SparkFun Pressure Sensor Breakout - MS5803-14BA is a breakout board that gives accurate pressure readings.
OpenPressure combines an ADS1220 24-bit ADC and a SAMD21 microcontroller to make a complete 24-bit, high precision pressure and temperature sensor interface.
Atlas scientific Dissolved Oxygen kit is an all in one research kit for your study on dissolved oxygen.
The FDC1004 is a single-chip IC for capacitance measurement for applications including proximity sensing and liquid level sensing.
Grove-Gas Sensor(MQ5) has a fast response time to detect any gas leakage.
Thermocouple amplifier is an outstanding analog-output amplifier that is easy to use.
Liquid flow meter measures/checks the flow of water to monitor and avoid wastage.
Adafruit MiCS-5524 Gas Sensor Breakout detects alcohol,CO and gas in one compact breakout board.
Grove - Gas Sensor(O2) is used to check the amount of oxygen present in the gases.
Grove - CO2 Sensor measures the level of carbon dioxide present in the atmosphere.
The H2OhNo! is a water sensor alarm kit that you build yourself. When water is detected across the sense pins an alarm goes off and an LED starts blinking.
These are standard MQ-5 gas sensors which are sensitive to LPG, LNG, natural gas, iso-butane etc. They can be directly connected to ADCs for ease-of-use.