Monthly Archives: July 2013

Measuring Rifle Recoil

Recently a major manufacturer of small arms needed to instrument a rifle in order to measure rifle recoil with various types of ammunition.  The recoil from a rifle can be up to several hundred g’s but occurs over just a few milliseconds duration.  A 3-axis strain-gage type accelerometer having a full scale of 2000 g was selected and the USB2250 was chosen to interface directly to this sensor.

The accelerometer required a 10 Vdc excitation, and this was supplied to each axis from the USB2250 excitation supply.  Each accelerometer axis provided a signal proportional to acceleration such that a signal of about 200 mV at 2000 g would be input into the USB2250.  In fact the USB2250 has several gain stages higher than 220 mV so that 16 bits of digital resolution could be applied to just a part of the accelerometer’s full range if needed.

The manufacturer of the accelerometer provided calibration data for each axis: the zero offset and sensitivity differed slightly but scale and offset factors for the USB2250 were readily calculated from the calibration data supplied and each axis was configured to read directly in G’s.

The Easy Sense software supplied with the USB2250 allowed streaming of the data to a laptop PC via the USB port.  The high sample rate possible by the USB2250 – up to 15,000 samples per second on all three input channels – allowed capture of the recoil shock profile as the rifle was fired. Please click here to find more information on the USB2250 and Easy Sense 2250 software.

rifle recoil rifle recoil

Tech Brief: What is Line Regulation?

Every pressure transducer that supplies a high level DC output signal such as 0 to +/-5 Vdc or 4-20 mA should have the line regulation specified.

Line regulation is the ability of the transducer to minimize the output signal error should there be a change in the power that the transducer receives. This can happen if a transducer is calibrated at one power supply voltage in the cal lab but is put into service with a different power source. What would be the error caused by the difference in power supply voltages? The line regulation specification lets you calculate this.

The P55, for example, specifies a line regulation of 0.02%. This means that any change in operating voltage – within its +9 to +55 Vdc limits – will result in a maximum error of 0.02% FS.

Suppose a P55 was calibrated for 0 to +5 Vdc output for 0 to 1 psid pressure, and the power in the cal lab was +12 Vdc. If that same P55 was installed in a system where the power supply was 24 Vdc, the maximum output error would be just 0.02% of the full scale of the transducer.

In this example the output shift due to line regulation would be just 0.0002 psi – a negligible error.

Line regulation in modern semiconductor circuitry has progressed to the point that output error due to power supply changes is essentially zero.  One use of this specification, however, is as a symptom in trouble-shooting.  A regulated transducer whose output changes greatly with power supply voltage probably has something malfunctioning in the electronics and should be repaired.

line regulation

Application Note: The Effect of Altitude and Weather on Vacuum Measurement

Validyne Products: P897V

Vacuum can be defined as a negative gauge pressure. That is, vacuum is the difference between the atmospheric pressure and some pressure lower than atmospheric, but measured with respect to the atmosphere. Atmospheric pressure, however, varies with altitude – lower pressures at higher elevations and higher pressures near or below sea level. The atmospheric pressure also varies slightly with the weather and this should also be taken into account. This application note will describe how to estimate the maximum possible vacuum as a function of altitude and weather and how to specify the best vacuum calibration. The P897V is Validyne’s first transducer that can be used for vacuum measurement.

The earth’s atmosphere is a blanket of gas comprised mostly of nitrogen and oxygen that is held by gravity. Because gas is compressible, the atmosphere is densest at sea level and decreases in density and pressure as altitude increases. The table below gives the atmospheric pressure for various altitudes.

vacuum measurement

From the above chart you can read the atmospheric pressure for typical elevations where vacuum might be measured.

The weather will also affect the atmospheric pressure, but only slightly. Extremes in weather amount to about +/-0.7 psi of pressure difference in the atmosphere throughout the year.

How to specify the best vacuum calibration? A few examples shows how this works:

Example 1: What is the best calibration for measuring a full vacuum in Houston, Texas?

Houston is at sea level so on an average day the atmosphere will be 14.7 psi. If we add another 0.7 psi for weather variations, a full vacuum – meaning the removal of all local atmospheric pressure – will be a maximum of 15.4 psi.

Calibration Specification: 0 Vdc = 0 vacuum and +5 Vdc = 15.5 psi vacuum. This will cover all possibilities for the local atmospheric pressure.

Example 2: What is the best calibration for measuring full vacuum in Mexico City?

Mexico City is at an elevation of about 7500 ft, so the average atmospheric pressure there is only 11.1 psi. Add in another 0.7 psi for the weather and the maximum available vacuum will be 11.8 psi.

Calibration Specification: 0 Vdc for 0 vacuum and +5 Vdc for 11.8 psi vacuum

Example 3: What about La Paz, Bolivia?

La Paz is at 10,500 ft elevation and so the atmosphere there is just about 10 psi. Adding another 0.7 psi for weather gives a maximum possible vacuum of 10.7 psi.

Calibration Specification: 0 Vdc for 0 vacuum and +5 Vdc for 10.7 psi at full vacuum.

