Category Archives: Transducers

Stand-Alone Pressure Transducer or Sensor + Electronics?

Introduction:

Validyne pressure transducers break down into two general categories:

Type 1 – A complete transducer with integral electronics

pressure transducer

 

 

 

 

 

Type 2 – A variable reluctance sensor and supporting carrier demodulator electronics.

pressure transducer

 

 

 

 

 

 

Type 1 category products include models P55, P61, P66, the P895 family and the DR800 and P532 process transmitters.

Type 2 category products include models DP15, DP360/363, DP103 with carrier demodulator models CD15, CD23/223, CD280 and CD17.

The transducers in both categories measure the same pressure ranges – so why would you choose one type over another?

Cost Effective DC Power and DC Signal:

Type 1 category transducers are generally more cost effective per point than are the sensor + electronics (Type 2) category. The Type 1 products come ready for DC power and produce a high-level DC signal, +-/5 Vdc or 4-20 mA. The Type 1 transducers include temperature compensation and are also available with higher accuracy because we can program corrections to sensor errors into the microprocessors in these products.

Type 1 products are generally ‘plug and play’ devices and are ideal for permanent installations.

Type 1 products, however, do not lend themselves to the changing of pressure ranges easily. It is possible to disassemble the sensor on a P55, for example, and replace a damaged diaphragm or install a diaphragm with a new range – but the correction factors and temperature compensation in the microprocessor will not be matched to the new assembly. Validyne can do this – and include new temperature compensation and error correction factors – but this takes time and has a cost.

Easy Range Changing:

The biggest reason to use Type 2 products is for convenient range changing. A DP15, for example, will be easier to disassemble and easier to replace a diaphragm than the Type 1 units. The sensor will be easier to calibrate with the zero and span adjustment ranges built into the external carrier demodulators. If fast frequency is important, the smaller variable reluctance sensors can be more conveniently close-coupled to piping than the larger Type 1 units and the electronics supporting Type 2 sensors have a higher low pass filter frequency available – up to 1 Khz.

Type 2 products are best suited to laboratory settings where pressure ranges are frequently changing, where a digital display is needed and where installation flexibility is important.

Type 2 products, however, do not have built-in temperature compensation, must be calibrated by the user with an appropriate pressure standard and are generally more expensive per measurement point.

Interfacing 4-20 mA Current Loops to the USB2250

Many pressure transducers and other field instruments use the two-wire 4-20 mA current loop for both power and signal. The 4-20 mA current loop is economical to install, using the same two wires for power and signal. It is also ideal for sending a signal over long distances – up to a mile or more – with high resistance to noise. Most data acquisition devices, however, are configured to accept voltage signals. This application note will describe how to interface a standard two-wire 4-20 mA current loop to the USB2250 sensor interface.

A typical 4-20 mA transmitter receives power from an external power supply. The power supply must be able to provide enough voltage and current to power the transmitter under all operating conditions. The transmitter will require some voltage just to produce a signal and the power supply must provide this plus any power needed to overcome any resistances placed in the current loop. The maximum amount of current required by the transmitter will be at least 20 mA, but it is best to select a power supply that will provide for 25 or 30 mA through the loop to allow for over-range indication by the transmitter.

To interface to a data acquisition device such as the USB2250 a resistor is placed in the loop and the voltage drop across the resistor will be connected to the USB2250 as a single-ended voltage input. To see how this works, assume the following conditions:

Minimum voltage required by the transmitter = 12 Vdc
Maximum loop current = 25 mA
Interface Resistor = 250 Ohms
Wire or other miscellaneous resistances in the loop = 20 Ohms

To calculate the voltage required for the power supply, we add up the voltage drops in the loop:

12 Vdc for the transmitter
Voltage drop through the wire = 0.025 * 20 = 0.5 Vdc
Voltage drop through the interface resistor = 0.025 * 250 = 6.25 Vdc

Adding these voltage drops together, the minimum voltage provided by the power supply must be 12 + 0.5 + 6.25 = 18.75 Vdc to push 25 mA through all the resistances in the loop. For a single loop the power supply will need to be rated for at least 18.75 * 0.25 = 0.47 W, but typically a single power supply will power many loops before wattage ratings become an issue.  We can use a 24 Vdc power supply – just as long as it is greater than 18.75 Vdc.

The diagram below shows how the current loop is interfaced to the USB2250 terminal block. Note that the power return is common to the USB2250 signal ground. The voltage drop across the resistor is 1 Vdc when the signal is 4 mA and 5 Vdc when the signal is 20 mA, being proportional in between.

4-20 mA USB2250

USB2250 scale and offset factors in Easy Sense software are used to convert the 1 to 5 Vdc input into engineering units. If, for example, the 4-20 mA signal is for a 100 psig pressure transducer, then at an input of 1 Vdc the pressure is 0 psig and at 5 Vdc the pressure is 100 psig.

The algebra is simplified by first determining the scale factor = change in pressure/change in voltage = 100/4 = 25.

Multiply the scale factor by -1 to obtain the offset factor: 25 * -1 = -25.

