Getting the most out of sensors

The decisions you make can only be as good as the information you use to make them. The same is true for your greenhouse controls. The data you collect can only be as good as the sensors making measurements. Whether it is a simple thermostat, a STEP controller or an advanced computer, these controllers all utilize sensors measuring some environmental condition(s), such as air temperature, light intensity, humidity or vapor pressure deficit, or cultural parameter such as nutrient solution or substrate electrical conductivity (EC) or pH. These measurements are then made to control greenhouse appliances and equipment such as unit heaters, fans and pads, vents, supplemental lights, misters, fertilizer injectors or dosers, or irrigation solenoids. Or, the data is being collected for logging information and keeping track of conditions for future references. Regardless of what it is used for, there are best management practices that ensure you get the most out of every measurement.

Air temperature is the most measured greenhouse parameter. Ensuring air temperatures are maintained within ranges that are conducive to crop growth and development is arguably the most important function a greenhouse performs. There are a few key points to getting accurate and reliable air temperature measurements. First, place the sensor where the plants are, not where it is convenient for you. This may seem obvious, but it is not uncommon to find temperature sensors placed several feet or more above plants. The further the sensor is from the plants, the less representative the measurement is of the air temperature the plants are experiencing.

In addition to placing temperature sensors by plants, we want to minimize the effect of light on temperature readings. Sunlight has radiant energy that warms solid objects. If sensors are not protected from the sun, or other light sources with radiant energy like high-pressure sodium lamps, readings will be higher than the actual air temperature due to the radiant energy warming the sensor. Sensors should be provided with some type of solar shield to keep direct light off the sensor and eliminate radiant heating. Along with shielding temperature sensors from direct light, it is also important to avoid stagnant air around the sensor, which may also result in a reading not representative of the greenhouse. Sensors should be aspirated, having air movement around the sensor. This can be achieved passively, with sufficient space in any solar shield to allow air to move freely across the sensor, or actively, with the use of a fan to move air across the sensor. If there are filters used in conjunction with aspiration, they need to be cleaned and replaced regularly to ensure sufficient airflow, much like your home’s furnace.

Photo courtesy of ©Bob | Adobestock

Another common sensor used in greenhouses are quantum sensors to measure light intensity. Some of the same best-management practices used for temperature sensors apply to quantum sensors. First and foremost, place these sensors at the plant canopy level to measure the light intensity plants are actually experiencing. Placing them higher may result in exaggerated measurements and give the impression there is more light than there actually is. Unlike temperature sensors, you obviously do not want to shield quantum sensors from the sun. However, you also do not want to place them in areas that are permanently covered by shadows, since this will result in lower-than-actual light intensities. It is also important to keep light sensors clean. Putting a small amount of deionized water on a tissue and gently washing the surface of the sensor will keep it free of debris that could reduce measurements.

All sensors should be periodically calibrated to ensure accurate measurements. However, some sensors (such as those for measuring pH and EC) should be calibrated more frequently due to the potential for more drift than compared to, say, a thermocouple or quantum sensor. Maintaining fresh reference solutions for pH and EC sensor calibration is key for maintaining accuracy, as old solutions may not be reliable references, especially once moisture is lost from them and their actual pH or EC values change. A simple way to ensure timely solutions are used is to write the date they were purchased on the bottle as soon as you receive them.

The sensors we use in our greenhouse for monitoring and controlling the environment and culture are not like crock pots. We can’t just set them and forget them. Rather, we should provide them with routine maintenance and smart placement so the information we are using to grow our crop is accurate.