Unformatted text preview: 3.2 Pressure Measurement Devices • Barometer • Manometer • Other types: • Bourdon tube • Pressure transducers (gage pressure, absolute pressure and differenAal pressure) • Strain-‐gage pressure transducers • Piezoelectric transducer Barometer • Atmospheric pressure is measured by a barometer; atmospheric pressure is oHen referred to as the barometric pressure. Invert a mercury-‐filled tube into a mercury
container open to the atmosphere: PB = Patm Pc = 0 (only mercury vapor above C, so the pressure is very low relaAve to Patm) => Patm = ρgh The basic barometer • The standard atmosphere is a frequently used pressure unit, defined as the pressure produced by a column of mercury 760 mm in height at 0°C (ρHg = 13,595 kg/ m3) under standard gravitaAonal acceleraAon (g = 9.807 m/s2). Barometer • The length or the cross-‐secAonal area of the tube has no effect on the height of the fluid column of a
barometer, provided that the tube diameter is large enough to avoid surface tension (capillary) effects. • Atmospheric pressure is the weight of the air above that locaAon per unit area. It changes with height (101.325kPa at sea level; 89.88kPa at 1000m; 5.53kPa at 10km) and with weather condiAons ImplicaAons at higher alAtudes: • • • • • • cooking takes longer since water boils at a lower temperature nose bleeding since Δp between Patm and blood pressure is larger air density is lower, so we get
less oxygen for the same amount of air a fan or compressor will displace 15% less air at 1500m airplanes need longer runways …. Manometer • ElevaAon change of –Δz in a fluid at rest corresponds to ΔP/ρg => fluid column can be used to measure pressure differences • Manometers are commonly used to measure small and moderate pressure differences • They contain one or more fluids such as mercury, water, alcohol, or oil • Heavy fluids are used if large pressure differences are anAcipated Basic manometer
measuring pressure in a gas tank 1) P1 = Pgas (gravitaAonal effects of gases are negligible) 2) P2 = P1 3) P2 = Patm + ρgh (ρ = density of manometer fluid) ⇒ Pgas = Patm + ρgh Cross-‐secAonal area of tube has no effect, as long as surface tension and capillary rise is negligible Manometer Manometer with stacked-‐up fluid layers: Pressure change across a fluid layer with density ρ and height h is ρgh => Differen5al manometer: measure the pressure drop across a flow secAon or a flow device When the
fluid flowing in the pipe is a gas, ρ1 << ρ2 => ΔP = ρ2gh Other pressure measurement devices • Bourdon tube: Consists of a bent, coiled or twisted hollow metal tube whose end is closed and connected to a dial indicator needle. When the tube is pressurized, the tube stretches and moves the needle in proporAon to the applied pressure • Pressure transducers: Use various techniques to convert the pressure effect to an electrical effect such as a change in voltage, resistance, or capacitance. •
Pressure transducers are smaller and faster, and they can be more sensiAve, reliable, and precise than their mechanical counterparts. • Strain-‐gage pressure transducers: Work by having a diaphragm deflect between two chambers open to the pressure inputs. When the diaphragm stretches, the strain gage stretches and a Wheatstone bridge circuit amplifies the output • Piezoelectric transducers: Also called solid-‐state pressure transducers, work on the principle that an electric potenAal is
generated in a crystalline substance when it is subjected to mechanical pressure. Various types of Bourdon tubes used to measure pressure ... Show What is a manometer? In this article, we’re going to introduce you to the manometer which is one of the oldest pressure measurement devices still in use today. A manometer is one of the most accurate devices for measuring pressure in the lower ranges. Since manometers are so accurate, they are often used as calibration standards. Manometer applicationsSome typical industrial manometer applications include: – Fluid flow measurements – HVAC systems maintenance – Residential or commercial furnace gas valve pressure measurement
Manometer basicsAlright… let’s get started. All manometers operate on the principle that changes in pressure will cause a liquid to rise or fall in a tube. There are several different types of manometers and we will discuss many in this article.
As mentioned earlier, all manometers operate on the principle that changes in pressure will cause a liquid to rise or fall in a tube.
Types of manometer1) U-tube manometerLet’s look at a U-tube manometer as it is probably the most common manometer in use today. We’ll discuss how it is used to measure pressure. Types of fluidsAs we said earlier, a manometer is filled with a liquid. Typical manometer liquids are mercury, water, and light oils. It’s worth saying here that mercury was a common manometer fluid in the past, but has largely been replaced due to its environmental and health hazards. Quite often the liquid is colored to help detect the fluid movement.