Check by determining the readings at each end point:

At 1 Vdc the reading will be R = (25 * 1) -25 = 0 psig

At 5 Vdc the reading R = (25 * 5) -25 = 100 psig

The USB2250 will read the input from the current loop and provide readings in psig or any other engineering units.

Wind Testing of Metal Buildings

Metal buildings are relatively inexpensive, easy to construct and are used extensively for agricultural purposes in the western United States. But metal buildings are light and often present a large flat surface to winds. In areas like West Texas, where local wind conditions can be severe, testing metal structures under field conditions is an important part of specifying safe building codes.

Texas Tech has constructed a field test station in an open field near Lubbock to provide data on wind pressures that are generated under local conditions. A 30 ft X 45 ft X 13 ft prefabricated metal building was anchored to a rigid undercarriage that is mounted on a circular rail track. The entire building can be rotated to create different angles of attack to the prevailing winds. The surrounding area is flat farmland and is representative of the wind conditions in that part of the state. A meteorological tower was constructed nearby to record local wind speed, direction, temperature and barometric pressure. The diagram below shows the concept of the rotatable building.
windtest
A series of 100 pressure taps were drilled in the various flat surfaces to measure the pressures developed along the outside of the building. Inside, Validyne DP103 pressure transducers were used to measure the pressures.  The DP103 is sensitive to low pressures and has a relatively fast response.  The DP103 transducers were calibrated to +/-0.18 psi.  The recording rate of the transducer pressures is 40 times per second by a PC-based data acquisition system. Up to 96 simultaneous pressure measurements are possible and wind speeds of up to 90 mph can be accommodated.

The pressure data that is collected under various wind conditions and angles of attack are used to calculate the loading force under actual wind conditions so that the anchoring requirements and profiles of prefabricated metal buildings can be specified for maximum safety.

Absolute or Gage Sensor – Which is Best to measure Absolute Pressure?

If you want to make an absolute pressure measurement, you have two choices of sensor type.  The first and most obvious choice is to specify an absolute pressure transducer.  Another choice is to use a gage pressure sensor and offset the output signal to exclude the local atmospheric pressure.

An absolute pressure transducer is referenced to absolute zero and such transducers are manufactured to incorporate a vacuum chamber into the sensor body. This adds to the cost of an absolute transducer and so a more economical approach is to use a gage pressure sensor that is open to the atmosphere and then offset the output signal by 14.7 psi.

A gage pressure transducer with a fixed offset for atmospheric pressure, however, has the disadvantage of being affected by the variations in atmospheric pressure due to the weather. It is possible to quantify this error and determine if a gage-referenced sensor with an offset will result in a satisfactory measurement.  For historical records we can see the range of barometric pressure variations due to the weather.  These vary depending on geographic location.

The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 30.01 in Hg, with record highs close to 32.04 in Hg .  The lowest measurable sea-level pressure is found at the centers of tropical cyclones and tornadoes, with a record low of 25.69 in Hg).

Honolulu, Hawaii is the place in the US with the overall smallest range of changes in barometric pressure, ranging from 29.34 to 30.32 in Hg. San Diego is the city with the smallest range of changes in the continental US, with an average range of about 29.37 to 30.53 in Hg.

As for the places with the greatest range of pressure changes, St. Paul, Alaska ranges from 27.35 to 30.86 in Hg. In the contiguous US, Charleston, South Carolina has the largest range of changes, with a 27.64 to 30.85 in Hg.

Choosing between an absolute transducer with a full vacuum reference and a gage-referenced transducer with an offset is simply a matter of determining how much error will result from typical ambient atmospheric pressure changes due to the weather.

Here is a table to help guide the selection process.

absolute pressure

 

Here are two examples:

You want to make a 200 psia pressure measurement and you are in the southeast US. The weather will likely be similar to Charleston, SC and so looking at the chart you would expect an error of 0.788% to be the result of ambient atmospheric changes if you were to use a gage pressure sensor offset for atmospheric pressure. This error is excessive so the extra expense of a true absolute transducer would be warranted.

You want to make a 2000 psia measurement in Los Angeles. San Diego has a similar weather pattern so from the chart you can see that the error incurred by using an offset gage pressure sensor would be just 0.029% FS. This is very small compared to the 0.25% accuracy of the transducer and so would be the most economical choice.

Look at Validyne’s offerings of absolute and gage sensors and transducers

Test and Measurement Grade Pressure Transducers (P895,P896,P897V)

General Purpose Pressure Transducers

AP10 Variable Reluctance Absolute Pressure Sensor

 

Guide to Bolt Torque Validyne Pressure Transducers and Sensors

Variable reluctance pressure transducers may be disassembled and the range changed by installing a new sensing diaphragm. When changing diaphragms, the correct body bolt torque is extremely important to performance.

The sequence for torquing the body bolts is just like putting the wheel on a car. Try to apply the torque in stages. Do not torque one bolt fully while the others are still loose.

Here is a listing of the bolt part numbers and correct body bolt torque settings, by transducer model number:

bolt torque

 

Here are the tools needed:

bolt torque

Validyne offers a wide array of Pressure Sensors and Pressure Transducers. Feel free to contact us for more information on any of these products to fit your application.