in one of our two older 120,000bfm kilns suffered extensive honeycomb. I am suspecting that the wet bulb performance may have been a factor. (We use a wick over a RDT. Yes, we rinse it out before use so that is soaks properly) I have read on this site that water temperature can affect readings. We have had issues with water flow control. We have used a basic ball valve tap to adjust flow. Since the last damaged charge, a needle valve was added to gain more flow control. I have read however, on this site that water being too cold or too hot can affect readings. What temperature is right? Should there be overflow in the reservoir or should it only fill at the rate that evaporation occurs? As a secondary question, should I just say 'screw it' and put in a recommendation to upgrade our kilns to computer controls with wafers? Is it worth the cost of the upgrade?
Please do not go to EMC Wafers, they are not what them seem in my experience with Oaks in the kiln. We have 5 kilns with them in it and are in the process of going back to wet bulb, which is how our other 10 kilns are. We use an adjustable float valve in our wetbox which meters in water to whatever level we prefer, we also use a sock not a wick over our RTD. I cannot see how you would control the temperature of the water in the box considering it is always going to be metering in slowly and be near the temp in the kiln, in other words I believe it does not effect the RDT. The largest factor is to check against your wet bulb with another thermometer/sock this should give you an idea and could tell you whether your RTD is junk. There is a way to test your RTD through resistance in certain temp water, you will have to research the process since I cannot remember where I found that info. Start by testing with thermometer then move on up the chain, I have found its always me overthinking what is wrong.
Confirm correct operation by comparing the dry bulb rtd and the wet bulb rtd with the wet bulb rtd dry (no wick, and placed near the dry bulb rtd). Readings at that point should be identical, or at least within sensor precision and accuracy.
There are thousands of kilns using wafers instead of WBs without any problems. The wafers are very effective if the clips or clamps are kept clean. Almost everyone used Lignomat wafers. Josh...what brand did you use? What brand measuring system did you use? It should not take long for an expert to figure out what you are doing wrong.
I agree with Bill on the simple test to confirm calibration.
In my experience, the big issue with WBs is that the water box is too big so that the air flow over the wick is very low, or the WB is located in an area that has low air flow.
However, almost always honeycomb in oak is the result of a surface check or end check getting worse. So, to control honeycomb, you need to control surface checking or end checking. These both occur at very high MCs...above 50% MC for red oak. So, if this wood was first air dried for a few weeks or longer, the issue is air drying problems and not kiln. Further, if air dried with small checks only, but you start with a high EMC in the kiln that adds moisture back to the surface quickly, you will almost instantly create honeycomb that will worsen even with normal drying.
Did you cut surface checking samples when you loaded the kiln to find out how much checking you had initially?
What was the initial MC of the lumber when it went into the kiln?
Did you get a sample that represented the wettest lumber in the kiln? If not, you may have raised the kiln temperature too soon.
What kiln schedule did you use? If you had checking prior to going into the kiln, did you use a schedule for checked oak rather than the regular schedule?
What was the daily drying rate? If the WB was in error, the rate would be too high at higher MCs.
So, I think you need to also look at other aspects of your drying operation before working on the WB.
Gene we use Lignomat controllers on all 15 of our kilns with 5 having wafers. The wafers are not as accurate as a wetbulb but you can get by with them just fine. You can clean them and calibrate them and pray over them and when dirt gets on them within the first day of drying they lose accuracy. So then you can clean them again, which takes time. The calibration alone can take 30 minutes to perform correctly. When you introduce more parts to electrical equipment you run into problems. With a proper wetbox, metering float and wetsock you have very few moving parts with a minimal amount of failure. We get our parts from SII. In short the wetbox is a sure fire timesaver and very accurate, whereas the wafer is not. And considering I know the cost of upgrading I believe it is better to keep the wetbox than change over to something that may or may not work for someone. Lignomat has been a great company to work with and I am not bashing their equipment, I am just explaining what experience we have had and would like to lend advice before someone spends a lot of money.
I did a study comparing wafers and WB and documented that wafers were more accurate. I am not sure why you are having problems, but it is correctable. I do not sell wafers, but they better than your experience.
