Substrate temperature

The temperature of the growing substrate is generally lower than the ambient air temperature because of the cooling effect of water evaporation. It is important to know the temperature of the substrate because it affects both pH and electrical conductivity.

The temperature of the substrate in the GrowCube is measured using three DS18B20 digital temperature sensors as pictured below.

Datasheet

DS18B20 sensors are very efficient in that they consume very little power, and several sensors be attached to the same signal wire. Each sensor has its own identification number which is added during manufacture. The software can therefore collect data from multiple sensors in a single step. However, in the GrowCube node, this creates a problem. Essentially, there is no way to know which of the probes is in which layer of the GrowCube.

Identifying your temperature probes

To fix the identification problem, you need to go through a procedure that lets the T5 know which probe is which. Essentially, the procedure consists of warming each probe in sequence so that the T5 can associate a warm probe with a particular GrowCube level.

Although a probe could be warmed in several ways, the simplest method is just to wrap your hand tightly around it. Your body heat will raise the temperature sufficiently to cause a noticeable difference.

To start the identification procedure, reboot the node using the reset button. When the menu appears, choose Identify temp probes as shown below. There follows a series of prompts that take you through the rest of the steps. After you have completed the instruction in the prompt, you press the control button to tell the T5 that you are ready to move on to the next step.

The main steps are

1. Hold the top layer probe
2. Hold the middle layer probe
3. Hold the bottom layer probe

Once the procedure is complete, the T5 will automatically reboot.

What can go wrong?

• Sometimes the electrical connections between the probe and the circuit board can become loose. This can prevent the T5 from reading the sensor. You can identify this situation by carrying out the probe identification procedure. It should report that three probes are found. If there are fewer that three probes recognised, going through the identification procedure will reveal which one has a problem. The solution is to open the sensor enclosure and to ensure that the probe is firmly seated on the pins as shown below. If it feels loose, push it onto the pins and then secure it with some glue. NB: Make sure that the red wire is on the left as shown in the image.

• There can be a similar problem with the connections between the probe cable and the connector that sits on the pins. Occasionally, the metal connectors are not pushed far into the black plastic housing. The solution is to use a small tool such as a jeweller's screwdriver to press the connectors down into the housing as shown below. You can go through the identification procedure again to check that the problem is solved.