SENSORS MEASURING TEMPERATURE, HUMIDITY AND AIR QUALITY IN CLOSED ENVIRONMENTAL SYSTEMS

SENSORS MEASURING TEMPERATURE, HUMIDITY AND AIR QUALITY IN CLOSED ENVIRONMENTAL SYSTEMS

Peter Safir

Bachelor of Science, The Azrieli College of Engineering in Jerusalem (JCE),

Israel, Jerusalem

ABSTRACT

In this article, I would like to discuss and analyze liquid and air temperature, and the use of humidity and air quality sensors for enclosed greenhouses. Also, the methods and subtleties of configuring and controlling temperatures and humidity with the help of the embedded system for these sensors will be disclosed.

Keywords: sensor, temperature, humidity, embedded system, CO2, SHT1X, Senseair S8, CJMCU-811, DS18B20.

Introduction

It is no secret, that one of the most important parameters when growing plants in indoor systems (and not only) is the right amount of humidity, ambient temperature, temperature of irrigation water, and suitable air quality. Many gardeners often opt to install CO2[1] gas bottles to improve the air quality and increase their harvest. In some cases, good light and watering can increase yields by as much as 20%. But in high concentrations, CO2 can be harmful to humans and plants. Therefore, in many greenhouses where CO2 levels are artificially high, it is usually necessary to ventilate the greenhouse to ensure it complies with acceptable limits for people to work there. t. After a series of experiments it has been determined that optimal concentration of CO2 to increase yield by 20%, is a concentration of 1200-1300 PPM[2]. We will discuss here the merits of sensors such as the Senseair S8 for CO2 control and the CJMCU-811 for air quality control. The optimal greenhouse temperature depends on the type of plants you want to grow. The temperature is of course very important for the wellbeing and harmonious development of these plants. Temperature extremes can affect plant nutrition, while extreme temperature fluctuations can cause diseases and stress in your plants. The optimal temperature for the development of greenhouse plants is somewhere between 22-26°C during the day and 2-between 5°C during the nighttime. In this article, I would like to discuss the DS18B20 temperature sensor. How to connect it, how to clear it from noise. As we have noted, significant fluctuations in humidity over a short period of time, can cause great harm to the cell structure of plants as plants growth occurs incrementally and rather slowly and changes to the atmospheric conditions can be harmful. The SHT1X sensor therefore can help us control the humidity in our greenhouse very effectively. For plant growth you need to create a totally stable environment for the plants, with constant temperature, humidity and air quality.

Figure 1.1 Sensor networks

Temperature sensor

Let us look at the DS18B20 temperature sensor. This is quite a popular sensor because of its cost. This sensor is specifically designed to measure liquids and has been made to be waterproof, although it can also be used to measure air temperature. We can use this sensor in any environment of our closed environmental system. And we do not need to write different code for different temperature sensors. It has a wide measuring range from -55C to +125C and it absolutely covers all our needs. Very importantly - we are already receiving digital data with a 12 bit resolution at the output of the sensor. This is very important for embedded systems such as the Raspberry PI[3] or others which do not have come with a built-in on board ADC[4] (analog to digital converter). For example, the Arduino development board already has a built-in ADC, which simplifies the project if you are looking for a sensor with an analog output. In our system we communicate data using the one-wire protocol[5]. Because one-wire has a 64-bit address and only one data cable, we can connect a huge number of these sensors using only one pin in our embedded system. On this data cable, where our temperature sensor is connected, we can connect not only temperature sensors, but also other devices which support one-wire data protocol. An embedded system such as Raspberry Pi has a built-in one-wire protocol support on certain models of GPIO pins[6], so connecting a sensor is relatively simple and causes no installation problems, but in other embedded systems such as Arduino or similar, one-wire is installed programmatically on any GPIO pin by adding a library to your project. From the one-wire protocol specification we can use only two cables to connect the sensor one cable is ground and, the second cable is data + power. The DS18B20 come complete with EEPROM memory, and built-in high and low alarm registers. If for any reason the temperature values cross these limits, the sensor will go into alarm mode. For this purpose, our microcontroller signals to the ALARM register of the sensor, and thus the sensor takes over the function of the primary check of the measured temperature. This takes the load off of the controller of our embedded system to a large extent and ensures the system is running smoothly. In our greenhouse if these sensors are be moved a considerable distance from the embedded board, it could lead to the danger of high voltage pulses on the connection cable between the sensor and our embedded system and this electronic noise could cause problems and may lead to distortion of the information received and, consequently, an incorrect measurement of the temperature in our greenhouse. To reduce noise, we can use a combination of digital filters such as the adaptive coefficient, plus the median filter. The adaptive coefficient will remove all external sharp impulses, and the median filter will smooth out the rest of the distortion appearing on our sensors, or its cables.

CO2 sensor

The Senseair S8 (004-0-0053) measures CO2 within a range of 200 ppm to 4000ppm, has an extended range up to 10000ppm. The sensor uses NDIR technology[7] for CO2 measuring, it measures CO2 and does not try to guess it. The sensor has automatic calibration, and an alarm pin like in a temperature sensor. At crossing the alarm parameter, which you set, the sensor will send a signal to your embedded system. Connected via UART, you can connect the sensor to the Rx and Tx pins of your microcontroller.

Air quality sensor

The CJMCU-811 air quality sensor. This sensor can measure TVOC[8], volatile organic compounds. The source of VOCs comes from a wide variety of substances, such as plywood, linoleum, paints, adhesives, household chemicals, cleaning products, perfumes, combustion products, and more. The measuring range is: 0-1187 ppb (parts per billion). The sensor is able to measure the concentration of volatile solvents by means of a metal-oxide-semiconductor MOX[9]. For the correct operation of the sensor, it has to be warmed up within 30 minutes after being switched on, as is the practice with most gas analytical sensors. The sensor has built-in automatic calibration. And most importantly, the sensor has no gas selectivity, i.e. it does not differentiate between alcohol or paint vapors. Thus, you only get a general idea of the polluted air, not of the source. Control and data transfer takes over via the I2C protocol[10]. In embedded systems such as the Raspberry Pi the protocol is built in and wired on the GPIO, but in other embedded systems you have to include libraries which implement it. You will also have to filter the incoming signal due to the length of the wires which are affected by parasitic external influences.

SHT1X humidity sensor

This sensor is very accurate and we receive accurate measurements of the humidity in the greenhouse in the 0-100% humidity diapason scale. The CMOSens® technology[11] makes it possible to combine two measurements: sensors on one crystal, in our case the CO2: and the temperature sensor. The sensor is controlled by the I2C protocol which does not require connection to any embedded system platform.

Conclusion

We have here considered some of the specifications, uses and requirements for precision sensors which we can easily use in our greenhouse. Parts of these sensors are waterproof and can be used in liquid environments. All the above described sensors are digital and do not require any additional modules to convert analog signal into digital. We believe the best combination for the noise filtering of these sensors is the adaptive coefficient, plus the median filter, because it is able to remove the unwanted noise in the most effective way.

References:

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