Introduction
The PH3 Calibration Station, developed by Centaur, is designed to facilitate the frequent calibration of PH3 sensors against gases with known concentrations. This essential procedure aims to enhance sensor precision by characterizing any potential drift in sensitivity and readings of the sensing elements.
2. The Calibration Circuit
The PH3 Calibration Station boasts a robust gas circuit comprised of high-quality components, including Teflon tubes and stainless-steel connectors and valves, ensuring durability when exposed to highly corrosive phosphine gas. The circuit is equipped with two precision regulators to maintain a consistent gas flow of 0.5 ~ 0.9 liters-per-minute to all of the coupled sensor hoods.
The schematic of the circuit is presented below (Figure 1):
Figure 1. The schematic of the circuit. Please note that this diagram is indicative.
The circuit can support one or more sensor hoods in series, allowing users to attach one or multiple sensors. Additionally, the setup accommodates PH3 gases at multiple concentrations (e.g., 500, 1000, and 2000 ppm). Given the toxicity of phosphine, it is imperative to have safety monitoring devices operational in the room during testing and ensure proper ventilation.
NB: Due to the toxicity of phosphine, it is advised to keep a safety monitoring device open in the room while running a test and make sure the room is well-ventilated.
3. Calibration Process Steps
The following steps will guide you through the entire procedure for testing a sensor and obtaining readings against a PH3 gas with a known concentration (e.g. 500, 1000, or 2000ppm):
Activate your gateway and sensors and initiate a new process on your Centaur Internet-of-Crops® account to begin collecting data from the units under calibration.
For each unit: Unscrew an available cover on the calibration station, remove the top sensor cover, and affix the sensor to the sensor hood by screwing it in well by hand. Don’t use tools.
3. Open the air source valve and set the precision valve (Vernier) at 0.5 ~ 0.9 liters per minute to introduce fresh air to the circuit, and the sensors. Maintain for 30 seconds.
4. Open the phosphine gas valve, directing phosphine flow away from the sensors and towards the exhaust.
5. Once phosphine flow stabilizes at 0.5 ~ 0.9 liters per minute, gradually and smoothly transition between the two valves, closing down the air valve and turning up the gas valve, to expose the sensors to the PH3 gas.
6. Expose the sensor(s) to the gas for a minimum of 5 minutes, while monitoring the graphs on the Centaur platform.
7. When all sensor readings reach a plateau, continue the gas flow for an additional 30 seconds.
8. Turn off the phosphine source and reopen the air valve to flush the circuit.
9. Observe the degassing process of the sensor returning to zero ppm*.
10. Repeat steps 3-9 for calibration against multiple phosphine concentrations.
11. Create sensor calibration tables on the Centaur platform (see below) and repeat the process to verify the freshly calibrated sensors’ performance.
This is an example graph of a sensors process during calibration. In this example, the sensor is calibrated against two different PH3 concentrations:
*During the calibration process, it is assumed that a sensor has returned to zero when there are consecutive readings in the range below 15 ppm.
4. Submitting Calibration Data to Centaur
After completing the calibration procedure against gases of known concentration(s), you must submit the information for inspection to Centaur technical support team. Once data are validated, you will receive your calibration certificates.
For submitting your information, please use this link.
NB: Ensure all applicable safety precautions should be followed during the calibration process due to the toxic nature of phosphine gas. Always maintain proper ventilation and safety monitoring.