Category Archives: P55

Stand-Alone Pressure Transducer or Sensor + Electronics?


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.

Soil Consolidation Testing

Soil consolidation testing is used to predict the ability of a certain soil to bear a load safely. Perhaps the most spectacular example of this is the leaning tower of Pisa. Construction was started in the 1173 and the tower took 200 years to complete. During the first 5 years of construction the soft soil on one side of the foundation began to consolidate under the weight of the stone structure and it began to lean. Construction was halted, for a variety of reasons, until 1272 and by this time the soil had stabilized and the rest of the tower was curved to compensate. Eventually, in the late 20th the tower structure was reinforced and remains a famous tourist attraction.


Large buildings today are built on foundation soil that has been tested for compaction before construction begins, and this is why modern buildings that are heavier and taller than the Tower of Pisa can be constructed.

Soil Compaction Testing:
To test soil for unsuitable compaction, a test of several samples is performed in a soils laboratory. The idea is to simulate the conditions of load and water pore pressure in the soil and then measure the amount it compacts, or is compressed under the load. Here is a sketch of a soil compaction test cell.

This seems complicated but the idea is that a static load is applied to piston that transmits the force to the soil sample contained within the test cell. Water is introduced and kept to the pressure expected for the construction site and this pressure is sensed and controlled by a pressure transducer. The force of the load is sensed by a series of strain gage load cells and the movement of the sample as it compacts is measured by an LVDT.
A compaction test may take a few hours or several days, depending on the soil conditions and applied load. The idea is to verify that the soil will be strong enough to bear the weight of the building to be built on that particular site.

Commercial Soil Compaction Testing System:


In this example a servo-controller is used to apply a continuous load to the sample instead of heavy weights which would make for a bigger and less stable fixture. The load cell just beneath the top platten measures the actual force applied and a spring-loaded LVDT measures the change in thickness of the soil sample as it consolidates.

Validyne pressure transducers are used to measure pore pressures and our USB2250 signal conditioning can be used as the data acquisition interface to record pressure, the force sensed by the load cell and the sample displacement from the LVDT signals. The USB2250 can accept all three sensor types directly, including other sensors such as themocouples and DC voltages. The variety of sensor types used in soil compaction testing – load cells, LVDTs and pressure transducers – make the USB2250 an ideal sensor interface. Validyne will work with customers to integrate the USB2250 into existing soil test software.

Selecting Accessories for the Recalibrating the P55 Pressure Transducer

The Validyne P55 pressure transducer has as its sensor a variable reluctance pressure sensor that can be re-ranged for different full scale pressure measurements. The sensor can be disassembled, a new sensing diaphragm installed and the unit re-calibrated to the new full scale pressure. Some 23 different full scale pressure diaphragms are available and this application note will describe how to select and order the parts needed to re-range the sensor and interface the signal to a PC.

Sensor Parts:
A typical P55 is shown below, with the external parts identified:

P55 Parts pressure transducer





First, remove the two Philips head screws holding the sensor to the P55 electronics housing. These are located on the underside of the housing. The wires from the sensor to the electronics are very short, so take care they do not break.

To disassemble a P55 sensor a torque wrench, T27 Torx socket and a vise are needed. The tools needed to disassemble the sensor are available from Validyne and are shown below:

torquewrench pressure transducer






The sensor can be disassembled by removing the four 10-32 Torx T27 body bolts. When disassembled, the sensor body pieces separate and the sensing diaphragm and o-rings are removed. These parts are shown below:

boltsorings pressure transducer






It is good practice to replace the body bolts and o-rings when changing the range of the P55. Various o-ring compounds are available (see ordering chart).

The sensing diaphragm may now be replaced with one of a different range. A typical sensing diaphragm is shown below:

diaphragm pressure transducer






To re-range a P55 sensor the full scale pressure must be known and the correct diaphragm part number ordered. The part number for a P55 diaphragm starts with 3- and is followed by a two-digit range code. The diaphragm in the photo above is p/n 3-22 and has a full scale range of 5.5 In H2O. The other available range codes for the P55 sensing diaphragm are shown in the chart below with their full scale pressures expressed in various engineering units.

P55Ranges pressure transducer








Re-assembly is simply the reverse of dis-assembly, taking care that the torque on the body bolts is 125 In-Lb. The vise is used to stabilize the sensor body during assembly and to allow the torque to be correctly transmitted to the body bolts.

Also be sure that the bleed screws are tightly seated – these use a 5/64” hex wrench, Validyne p/n K950-0781. The sensor is reattached to the housing using the two Phillips head screws.

Calibration Accessories:

The next step is to calibrate the P555 against a pressure standard. Validyne can supply model T140K calibrator kit that includes a pressure pump and reference standard – an example is shown below.

T140K pressure transducer








The T140K calibrator kit is available in six different versions covering the available DP15 full scale pressure ranges. To calibrate theP55 connect it the SI58 digital interface and have a voltmeter to observe the analog output signal of the P55 as it appears on the binding posts of the SI58. 

SI58 pressure transducer






The SI58 connects to any USB port on a PC and is supplied with software that allows changing the internal registers of the P55 to achieve an accurate calibration. Connect the re-ranged P55 to the SI58 and the SI58 to a PC. Connect a multimeter to the SI58 binding posts to observe the P55 output signal.  

P55Cal pressure transducer





Load the calibration software and follow the instructions for applying zero and full scale pressures using the T140K calibrator. The software will adjust the P55 microprocessor correction factors to produce an accurate calibration with the new sensing diaphragm.

The SI58 software also allows the user to compensate the P55 through temperatures. The temperature range can be selected by the user as applied by an environmental chamber. 

SI58 Software pressure transducer


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.