Monthly Archives: August 2013

Instrumenting Curtain Wall Tests

Validyne Products: DP103, USB2250

Curtain walls are the outside glass coverings of skyscrapers and large buildings. A curtain wall allows floor-to-ceiling windows and provides a shiny exterior to the building. Older buildings had windows that were openings in the wall structure. Curtain walls are so named because they are hung away from the structural part of the building – like curtains – and bear no structural load. The curtain wall must withstand the outside elements: wind, rain and temperature extremes and so curtain walls must be tested prior to being installed on a large building. This application note details a procedure for curtain wall tests.

The standard test sequence is to build a sample section of the curtain wall and expose it to wind and rain generated at the test facility. This sequence of photos shows what this looks like.

Installing the sample curtain wall section on a scaffold.

curtainwall-5 curtain wall tests

 

 

 

Bringing up the wind machine.

curtainwall-2 curtain wall tests

 

 

 

 

 

 

 

Blowing air and water at the sample.

curtainwall-3 curtain wall tests

 

 

 

 

 

The idea is to simulate the worst predicted weather for a given location and verify that the curtain wall design is robust enough to survive. The curtain walls are instrumented so that the deflection of the glass sections that make up the wall can be measured at different wind force loads.

The deflection is measured with a lanyard potentiometer. This is a displacement sensor that resembles a tape measure – a length of cable extends out from the lanyard pot that is spring loaded. The cable is wound around a spring-loaded potentiometer so that as the cable move the resistance of the potentiometer changes.

The wind loading is measured by a DP103 pressure transducer connected to the outside of the test section. A sensitive transducer is used because even a lot of wind velocity does not generate a very high pressure. Air moving at 100 MPH, for example, generates about 4.8 In H2O of pressure. A DP103 can be used to measure this and much lower pressures if needed.

Both the lanyard potentiometer and the DP103 transducer can be interfaced to a computer using the Validyne USB2250. This will provide the DC excitation required for the lanyard pot and the AC excitation needed for the DP103 sensor. The USB2250 will provide digital readings in In H2O for the DP103 and inches of deflection for the lanyard pot. These readings can be recorded in Easy Sense software and correlated so that the window glass can be evaluated for the conditions simulated during the test.

The sketch below shows the actual wiring connections for these sensors to the USB2250:

curtainwall-1 curtain wall tests

Application Note: Basics of Air Velocity, Pressure and Flow

Air velocity can be measured by sensing the pressure produced by the movement of the air. This application note will describe the basic relationships between air velocity and the pressure generated by air flow.

Anyone who has put their hand out the window of a moving car has experienced the force applied by moving air. This force can be sensed as a pressure by connecting a tube from the positive port of a differential pressure transducer like the P55 differential pressure transducer and pointing the open end of the tube directly into the oncoming air stream. The figure below shows how this might look.

Differential Pressure Transducer Air Velocity

The tube that is placed into the air stream is called a pitot tube after Henri Pitot, the French engineer of the early 18th century who invented it. As the velocity of the air increases the pressure also increases inside the pitot tube with respect to the ambient atmosphere. A differential pressure sensor like a DP15 Range Changeable Pressure Sensor can be plumbed to measure this when the pitot tube is connected on the + port and the – port is open to atmosphere. Note that the pitot must be pointed directly into the flow – if the tube is mounted at some angle to the direction of flow, the transducer will not sense the full pressure developed by the air velocity.

The pressure developed by the air velocity is called the velocity head, and it is affected by the density of the air. The density of the air, in turn, is a function of the local atmospheric pressure and the temperature. The equations that relate all these factors are:

airflow-3 air velocity

Note that to determine the air velocity the density must first be known. This is the second equation and relates ambient atmospheric pressure and temperature to density. The temperature in degrees Rankine is an absolute reference and is T in degrees F + 460. Assuming average conditions of 70 F and a barometer of 29.92 In Hg, the density of air is 0.075 Lbs/Cu Ft.

If, for example, we measure a differential pressure from the pitot tube of 2.00 In H2O, then the air velocity calculates to 5671 ft/min or 94.5 ft/sec.

Air velocity is a function of air density and differential pressure, but determining air flow requires that the geometry of the piping be taken into account. The pitot tube can be used as before, but  the negative port of the pressure transducer is now connected to the pipe or duct so that the internal pressure is taken into account by the measurement of the velocity head.

Differential Pressure Transducer air velocity

Note that it is still critical that the pitot tube be installed so that it is pointed directly into the oncoming flow stream.

Ideally, determining the flow in terms of volume should simply a matter of multiplying the cross sectional area of the tube or duct by the air velocity. If the dimensions of the ducting are known, then the cross-sectional area can be easily determined and the volumetric flow calculated.

