Frequently Asked Questions

What is GDD (Growing Degree Days)?

How is Growing Degree Days (GDD) calculated?

Growing Degree Days (GDD)

Growing degree days are used to estimate the maturity of crops during a growing season. We calculate GDD10 values (GDD50 for Fahrenheit), which means that the average daily temperature is accumulated only if it is above 10°C (50°F), and below 30°C (86°F). More specifically, we use the standard GDD formula:

GDD10 = max(0, Tavg−10)

GDD50 = max(0, Tavg−50)

where Tavg is a day's mean temperature. Each day's GDD value is then added to the current total for the period, and this summed result is displayed on the web page.

Click link to see Harvest systems that can be used to remotely display GDD  Harvest ITU


What are Richardson Chill Units?

How are Richardson Chill Units (RCU) calculated? 

Richardson Chill Units (RCU)

Plants need a certain amount of cold weather during the winter in order to mature properly later on. Straight chill units simply count the number of hours below 7°C, but we use the Richardson Chill Units to provide a more accurate model for orchards and vineyards. First we calculate the average temperature for each hour, and then use the table below to accumulate the RCU:

Temperature (°C)Temperature (°F)RCU (per hour)
T < 1.5 T < 34.7 0.0
1.5 ≤ T < 2.5 34.7 ≤ T < 36.5 +0.5
2.5 ≤ T < 9.2 36.5 ≤ T < 48.6 +1.0
9.2 ≤ T < 12.5 48.6 ≤ T < 54.5 +0.5
12.5 ≤ T < 16.0 54.5 ≤ T < 60.8 0.0
16.0 ≤ T < 18.0 60.8 ≤ T < 64.4 −0.5
T ≥ 18.0 T ≥ 64.4 −1.0

Click link to see Harvest systems that can be used to remotely display RCU  Harvest ITU


How is soil moisture calculated?

Soil Moisture Measurement using the Topp Equation

In June of 1980, G. Clarke Topp and his team members, J.L Davis and P. Annan, published a watershed paper which included what would come to be known as the Topp equation. The equation is so commonly used that some people don’t even know they’re using it. The Topp Equation makes it possible for us to measure an electrical property of the soil (the dielectric permittivity) and correlate that electrical property with the water content in the soil. The beauty of the equation is that it is pretty accurate in most typical soils, saving us the job of doing custom calibrations.

The equation specifically is: 

T (m3/m3) = 4.3 X 10-6 * e3 - 5.5 X 10-4 * e2 + 2.92 X 10-2 * e -5.3 X 10-2

Topp and his team showed that the volumetric water content of soil can be determined from the apparent dielectric constant of the soils, independent of soil type, soil temperature, and soluble salt content.

True Time Domain probes like the Acclima measure dielectric permittivity and report that value as well as calculating and reporting the Volumetric Water Content (VWC) using the Topp equation.

Reference - Topp, G.C., J.L. David, and A.P. Annan 1980. Electromagnetic, Determination of Soil Water Content: Measurement in Coaxial Transmission Lines. Water Resources Research 16:3. p. 574-582.

Click link to see Harvest systems that can be used to remotely display soil moisture  Harvest ITU

Click link to see Acclima true Time Domain soil moisture probes  Acclima True TDR Probes


What is dew point?

How is dew point calculated?

Dew Point

The Dew Point is the temperature at which a pocket of air (in this case around the temperature/humidity sensor) would form dew.

To calculate the Dew Point temperature, the unit requires a temperature and humidity sensor to be attached.

The Dew Point temperature is directly related to the temperature of the air in a given humidity. If the humidity is 100% then the Dew Point temperature will be equal to the temperature of the air. As the humidity decreases, so does the Dew Point temperature.

The Dew Point is helpful for predicting if and when dew will form on the vines. It is also important to note that if the Dew Point falls below freezing (0°c) then it is known as the frost point. This is a good indication that any dew forming will instead be in the form of a frost.

Click link to see Harvest systems that can be used to remotely display dew point  Harvest ITU


What is the best soil moisture sensor?

True Time Domain Reflection (TDR) technology provides the most accurate results of any electronic moisture probe. It is far superior to older style capacitive probes which give inaccurate readings when soil conductivity changes (eg after adding fertiliser). Unlike capacitive probes, True TDR probes do not need external calibration.

Click link to see Acclima true TDR probes  Soil Moisture Sensors

Click link to see Harvest systems that can be used to remotely display soil moisture Harvest ITU