Grains Per Pound (GPP): What it is and Why it matters

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Grains Per Pound

“Grains per pound” is a unit of measurement which is primarily used for gauging moisture in the air. Along with dew point temperature and relative humidity (%RH), it is one of the most commonly-used measurements of air moisture. An accurate GPP measurement allows growers to gauge the actual indoor humidity of a grow room. As you can imagine, this information is an invaluable resource to allow for adjustments in order to optimize the cannabis growth environment. With that in mind, the aim of this article is to provide a comprehensive overview of what GPP means, how it is calculated, and how useful it can be as a resource for indoor cannabis growers.

To begin with, we will outline a precise definition of the term “grains per pound”. Then we will take a look at the importance of maintaining accurate grains per pound measurements within the grow room. Next, we’ll examine a few alternative methods of measuring moisture in the air, including relative humidity. Finally, we will examine how the measurement of grains per pound is calculated.

What exactly is Grains per pound?

Firstly, it’s essential to provide a working definition for the expression “grains per pound”. Grains per pound is a term that refers to the amount of moisture present within dry air. And it means grains to pounds air.Grains in a pound is an important indicator to plant.

The use of grains per pound as a measurement has its origins all the way back in the Bronze Age. When “grain” referred to literal grains of wheat or barley, etc. As such, this was a singularly unspecific term. It gained its current definition in 1959, when the International Yard and Pound agreement standardized it at its current rate. These days it is a wholly inflexible and accurate measure.

Specifically, the designation “grains per pound” represents the quantitative relation between the mass of moisture and the mass of dry air in a given sample. The moisture is measured in “grains”, a unit equal to approximately 65 milligrams. Therefore, a pound of air is comprised of 7,000 grains. In turn, a pound of air equates to 13.33 cubic feet. To put it in simple terms, grains per pound is a measurement typically used to determine moisture levels in air.Know how many grains are in a pound can help you understand the humidity of greenhouse

The importance of measuring GPP in grow room

The total amount of grains per pound can be an invaluable signifier for the level of moisture present within an indoor cannabis growth environment. The presence or absence of moisture in varying levels has been proven to exert a dramatic effect on all manner of biological and/or chemical processes, including those related to cannabis plant growth.

As a grow room is typically a closed, controlled environment, it is very important to monitor the GPP level. The reason for this is that cannabis growth produces moisture as a by-product. And an excess of this within the air can impinge on growth processes and even harm your cannabis plants.

Role of GPP

GPP is used to calculate levels of moisture within a pound of “dry air”. Dry air is a hypothetical measurement of air completely devoid of moisture. Dry air containing typically fixed concentrations of 78% Nitrogen, 21% Oxygen and 1% other gases, including Argon, Carbon Dioxide et al. As it would be impossible for air to be completely dry (even the coldest air contains some degree of moisture). So, air was be used as a theoretical signifier for the purposes of calculating GPP.

As such, the value of the grain per pound figure can be used to determine the actual humidity of a grow room. It is an invaluable resource for growers. Grains per pound indicating whether dehumidification needs to take place, or whether humidity needs to be increased to ensure optimum moisture levels for plant growth. This is because, for example, excess humidity can cause the growth of mold, bud rot and other harmful pathogens. This damages cannabis crop yield, not only harming the plants themselves but also their potential for profit-making.

Many cannabis growers control humidity using humidifiers or dehumidifiers, but there are also plenty of other techniques they might employ. For instance, regulating ventilation and air currents is an effective and simple method. Similarly, the use of lighting can be invaluable as a means of increasing or decreasing temperature and, therefore, humidity.

Alternative methods of measuring humidity

While the focus of this article is on grains per pound, it’s important to be aware that this is by no means the only measure for moisture levels. Another commonly used example is dew point saturation temperature. This refers to the temperature at which air is “saturated”. Meaning when it is retaining the maximum amount of moisture, at which point it condenses into liquid water. This is calculated formulaically, using the air temperature and the relative humidity.

Relative humidity refers to the amount of vapor present in the air in comparison with the total amount it could potentially hold. Because the humidity in question is also “relative” to the fluctuations in air temperature, this measurement is considerably more malleable and subject to change than that of the GPP. However, the two are frequently confused and conflated.

How To Calculate Vapor Pressure Deficit?

Now, we know that the maximum amount of vapor the air can hold is its SVP. And the actual amount of vapor currently in the air is its AVP. Therefore, we simply need to subtract the maximum value from the actual value to find out our deficit value.

To simplify it in formula, here is how to calculate Vapor Pressure Deficit;

VPD = SVP – AVP

P.S. Keep in mind, you can calculate the air VPD and leaf VPD separately, using the grow room’s temperature and humidity levels. Leaf VPD will help you understand the cooling effect of evaporating water on the plant and find a way to control its growth better. You will need an IR temperature to calculate leaf VPD.

What is the difference between relative humidity and grains per pound?

It is important to distinguish between grains per pound and relative humidity. As both are commonly used units of measurement which are tangentially linked, and therefore easily confused. Relative humidity is a measure of the percentage of water present in the air, while the GPP is a measurement of the specific weight of the moisture in a pound of air. That is why %RH is referred to as “relative” as opposed to the “absolute” nature of GPP. As mentioned above, %RH fluctuates dependent on temperature and other variable factors unrelated to the amount of moisture, whereas GPP does not. Bearing that in mind, GPP can be read as a more accurate reflection of the total amount of moisture in the air.

Calculation of grains per pound

Calculating the value of moisture grains per pound of dry air might initially seem like a complex procedure. However, with the right materials it is in fact a surprisingly straightforward process.

As mentioned previously, “psychrometry” is the technical term given to the study of moisture levels in the air. Therefore calculating GPP requires use of a psychrometric chart. The variables which will serve as axes for the psychrometric chart are the temperature of the air (the aforementioned dew point saturation temperature) and the relative humidity. The point at which these two variables intersect will provide the grains per pound.

At the end

As this article has demonstrated, grains per pound is an invaluable measurement when it comes to maintenance of indoor environments for cannabis growth. It provides the most accurate reflection of humidity, and therefore enables growers to monitor and control the conditions of their plants, ensuring optimum growth potential. The impact of moisture on cannabis growth is immense, but thanks to GPP it is manageable.

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