Pressure Measurement and Calibration for Process Instrumentation

Pressure units Pressure = Force / Area SI units: Newton/square meter = Pa (Pascal) kg/cm2 is based upon SI units, but Pa is the officially recognized unit. British units: pounds/square inch = psi Many units are based upon the pressure exerted by a column of liquid. inHg, mmHg, inH2O, etc. Converting between units is easily done by multiplying by an appropriate factor.

Ideal Gas Law Combining Charles’ and Boyle’s Law results in the Ideal Gas Law P=kT/V Chemists often use PV=nRT k (or nR) is a property of the gas Example: Volume smaller and/or temperature higher will increase pressure. 이상 기체 상태 방정식은 다음과 같은 식으로 표시된다. PV = nRT (P : 압력 V : 부피 n : 기체의 몰수 R : 기체상수 T : 온도(K) ) 이 식은, 기체의 법칙, 즉 다음과 같은 법칙을 일반화시킨 것이다. 보일의 법칙 : 온도가 일정하면 압력과 부피는 반비례한다. 샤를의 법칙 : 압력이 일정하면 부피는 온도에 비례한다. 보일-샤를의 법칙 : 부피는 압력에 반비례하고 온도에 비례한다.

Types of pressure measurement Gage Pressure relative to atmospheric pressure “PSIG” indicates this type of pressure Differential Pressure difference “PSID” indicates this type of pressure Absolute Pressure relative to a perfect vacuum “PSIA” indicates this type of pressure

Definition of Pressures Every pressure is the difference between two pressures Pressure that is measured relative to vacuum is: Absolute Vacuum Atmospheric Pressure

Definition of Pressures Every pressure is the difference between two pressures Pressure that is measured relative to atmosphere is: Positive and Negative Gauge (Relative) Vacuum Atmospheric Pressure

Definition of Pressures Every pressure is the difference between two pressures The difference between two pressures, neither of which is necessarily vacuum or atmosphere is: Differential (Relative) Vacuum Atmosphere Pressure

Pressure measurement The manometer gage Atmospheric Pressure Atmospheric Pressure The manometer gage Under static conditions, both sides will be at equal heights. Pascal’s Law

Gage pressure Applied Pressure Atmospheric Pressure Apply pressure to one side of manometer, leave other side vented to atmosphere h h g * r = P    ρ : 유체밀도      g : 중력가속도      h : 높이의 차

Differential pressure (High) Pressure (Low) Apply two pressures to both sides of the manometer again... h h g * r = D P

Absolute pressure Pressure relative to a vacuum Barometers by definition measure absolute pressure Vacuum 760 mm Atmospheric Pressure Mercury

Pressure calibration basics Pressure is the second most commonly measured PV. Pressure controls used in virtually all process and manufacturing industries Petrochemical, food and beverage, pharmaceutical, etc. Environmental, motor and process pressures Many different measurement technologies Strain gauge Capacitive Piezoelectric Deadweight balances

Pressure transmitter linear Converts pressure to linear or square root responding 4-20 mA or dc voltage Millions in service, require service and calibration “Pressure controlled current regulator” + – Sensor Input Gage Absolute Differential 24 V Loop Supply Pressure Transmitter 3 to 15 PSI 7 6

Pressure transmitter, square root Converts pressure to square root responding 4 - 20 mA or dc voltage The square of the input % of span produces that percent output With 25 % of span applied, 50 % output + – Sensor Input Gage Absolute Differential 24 V Loop Supply Pressure Transmitter 0-100 inH20 7 6

Analog pressure transmitters Fixed ranged Some turn-down and re-ranging via jumper settings Adjustment manual, zero (4 mA) and span (20 mA) Some include linearity and dampening adjustment Signal conditioner technology, little or no digitization Analog Transmitter Zero Span Analog Electronics Input Sensor 4-20 mA Output

Fluke 740B Series Documenting Process Calibrators Pressure transmitter calibration using a portable calibrator: connection 49

Configure the calibrator Fluke 740B Series Documenting Process Calibrators Configure the calibrator Set the correct source and measure functions. The measure portion of the screen should be in mA. The source portion of the screen should be in pressure. Define the span parameters, acceptable tolerances and test strategy (example: 0 – 50 – 100 %) for the test. 49

Documenting the as found test For each test step: Capture the applied pressure. Measure the actual mA value. Calculate and document the error % of span for the measured mA. Record the tag, serial number and operator for the as found result.

Adjusting to within tolerance, as left test If the as found test passes within limits: No adjustment required. Return transmitter to service. If the as found test fails: Adjust the zero and span. Run the as left test, confirm, document adjustments.

I to P Transmitters I to P converts a 4-20 mA signal to a 3-15 PSI signal Often used with control valves Used as a bridge between 4-20 mA loop and 3-15 PSI pneumatic technology Typically operate from a 20 PSI or greater pressure supply “A current controlled pressure regulator” Pressure Output 4-20 mA Current input Supply Pressure ~20 PSI

I to P calibration using a portable calibrator: connection

Configure the calibrator for the test Select measure pressure and source mA. Define the test parameters. Document the results.

Adjust and as left test Apply the 0 % pressure value and adjust the zero. Apply the 100 % pressure value and adjust the span. Verify 50 % linearity. Run the as left test. Document the results.

Smart transmitter calibration Smart pressure transmitters have no physical zero and span adjustment. Smart Transmitter PV URL PV LRL Sensor Trim Pressure Zero Loop Test Output Trim Input Sensor Sensor Input Range Computation Instrument Output 4-20 mA Output PV PVAO

Smart pressure transmitter Three stages Sensor block Converts pressure signal to digital (A/D) Range computation Holds the programmed settings for zero (LRV) and span (URV) Outputs PVAO digital mA output signal Output section Receives the PVAO command and output the mA signal (D/A) Input and output section are individually calibrated to provide overall performance. Sensor Input Range Computation Instrument Output 4-20 mA PV PVAO Sensor Trim PV URV PV LRV Loop Test Output Trim A HART instrument is more complex, having three distinct stages.

Example smart calibration Almost identical to analog transmitter connections HART cable connection required for digital adjustment. Rosemount 275 or 375 can be used in place of Fluke 744 for digital adjustment (requires an analog calibrator).

Configure the calibrator Acquire the test parameters (HART query). Configure the input/output of the calibrator. Define the test parameters. Configure the measure and source function of the calibrator. Define the performance test. Acquire HART configuration.

Calibration, as found test Document the pre-adjustment transmitter condition.

Calibration, digital adjustment Perform HART trim on the transmitter.

As left test Verify post adjustment condition of transmitter. If still out of tolerance (OOT) Repeat adjustment. Re-run as left until it passes within tolerance. If it cannot be satisfactorily adjusted Was the test tolerance too tight? Were there leaks in the fittings? Repair or replacement if necessary

Summary Pressure is sensed via a number of different methods. Pressure indicators vary in functionality. Many configured as switches Pressure transmitters vary in range and type. Analog and digital Gage, differential, absolute Analog transmitters are adjusted with a manual tool. Smart transmitters need a digital adjustment tool.