About Soil Sensors
Several months ago, I talked about the new Apollo PLT-1 Plant Sensor and it’s utility in Home Assistant. The PLT-1 is a device that measures soil temperature, humidity, light levels, UV index, as well as air humidity and temperature. The measurements are correct and on point, without significant offsets. Since it’s ESPHome-based, the PLT-1 is very tweakable and configurable, allowing you to adjust the readings and recalibrate the device if needed.
The PLT-1 is available in two variants, with and without a battery. The battery version (PLT-1B) uses a rechargeable 18650 battery which can keep the device going up to 6 months with some clever operational logic. However, the biggest drawback of the PLT-1 soil sensor is its IP rating. It’s not protected against any level of dust or water ingress, which does limit it’s utility for outdoor use.
In this article, I’m sharing my experience with the popular ThirdReality Smart Soil Moisture Sensor, model 3RSM0147Z. This soil sensor measures soil moisture and temperature only and uses a single AA battery for power. ThirdReality claims it can live for up to 3 years on a single battery and you can use it in any gardening location including outdoor gardens, greenhouses, plant nurseries, and potted plants.
Technical Specification
- Name: ThirdReality Smart Soil Moisture Sensor
- Model: 3RSM0147Z
- Connectivity: Zigbee 3.0
- Battery Type: 1xAA
- Temperature Range: -10℃~50℃ (14℉~122℉)
- Temperature Accuracy: ±1.5℃
- Humidity Range: 0～100%
- Humidity Accuracy: ±3%
- Price: $19.99 Amazon
Device Disassembly
The ThirdReality Smart Soil Moisture Sensor arrived in a long box containing the device itself, a user manual and a single AA battery pre-installed. The device measures at ‎18.03×2.54×2.54cm and weights about 68 grams. The probe itself measures at 100mm or 10cm, which makes the entire device quite long.
The device comes with an alkaline AA battery pre-installed, whose contacts are separated by a thin plastic foil you need to remove before use. The top cap unscrews easily from the base with a single twist and pull motion. There’s a rubber O-ring installed to keep dust and moisture from entering the enclosure. It’s a very simple device with a simple design.
The battery inside the ThirdReality Soil Moisture Sensor is easily replaceable. ThirdReality claims this sensor can live up to 3 years on a single AA battery. I honestly don’t think it’s possible to reach anywhere near 3 years, but anything above 1.5 years I would consider great.
On the underside of the PCB, the main connectivity module is installed along with the pairing button. The device enters pairing mode immediately once the protective foil is removed, but should you need to, you can reset it with this button. I could not discover the actual model of the SoC without ruining the radio frequency shielding, which I was not willing to do as reception could potentially suffer.
Home Assistant Integration
The ThirdReality Smart Soil Moisture Sensor is officially supported in Zigbee2MQTT and ZHA in Home Assistant. ThirdReality is in good standing with Home Assistant and they always create their own external converters and quirks for handling their devices.
Zigbee2MQTT
Once paired to Zigbee2MQTT, the device is identified correctly as a soil sensor with model 3RSM0147Z by manufacturer ThirdReality. It’s set as an End Device and the correct device picture is applied. Here’s what it exposes in Home Assistant via Zigbee2MQTT:
All entities are self-explanatory. The sensor reports soil temperature, humidity, remaining battery and signal quality. The update interval of the ThirdReality Soil Sensor is set at 5 minutes by default.
ZHA
The same entities are exposed when the device is paired in ZHA. For once, nothing is missing and everything is available to the user. Temperature, humidity and battery are all reported correctly.
Installation and Testing
In order to benchmark the ThirdReality Soil Moisture Sensor, I installed it in one of my office plant pots alongside the Apollo PLT-1. Both devices use capacitive probes for measuring soil conditions, which are much more durable and less affected by soil salinity than resistive probes. Capacitive probes measure the water content in the soil by detecting changes in the dielectric permittivity of the soil.
Even though I used the wired version of the Apollo PLT-1 in my tests, I took a photo of the battery version too, just to showcase the size difference. Here’s how they look like installed in a plant in my office:
The Apollo PLT-1B is much larger and bulkier, since it relies on a larger 18650 battery. It spans horizontally instead of vertically like the ThirdReality sensor, so it will be trickier to install in smaller pots. The ThirdReality sensor is not small though, and towers over smaller sensors like the Xiaomi MiFlora Plant Sensor.
There are other differences between the two sensors, but I tried my best to create an equal environment for obtaining accurate measurements. I recalibrated the PLT-1 by recording the dry voltage of the sensor, a procedure I detailed here. I also configured the update interval of the PLT-1 to 5 minutes in order to match the ThirdReality Soil Sensor.
After several days of collecting data, here’s what I got in Home Assistant:
Both devices show a similar trend in moisture readings. The graphs are nearly mirrored, though discrepancies exist. When I watered the plant (morning of March 16th), the ThirdReality sensor registered a significantly higher moisture level, subsequently declining and stabilizing near the PLT-1’s reading. This was the sole instance where the ThirdReality sensor exceeded the PLT-1’s measurement.
Most of the time, the ThirdReality sensor reported lower values than the PLT-1. While the difference varied, my calculations indicate a median difference of approximately 5%. Whatever the actual number is, it’s fair to point out that the ThirdReality Soil Sensor accurately reflects the soil moisture trend. It’s also important to clarify that my analysis relies on the assumption that the calibrated PLT-1 provides accurate measurements. I do not have lab-level equipment to obtain an accurate number.
The temperature graph was much more similar compared to moisture. The reported values closely mirrored each other, but again the ThirdReality sensor reported about 0.5°C lower on average. ThirdReality claims that the temperature reading is only an estimate, but it seems to report quite correctly in my tests.
Final Thoughts
If you are looking for a simple and straightforward soil moisture sensor, than I believe the ThirdReality 3RSM0147Z is a great pick. It measures soil moisture quite accurately and follows the trend of the Apollo PLT-1. While the actual number may not be 100% correct, it’s accurate enough to give you a clear picture of the current state of the soil.
Further, the ingress rating of this device is quite good. The top cap is molded from one whole piece which prevents any water from entering the enclosure. You can freely water you plants with the ThirdReality soil sensor still installed.
And last, if Zigbee is your protocol of choice, than I think there isn’t a better device out there. It costs much less compared to similar sensors and does what it promises. If you prefer using ESPHome-based devices and the flexibility they offer, than the Apollo PLT-1 is great. If you don’t mind Bluetooth as well, than check out the Xiaomi Mi Flora.
Here’s where you can get the ThirdReality Soil Sensor:
Thanks for the complete review ! This device is way less bulky than the one I use from aliexpress which uses 2 batteries. I’ll probably give it a go next time I need one 🙂