How flow sensors work?

Flow sensors are used for wide range of fluids in multiple industries for various measurements. There are many types of flow sensors and they all measure “volume or area per unit time”. And depending of the sensor type there are multiple ways this volume per unit time can be measured, but they all use basic concept of fluid flow principles with concept such as the Bernoulli’s principle. Some of the basic flow sensors used today are orifice meter, venture meter, flow nozzle, and pitot tubes which us the principle of difference in pressure from the Bernoulli equation. Then there are sensors which use direct force to measure the flow which include rotameter, turbine meter, propeller flow meter, coriolis mass flow meter. Using pressure differences and direct force are the most common methods used to measure flow rate but there are other complicated methods such as ultrasonic flow meters, magnetic flow meter, calorimetric flow meter, gear flow meter, thermal flow meter, and couple more.

All the flow sensors which use pressure difference use Bernoulli equation which is 

Where the condition on both sides of the sensors are inversely related to each other, therefore the equation can be manipulated into the pressure drop across the flow sensor is equal to velocity of the flow squared. When calculating the flow using the different sensors the area of the opening on both sides of the sensor, density of the liquid, and pressure readings from the sensors are known an can be plugged into the Bernoulli equation to find the velocity because V₁ = V₂ = V. Below are few images portraying how some of this sensors operate and how the pressures differences (dp) can be measured.     

Orifice Plate Flow Sensor

Venturi Tube Flow Sensor

Flow Nozzles Flow Sensor

Sensors using direct force to measure velocity use methods of balancing forces with in systems where the force applied by the fluid flowing through the sensor is measured and manipulated with a proportion factor to get the flow of fluid in the system. Below are some images which portray how the forces applied by the fluid are with few different types of direct force flow sensors.

Rotameter: resistance of gravity force of the bolt is being measured here.

Turbine meter: work is being measured here where work equals force times distance, and the distance id known so the force can be calculated from the work measured by the sensor.

Sources:

http://www.princeton.edu/~asmits/Bicycle_web/Bernoulli.html

https://controls.engin.umich.edu/wiki/index.php/FlowSensors

Temperature Sensors

While initially, sensing temperature may seem like a simple concept (and it's been getting done for a very long time now) the actual details of what we are sensing can become quite confusing. In its most basic sense, temperature is a measure of the movement of molecules within a body of matter. That matter can be solid liquid or gas, and more frequent movements translate to higher temperatures (with absolute zero at the bottom of the scale, meaning no particle movement at all) The reasons we care about temperature are varied, but mainly we want to be comfortable or we want to ensure that a certain process is happening by manipulating the temperature. Of course our perception of comfort is based on our own sensing of temperature by biological processes far more complex than the processes we use to quantify temperature.

Obviously the thermometer is the simplest way to sense and understand temperature. We figured out pretty early on that certain fluids expand when they are heated, so that their volume is a function of temperature. By controlling and marking this expansion of volume we were able to quantify temperature. As John points out, we were later able to recognize the same principle applies to solids, and we began to use this to control the movment of thin metal strips, which would in turn control some basic circuitry, mainly for switching on and off some electrical process. Of course, this is the principle by which most early thermostats operated.

As we progressed in knowledge of electrical properties of materials, it was found that the resistance of certain conductive materials decreases as we increase the temperature, and so we developed thermistors made of ceramics or polymers to effectively convert a change in temperature to a change in resistance that could be easily measured. We also knew that the current changes between to metals at different temperatures, so a slightly more complicated Thermocouple circuit was developed, where this change in current can be measured across dissimilar metals. Furthermore we found that different metals behaved more predictably for certain temperature ranges, so a number of sets of metals were used for varying applications. Finally, the resistance in a coiled wire was found to be more accurately measurable, so RTD's (made of a homogeneous wire wrapped around a ceramic core) became prominent. These could be configured differently to accomodate different temperature ranges and provided high accuracy and repeatability.

Mike does a great job of explaining the usefulness of all of the sensors mentioned above. However, there are still some applications in which even RTD's and Thermocouples cannot be used. For example measuring the surface temperature of an object that is moving or one that cannot be disturbed becomes quite difficult with the sensors mentioned above. Rather than measuring a change in a specific material's response to a temperature, Pyrometers are able to measure a temperature directly as a function of an object's electromagnetic radiation. The methods by which this is acheived can become rather complex, but in their simplest form all pyrometers consist of a lense to concentrate radiation into a point or array of points and an absorber, which chemically translates radiation into terms of flux. It is at this point that the line between sensing and transducing becomes blurred for me. David gives a good explanation of these principles as applied to an array of infrared sensors (thermography).  By some means of transduction the radiation is translated to thermal energy and the thermal energy is translated to an electrical current. The figure below helps explain the process.

References:
http://drexel.summon.serialssolutions.com/search?s.q=thermal+radiation+sensor

http://site.ebrary.com/lib/drexel//docDetail.action?docID=10438390

http://www.electronics-tutorials.ws/io/io_3.html

http://en.wikipedia.org/wiki/Pyrometer

Temperature Sensors

There are many types of temperature sensors. They are listed in two categories. The categories are Mechanical and Electrical temperature sensors. Examples of mechanical temperature sensors are the thermometer and Bimetal. Examples of electrical temperature sensors are the thermistor, thermocouple and resistance thermometer.

I’m quite sure that almost anyone, who has graduated the fourth grade, can use a basic thermometer. It is simple. Mercury expands and contracts according to the temperature and thus this expansion and contraction can be measured. A bimetal temperature sensor however, is a little less known to most people. Bimetal temperature sensors use strips and disks of metals which convert into mechanical displacement.

Electrical temperature sensors include thermistors, thermocouples and resistance thermometers to name a few. Thermistors are types of resistors which in turn measure resistance with ensuing changes in temperature. The basic equation of  controls the sensor. Where R is resistance, T is the temperature and k is a coefficient based on other attributes of the sensor. Thermocouples consist of two conductors that yield a voltage with differences in temperature. The governing equation is . Again the equation is based on the metals in the thermocouple. Tables are easy to look up for the coefficient in the aforementioned equation. A resistance thermometer, also referred to by the acronym RTD (Resistance Temperature Detectors), correlates resistance to changes in temperature. Again variations in metals used in the product dictate the subsequent equation:  , where  .

Typically a thermostat in a small residential home uses mechanical temperature sensors in the form of bimetal technology. As fellow student Mike S. points out it is the relative cheapness of these sensors that makes them very useful in residential applications. But as a corollary, for uses in commercial and industrial applications where high accuracy, low drift, wide operating range and precision are needed, you will typically find some sort of electrical temperature sensor. These sensors tend to be more expensive based on the fact that they have to be engineered and calibrated to handle the above-mentioned qualities.

I wouldn’t be surprised to see, that when the prices of these electrical sensors come down with advancements in technology, we will see them installed into small residential homes.

It was interesting to see from other posts like C. Meraz’s post, that other sensors such as pressure sensors typically measure something other than pressure, like resistance and capacitance and then translate these changes from the change in temperature that causes this change in resistance or capacitance.

Sources:

http://en.wikipedia.org/wiki/List_of_temperature_sensors

http://www.temperatureworld.com/shci.htm