Well it is great that you have done a study on this, but I have actually used the wafers over and over again and can confirm that their accuracy decreases when you start the kiln. I suppose you would like to also tell me that lumber probes are more accurate then cutting a core. Wafers and probes work off the same electrical principal and both are not accurate above 15% and most off all time consuming. In the case of drying 1.2 million a month we cannot afford to waste our time on a system that is inconsistent. You are more then welcome to study a real working system under real time constraints. As much as I respect your opinion I disagree with you on the basis of experience with the system.
I would not tell you that the lumber probes are better than weight using moisture sections, samples boards, etc. They are not. In fact, sometimes the pins are way off. However, sample boards must be chosen and prepared correctly, or else they will be off. The correct way is the technique specified by the developers of the kiln schedules.
So in fact you are stating that moisture probes are inaccurate which indeed means that wafers are inaccurate on the basis that each work off the same electrical system. With lignomat probes, the same transmitter collects information from the EMC station which holds the wafer. It is still two hunks of metal with cellulose conducting an electrical signal, which is inconsistent due to the structure of the cellulose fiber and the environmental changes in which the wafer is put through. Whereas the wet sock we use is a consistent material with an RTD and one electrical connection with very little variability. When troubleshooting this system it is quick and easy which saves time and calibrating/checking this system is quick and easy which saves time. In the world of lean manufacturing this system reduces waste of my time and allows me to focus on other tasks and be more effective, which in turn makes my company more profitable.
Probes driven into the lumber are not very accurate when the MC is above 28% MC. Yet knowing the MC at these higher MCs is very important as that is when all defects, except cup, are formed. A wafer in a kiln is measuring the moisture in the kiln when conditions are under 100% RH, so that the wafer does not have the problem of trying to work at high MCs, but rather it is working oftentimes between 3% to 20% MC. In this range, the resistance of the wafer is quite accurately correlated to the EMC (or RH) of the air.
It is surprising how consistent the relationship between RH and MC is when under 22% MC. Species is not a factor in most cases. With a wafer, as it is really a paper product, the relationship is extremely consistent.
Certainly, the DB and WB are the most accurate method of measuring the kiln conditions precisely, assuming that the wick is muslin with 600 fpm air flow and the water is pure (not well water or city water or boiler water). In practice, most WBs are inadequate because the air flow is too low, the bulb is too close to water that is very hot, the wick is dirty and should have been changed, etc.
It is very favorable to note that the WB temperature in the kiln is constant, if the vents are closed. (Constant from a commercial lumber drying standpoint.) The reason is that with closed vents, the process is adiabatic...in case you were wondering. That is why we have only one WB in many kilns...sometimes two in a very long kiln (over 80' long). However, the DB can vary greatly creating varying RH and EMC. So, an EMC wafer can give different readings in different locations, depending on the variation of DB. Likewise, the depression and DB will vary in different locations. For this reason, when comparing DB-WB systems and EMC wafers, the sensors must be in essentially the same location.
For what it is worth, I measure the EMC of twenty-two wafers using the Lignomat wafers and clamps with all the wafers being but in a room that was precisely controlled to 65% RH and had no temperature variation. All the EMC readings were within 0.1% EMC.
One really neat feature of drying is to look at the EMC difference from one side of the load to the other. This number gives us info about the rate of drying. We can get the same info if we used two DB readings, one on each side of the load, but most controllers switch to the entering DB when the fans reverse. Using two DB readings resulted in the TDAL kilns I developed for Southern pine drying over 30 years ago.
One final bit of info. Assume a kiln is at 130.0 F DB and 120.0 F WB. This is 73% RH, 10.0 F depression and 12.1% EMC. Now, change the WB to 119.0 F. This one degree WB change creates 71% RH (a 2% RH drop) and 11.5% EMC (a 0.6% EMC drop). The basic question is "How close must the DB, WB, RH, depression, and / or EMC be to the value in the schedule? Must we be within 0.1 degree F; 1% EMC, 3% RH, or what? The schedules (developed over 60 years ago) are not very precise. In fact, they were developed with a built in safety factor as the developers knew that the kiln instruments 60 years ago were poorly calibrate and not precise. So, are we fooling ourselves somewhat today when we think hat the depression must be 10.0 F? Further, back 65 years ago, we saw a lot of kilns running at 200 fpm air velocity, yet today we see a lot at 350 fpm. This higher velocity means faster drying even though the DB and WB are at the specified value. It is for these reasons that I developed the drying rate concept for monitoring drying so that we followed the lumber's response (daily moisture loss rate) to the kiln's conditions.