Consider a hilly area in which new homes are being constructed. As rain falls on the site, without any controls, water running off the site would carry eroded soil particles to a nearby creek, polluting it badly. A detention pond is built to catch the particle-laden runoff at the bottom of the hill, to allow the soil particles to accumulate in the bottom of the pond while cleaner water overflows out of the pond and into the creek. For simplicity, assume that the rain is steady, all flows are steady, and ignore any transient effects on pond and water concentrations of soil particles. The water generated by the rain storm has a flow rate of 5.00 m3/min and entrains a concentration of particles of 4000. mg/L as it runs over the land. The nearby creek has a flow rate of 120. m3/min and normally carries an ambient concentration of 15.0 mg/L of soil particles. The detention pond will catch and remove most of the soil particles from the runoff, leaving outflow water (the water exiting the pond and flowing to the creek) with a concentration of 400. mg/L. Assume that 5% of the water that comes into the pond will seep down through the bottom (filtering out all particles), while the rest will over flow into the creek. a. Calculate the mass of mud accumulated in the detention ponds after three hours of this rainfall/runoff.
b. Calculate the concentration of particles in the creek after mixing with the overflow from the pond.

Infrared sensor is most used sensor in wireless technology where remote controlling functions and detection of surrounding objects/ obstacles are involved. this post will discuss about what is infrared sensor, its working principle, how it works, types, applications, advantages and disadvantages. what is infrared sensor

ir sensor is a simple electronic device which emits and detects ir radiation in order to find out certain objects/obstacles in its range. some of its features are heat and motion sensing.

ir sensors use infrared radiation of wavelength between 0.75 to 1000µm which falls between visible and microwave regions of electromagnetic spectrum. ir region is not visible to human eyes. infrared spectrum is categorized into three regions based on its wavelength i.e. near infrared, mid infrared, far infrared.

wavelength regions of infrared spectrum

near ir – 0.75µm to 3 µm
mid ir – 3 µm to 6 µm
far ir – > 6 µm
1 introduction to infrared sensor

fig. 1 – introduction to infrared sensor

working principle of infrared sensor

infrared sensors works on three fundamental physics laws:

planck’s radiation law: any object whose temperature is not equal to absolute zero (0 kelvin) emits radiation.
stephan boltzmann law: the total energy emitted at all wavelengths by a black body is related to the absolute temperature.
wein’s displacement law: objects of different temperature emit spectra that peak at different wavelengths that is inversely proportional to temperature.
key elements of infrared detection system

the key elements of infrared detection system are:

ir transmitter

ir transmitter acts as source for ir radiation. according to plank’s radiation law, every object is a source of ir radiation at temp t above 0 kelvin. in most cases black body radiators, tungsten lamps, silicon carbide, infrared lasers, leds of infrared wavelength are used as sources.

transmission medium

as the name suggests, transmission medium provides passage for the radiation to reach from ir transmitter to ir receiver. vacuum, atmosphere and optical fibers are used as medium.

ir receiver

generally ir receivers are photo diode and photo transistors. they are capable of detecting infrared radiation. hence ir receiver is also called as ir detector. variety of receivers are available based on wavelength, voltage and package.

ir transmitter and receivers are selected with matching parameters. some of deciding specifications of receivers are photosensitivity or responsivity, noise equivalent power and detectivity.