Note that there are areas below sea level and these locations would experience somewhat higher pressures than at sea level: 15 to 15.2 psi. Adding in the weather and a calibration of 15.9 or 16 psi at full vacuum should be sufficient.

Specifying Pressure Ports on the P55 Transducer

pressure ports

The pressure ports are the piping connections on the transducer body that are used to connect the transducer to a pressure source. The standard connection for the P55D is 1/8” female NPT pipe threads. These are standard plumbing connections and adapters may be purchased from a variety of sources to connect to plastic or steel tubing. Swagelok adapter fittings are often used with the P55.

There are two 1/8” NPT ports for the standard P55 – one port for each side of the differential pressure transducer. These are marked + and -. The + port is normally connected to the higher pressure and the – port to the lower pressure in the system to be measured. The transducer so configured will measure the difference between the two pressures. When the – port of a P55D is left open to atmosphere, the + port will measure gage pressure.

The standard P55D also has bleed ports opposite the 1/8” NPT pressure ports. The bleed ports are 8-32 set screws with a small gasket that is used to allow the air inside the sensor cavity to be displaced with liquid. This is useful for making fast dynamic measurements in fluid-filled systems.

An alternate pressure port configuration provides for two 1/8” NPT female threaded ports on
each half of the sensor. In this configuration there is no bleed port but rather a second 1/8”
NPT port. This can be used to connect additional plumbing to the transducer or to flush the  sensor cavity, etc. A pipe plug can also be used to plug off one of the open NPT ports. P55A absolute sensors have a single 5/16” female straight thread port. An adapter is supplied so that a 1/8” male NPT pipe thread interface is provided.

Both the differential and absolute versions can be ordered with a 1” long 1/4” OD tube stub
that can be used to connect directly to a Swagelok fitting. This is useful if threaded connections are not appropriate. 

Application Note: Selecting O-Ring Material

Validyne transducers and sensors bolt together around the sensing diaphragm. In order to seal the sensor cavity so that pressure can be measured, O-Ring seals are used. The O-Rings come in contact with the fluid inside the transducer so it is important that the fluid and the O-Rings are compatible. Various O-Ring compounds may be specified in the transducer model number to insure that the seal will remain viable when in contact with the fluid during service. This application note will discuss how to select the best O-Ring material for your application.

The standard O-Ring material used in Validyne transducers is Buna-N. This compound is compatible with water, oil and most gases. Since most fluids involved in pressure measurement are usually water or oil-based, Buna-N is normally sufficient. But if you are not sure, it is best to check a compatibility table to select the best O-Ring compound.

The industry standard reference for O-Ring fluid compatibility is the Parker O-Ring Handbook, as published by Parker-Hannifin, manufacturers of O-Rings and fittings. The O-Ring Handbook has an extensive set of tables that give the compatibility of various O-Ring compounds with chemicals commonly used in industry. The O-Ring Handbook is now accessible on line at:

http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf

To use the Parker compatibility tables simply look up the chemical or fluid in question (they are given alphabetically) and determine the compatibility rating of the various available O-Ring compounds.

The Validyne transducer model number designations for the available O-Ring compounds are as follows:

Validyne Model …………. Common Term …………… Parker O-Ring Book Designation
……… N ……………………… Buna-N ……………………………. Nitrile
……… E ……………………… Ethylene Propylene …………… Ethylene Propylene EPDM
……… S ……………………… Silicone …………………………… Silicone MQ
……….T ……………………… Teflon ……………………………… Not Offered by Parker
……… V ……………………… Viton-A ……………………………. Fluorocarbon
……… K ……………………… Kalrez ……………………………… Fluorosilicone

The compatibility designation in the Parker book is 1 through 4 where 1 is considered compatible and 4 is considered unsatisfactory.

Here are some examples:

Fluid: Carbon Dioxide
Compatibility: All of the O-Ring compounds are shown as compatible – Buna-N is a good choice.

Fluid: Brake Fluid DOT 4
Compatibility: Ethylene Propylene is the best choice as it is compatible with the fluid and available as a selection in the Validyne model number.

Fluid: Dichlorobenzene
Compatibility: Fluorocarbon is given as compatible in the Parker table and so Viton-A should be chosen in the Validyne model number. Note that the Parker book shows two different dichlorobenzene compounds, but both are compatible with fluorocarbon.

Teflon is normally a good choice for compatibility with all kinds of fluids but we have found that the lack of elasticity in a teflon O-Ring can cause issues with transducer accuracy and performance. Teflon has a tendency to cold flow over time and this affects the clamping force on the transducer sensing diaphragm. An O-Ring with elasticity is always the better choice and a satisfactory compound can usually be found for all but the most corrosive of fluids.

If the O-Ring in the sensor is not compatible with the fluid, the first sign will probably be leaking or even complete loss of pressure. Disassemble the sensor and inspect the O-Rings for swelling and replace with a compatible compound. Validyne offers O-Rings separately for each model transducer or sensor and these are usually in stock. You can find more details on O-Rings here.

o-ring material