There is a problem with this, however – the velocity of the air is not uniform at all points along the cross-sectional area of the tube. This is because friction between the moving air and the inside surface of the pipe or duct slows the velocity down. The air velocity in a pipe, for example, is highest near the center but slows towards the inside walls. To make things even more complicated, the shape of the velocity profile is also affected by the type of flow – turbulent or laminar – and the proximity of other fittings and protuberances inside the piping.

airflow-4 air velocity

Averaging pitot tubes have been developed that sense the velocity head at several points along the cross-section of the air pipe or duct and deliver a differential pressure that more accurately reflects the average velocity profile.

airflow-5 air velocity

The extent to which the averaging pitot tube differential pressures deviate from the actual velocity profile pressures is expressed by a correction factor supplied by the manufacturer. This factor may depend on duct geometry and the flow regime present – sizing a pitot tube correctly has become an art. But the pitot tube factor must also be included along with the other variables in the final flow equation.

The Validyne P55 Pressure Transducer can be used to measure flow. Click here now to contact us for more information. 

p55

Tech Brief: Identifying Transducer Diaphragms

Validyne transducers can be disassembled and the sensing element replaced.  This is done to repair an over-pressured sensor or to re-range the transducer for a higher or lower pressure measurement. Validyne transducer diaphragms will have a series of numbers etched on to them that identify their  sensor type, pressure range and manufacturing lot code.  A sample DP15 diaphragm is shown below:

diaphcodes transducer diaphragms

As can be seen, there are two sets of numbers etched along the top and left-hand edges.  The lot code appears on the left edge and is used for production control purposes – it contains no information useful to the user.

The top set of numbers – in this example 3-22 – are the important numbers.  The 3 in 3-22 identifies the sensor type into which the diaphragm can be installed. The -22 is the range code and tells what full scale pressure the diaphragm can measure.

The sensor types and available pressure range codes for each Validyne transducer model are shown in the table below:

diaphchart transducer diaphragms

The range code pressure chart is shown below:

diaphrng transducer diaphragms In the example above, the 3-22 diaphragm can be used in transducer models DP15, P55, PS309, P300, P306 and P305.  The -22 range code indicates that this diaphragm has a full scale pressure range of 0.20 psid.

Tech Brief: Transducer Isolation

Validyne pressure transducers are designed and built to be electrically isolated from the fluids they measure.  This is done by protecting the inductive sensing coils inside the sensor body with a welded cover and using isolation feed-throughs for the coil wires.  This is done because often the plumbing connected to the transducer will be at a different ground potential than the electronics powering the sensor.  Isolation insures that any difference in ground potential between the sensor body and the electronics receiving the transducer signal will not cause instability in the readings or damage to equipment connected to the transducer wiring.

A loss of isolation will affect the transducer readings, often causing drift or noise in the pressure signal.  If a transducer is malfunctioning or exhibits an unstable signal the first thing to check is the transducer isolation.  This is very easy to do:

1. Take a standard electrical multimeter and put it in the DC resistance measurement mode.

2. Remove the mating connector from the transducer, exposing the pins on the transducer connector.

3. For VDC output type transducers (P55, P24, etc) place the red probe of the multimeter on pin A of the exposed transducer connector (or pin E for 4-20 mA versions).  Also use Pin A for all variable reluctance sensors such as the DP15 or DP360/363.

4. Touch the black probe to the metal sensor body.

5. Measure DC resistance.

The DC resistance should be infinite or at any rate very high.  An open circuit indication also means the sensor isolation is good.  If the DC resistance is below 100 K Ohms, the sensor will likely exhibit drifting or noise.  A low resistance or a short means the sensor will not provide an accurate reading.

transducer isolation

Validyne Goes Mobile With Smart Phone App!

phone app

 

 

 

phone app

Now you can put Validyne on your iPhone with our new product phone app.  You can download from the link above.  iPad and Android versions of this app will be available soon.  Now you can carry everything you need to know about Validyne products with you wherever you go.

The  app lists all Validyne products and alphabetically and you can scroll through them to find the product you need.  Tap on the model number and you will go to a details page with links to the data sheet and instruction manual for that product.

The Validyne YouTube Channel is also accessible and can be used for trouble-shooting and product presentations directly on your smart phone.

The app also includes our Transducer Selector – enter the pressure measurement type, pressure range, fluid and line pressure and the app will produce links to the appropriate Validyne products.

There is also a contact page where you can send email or have your iPhone dial us directly.

Download the app today and take us wherever you need instant information about Validyne products.

phone app