Measuring pressureThe tube is filled until both sides are approximately half full. When the pressures are equal, the column of liquid on each side will be at the same height. This is usually marked as zero on a scale. With both sides of the manometer open to the atmosphere, the fluid level on one side will be the same as the level on the other side because P1 equals P2.
Ok… Now suppose one end of the U-tube manometer is connected to an unknown pressure P1 whose value must be determined. The other end is left exposed to the atmospheric pressure, P2. The difference in the height of the liquid on the two sides of the tube is the differential pressure. In this case, the manometer provides a gauge pressure measurement because it is referenced to the atmosphere. OK… So the total difference in liquid height is 4 units. And this is where the pressure measurement becomes interesting! What does the 4 unit difference represent? Let’s assume our U-tube manometer is filled with water. That means that our pressure differential is 4 inches of water column. From our previous article, DP Flow Transmitter Testing and Re-Calibration, you know that inches of water column is a unit of pressure measurement. Using conversion charts, we could easily convert 4 inches of water column to 0.144 psi(g). As you can appreciate, a U-tube manometer filled with water is only capable of measuring very small pressures. To illustrate, let’s apply a pressure of 5 psi to the P1 side of the U-tube manometer. Oops… we don’t want that to happen!
Increasing measurement accuracyWhat if we wanted to accurately measure very low-pressure values? If we replace the water with a Meriam Red Oil fluid we will get a greater difference in liquid levels. Why is that? Water has a specific gravity or relative density of 1.0 while Meriam fluid is an oil and has a specific gravity of 0.83. We will get a much larger difference in liquid level with Meriam fluid resulting in more accurate pressure measurement. In some cases, vendors will provide a U-tube manometer scaled directly in pressure units such as kilopascals (kPa).
Meniscus errorManometer users must be aware of meniscus error. Meniscus occurs when the particles of the liquid stick to the sides of the glass tube. Depending on the direction of pressure applied, the meniscus can be concave or convex. Either way, a reading error may occur.
Let’s have a look at a couple of other types of manometers. One common type of manometer is a barometer used to predict weather conditions by measuring atmospheric pressure. 2) BarometerA Barometer consists of a glass tube with one end sealed. An evacuated tube has its open end submerged in an open container of mercury. The pressure exerted by the column of mercury is balanced by the pressure of the atmosphere. The glass tube is calibrated in pressure units. Any liquid could be used in a barometer, but mercury is used because of its high specific gravity. A mercury barometer needs to be at least 30 inches tall. A water-filled barometer would have to be more than 33 feet high!
3) Inclined manometerA manometer that provides even better accuracy than the U-tube manometer is the inclined manometer. This manometer has a well that contains the liquid and a transparent column. The column is mounted at an angle. The pressure is indicated by the vertical amount the liquid rises or falls in the column. Because of the incline, a small change in pressure will cause greater movement of the liquid in the column.
Summary– A manometer is one of the most accurate devices for measuring pressure in the lower ranges. – Typical manometer applications include measuring pipe fluid flows, HVAC system pressure, and gas pressure. – All manometers operate on the principle that changes in pressure will cause a liquid to rise or fall in a tube – Typical manometer liquids are mercury, water, and light oils. – One end of a U-tube manometer is connected to an unknown pressure while the other end is left exposed to the atmospheric pressure. The difference in the height of the liquid on the two sides of the tube is the differential pressure. – Other types of manometers include the barometer and the inclined manometer. If you have any questions about manometer and its applications, add them in the comments below and we will get back to you in less than 24 hours. Got a friend, client, or colleague who could use some of this information? Please share this article. You might want to review 3 of our other articles: Automation Engineer Posted on Nov 23, 2020
and gain an unfair advantage
RealPars is the world's largest online learning platform for cutting-edge industrial technologies. Questions? Rotterdam Science Tower, Refund & Cancellation Policy © 2022 RealPars B.V. All rights reserved. Created with coffee and tea in Rotterdam. Is a device consisting of a liquid filled tube used for measuring pressure?One of the earliest pressure measuring instruments is still in wide use today because of its inherent accuracy and simplicity of operation. It's the U-tube manometer, which is a U-shaped glass tube partially filled with liquid. This manometer has no moving parts and requires no calibration.
What is manometer measure?Manometer is a device that measures fluid pressure.
What is a manometer in physics?A Manometer is a device to measure pressures. A common simple manometer consists of a U shaped tube of glass filled with some liquid. Typically the liquid is mercury because of its high density.
Is a manometer used to measure atmospheric pressure?How Pressure Is Measured. A manometer is a device that measures air pressure using a container with a "U"-shaped tube open at one or both ends.
|
A ___ is a device consisting of a liquid-filled tube used for measuring pressure.
Urheberrechte © © 2024 membukakan Inc.
