Rarely does a week go by without a restoration contractor asking me about grain depression. The question usually goes like this:
“I am being asked to remove a dehumidifier from my drying invoice as it did not have at least a 5-grain depression in a 24-hour period. I needed that dehumidifier on the job as we were not done drying, and I know it was working based on my other psychrometric readings, so how do I get paid for that dehumidifier?”
Sound familiar to anyone else?
There is considerable discussion these days about grain depression, and whether or not it is an accurate measurement of a dehumidifier’s performance. Should restorers be paid for dehumidifiers that are not creating grain reduction every day on a drying project?
I think restorers should be paid for them regardless of grain depression. I feel only using grain depression to determine a dehumidifier’s effectiveness is a very limited approach, and we should remove this reading from our moisture documentation. If that be the case, what is the best way to determine if a dehumidifier is working properly?
In order to better answer these questions, we need to first define the goal of restorative drying. Page 49 Section 10.6.6 in the 5th Edition S500, states: “The goal of any drying project is return wet materials to their pre-determined dry standard, or within 10% of that standard.” Thus, the focus of all drying must be the removal of excess moisture from materials. When the focus becomes more on grain depression than on moisture content, we have gotten way off track and lost sight of the goal. However, understanding that equipment usage should be justified, let’s also define a few basic psychrometric measurements that are evaluated as part of the drying process.
Per the 5th Edition IICRC S500:
Relative Humidity (RH): the amount of moisture in a volume of air at a given temperature, measured as a percentage. Simply stated, the percent of moisture the air can hold at that given temperature. Change the temperature, it changes the percentage of moisture that air can hold.
Humidity Ratio: the ratio of the mass of water vapor to the mass of dry air, expressed as grains per pound (gpp). Weightwater vapor / Weightdry air. In simpler terms, how much wet air weighs as compared to the same volume of dry air.
Dehumidification: the process of removing moisture from the air.
Grain Depression: …hmmm, this term is not defined nor used anywhere in the current or previous (2015) edition of the IICRC S500.
How can we as an industry use the term grain depression if it is not recognized in the S500 Standard? Perplexing to say the least. If the goal of dehumidifiers is to provide dehumidification, then the measurement of their performance would be their ability to remove moisture from the air, or reduce relative humidity. Unfortunately, it is not that simple when using grain depression as a performance metric.
Grain depression is difference between the grains in the ambient air minus the grains exiting a dehumidifier. Theoretically, if the grains leaving a dehumidifier are lower than the grains entering the dehumidifier, the dehumidifier is working and therefore reducing the weight of water in the air, creating a grain depression. Good in theory but not in practice. Several issues exist with the accuracy of this measurement.
The first challenge when measuring grain depression is the accuracy of the instruments taking the measurements. Less expensive thermo-hygrometers have an error rate of more than 3% RH, and are even more prone to error at higher temperatures, common on many drying projects. Better quality thermo-hygrometers can have an error rate of 2% RH. Take a meter that hasn’t had reasonable acclimation time or needs calibration and the error rate could be even greater. This alone can cause inaccurate readings and quite frequently does. Example:
Another consideration when evaluating grain depression is the actual CFM (cubic feet per minute) of the dehumidifier. A large dehumidifier may process 230 CFM versus an extra-large unit that could process 310 CFM. They could both create the exact same grain depressions, but the larger unit is processing almost double the amount of ambient air, therefore outperforming the lower CFM unit.
In very dry air, low-grain refrigerants (LGRs) will not continue to reduce grains; they effectively become a space heater. Yet one that is still controlling indoor humidity and providing warm dry air to the drying environment. But if grain depression is the only reading that matters, it would be hard to justify their use on a job.
Ever had grains go up instead of down? Absolutely. This often happens when indoor temperatures are very warm, essentially baking moisture out of materials. As the moisture leaves wet materials and enters the air, grains could be elevated for a period of time. This certainly doesn’t mean a dehumidifier is not working, it just means more moisture has been added to the drying atmosphere.
So ultimately, how should the industry, as a whole, evaluate a dehumidifier’s performance?
The 5th Edition IICRC S500, Section 12.5.6 states:
The primary indicators related to dehumidifier performance are the ongoing control of the drying environment and a resultant reduction of material moisture level or moisture content.
Therefore, the goal of restorative or structural drying is to return materials to their normal moisture content. I believe using grain depression as a measurement of a dehumidifier’s performance is an incomplete assessment. The restoration industry, contractors, and carriers alike should be more closely monitoring drying temperatures and vapor pressures, as they play a significant, critical role in the drying process. Dehumidifiers are simply there to reduce relative humidity, which they do by either increasing temperature or reducing grains.