Category Archives: Vacuum

Selecting Accessories for the Recalibrating the P55 Pressure Transducer

Introduction:
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

 

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.