The BME280 Atmospheric Sensor from Whadda is the easiest way to measure barometric pressure, humidity and temperature.

A Water Level Sensor 1/2Npt with 0-12VDC, from ProK.

LPS331AP Pressure/Altitude Sensor Carrier with Voltage Regulator

The Atlas Scientific pH circuit designs currently available on the Internet are outdated and use a significant number of overly complex analog components. Atlas Scientific has reinvented and refined the pH circuit of the past to a more modern and simple to use model.

Converter of Level Logic 3.3V and 5V 8 Channels.

This package includes a Sensor of Level of Water and a Sensor of Humidity from Solo. If the plates have water covering the sensor components, an analog value will be available on the "SIG" connection. The Sensor of Level of Water can detect up to 4cm of water. This way, you can check if your aquarium, or your pet's water bowl, still has enough water. You can use the Sensor of Moisture of the Soil to check your plant's soil or growing environment, for example.

Pressure Sensor MD-PS002 - Absolute Pressure

A Water Flow Meter Sensor 6mm Hose designed for your Drinking Water Machine!

Life has its ups and downs, so why not measure them? The MPL3115A2 is a MEMS pressure sensor that provides Altitude data to within 30cm (with oversampling enabled).