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Pressure

 

Keep plants operating efficiently and safely

Pressure instrumentation is found in virtually every process plant. Periodic calibration of these pressure, level, and flow instruments is required for efficient operation. 

 

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Fluke provides a broad range of pressure calibration tools to help you quickly and reliably calibrate your pressure instrumentation. 

Fluke 719 Electric Pressure Calibrator

These pressure calibrators accurately measure pressure by using:
• Internal sensors, or
• External pressure modules

A pressure source may be provided by:
• A self-contained internal pressure pump, or
• An external source such as an accessory pump or a pressure bottle / regulator.

A summary of the pressure calibration capabilities of Fluke Process Tools is shown below.

How to use the 718 to calibrate a pressure switch

Pressure Switch Calibration

1. Depressurize and isolate the pressure switch from the process. 
2. Plumb the 718 and make connections as per the illustration.
3. Turn on the 718 and open the vent valve. Press the Zero button to clear the zero offset. Close the vent.
4. Press the Switch Test button to enter the switch test mode.
5. Apply pressure slowly with the hand pump until you approach the setpoint. Using the fine adjust vernier adjust the pressure until the switch opens and OPEN is displayed on the 718.
6. Release the pressure slowly using the fine adjust vernier until RCL is displayed.
7. Press the Switch Test button once to read the pressure values for switch opening and again to see the pressure at switch closing.
8. Press and hold the Switch Test button for 3 seconds to clear the test results and start over.
9. Adjust the pressure switch setpoint until the switch contacts open and close at the desired pressure.

Typical Pressure Applications

How to calibrate a P/I transmitter

Calibrating a P/I transmitter

With a built-in hand pump, precision measurement of both pressure and current, and a 24 volt loop supply, the Fluke 718 Pressure Calibrator is a complete, self-contained tool for the calibration of P/I transmitters. To calibrate a 3-15 psi / 4-20 mA transmitter using a Fluke 718 30G Pressure Calibrator:

1. Depressurize the transmitter, and then plumb the transmitter to the 1/8 inch NPT pressure port of the 718. Connect the test leads per the figure above.
2. Turn the calibrator on. (If you need to power the transmitter, hold down the UNITS key while turning on the calibrator.)
3. Press the UNITS key until PSI shows in the display.
4. With the 718’s bleed valve open to atmosphere, press the ZERO key. Close the bleed valve. Set the pressure / vacuum valve to +, for positive pressure.
5. Use the hand pump to apply roughly 3 psi to the transmitter. Partial pump strokes will apply small increments of pressure. Use the fine-adjust knob to get reasonably close to 3.00 psi.
6. Press the HOLD key, and record the psi and mA readings. Press the HOLD key to resume reading.
7. Calculate and record the error, using: Error = ([ (i-4)/16] – [(P-3)/12]) * 100 where Error is in % of span, i is your measured current in mA and P is your measured pressure in psi.
8. Repeat steps 5 through 7 at mid-range, around 9 psi, to check linearity at mid-span.
9. Repeat steps 5 through 7, now at 15 psi, for a check at 100 % of span.

If your calculated errors are within tolerance, the transmitter has passed your As-found test, and you are done. If necessary, perform your zero and span adjustments, then repeat steps 5 through 9 for an As-left test. Depressurize the line, and disconnect the 718.

A complete family of pressure modules

A family of 29 pressure modules covers the most common pressure calibrations from 0 to 1" H2O (0 to .25 kPa) to 0 to 10,000 psi (0 to 70,000 kPa). Gage pressure modules have one pressure fitting and measure the process pressure with respect to atmospheric pressure. Differential pressure modules have two pressure fittings and measure the difference between the applied pressure on the high fitting versus the low fitting. Each module is clearly labeled for range, overpressure specification, and media compatibility. A metric adapter is included with all but the P29 through P31 high pressure modules.

Quick and easy measurements

Fluke 700 Series pressure modules are easy to operate. To measure pressure, the technician plumbs the pressure module to a pressure source, and connects the pressure module cable to the calibrator. Pressure is applied, measured by the pressure module, and displayed digitally on the calibrator. At the touch of a button, the pressure may be displayed in up to 11 different engineering units. When used with the 750 Series Documenting Process Calibrators, pressure readings can be date/time stamped and stored electronically for later retrieval. This saves time, eliminates errors, and supports compliance with quality standards and regulations.

Pressure module performance

Fluke 700 Series pressure modules are highly accurate, with total specifications that apply from 0 % to 100 % of full span and from 0 °C to 50 °C (32 °F to 122 °F)—a feature that sets them apart from other pressure calibrators. Many ranges have total uncertainties of 0.05 % of full scale and reference uncertainties of 0.025 % of scale. This performance is possible through the innovative application of mathematics and microprocessor power. Fluke pressure modules have silicon piezoresistor sensors which consist of a resistive bridge fabricated in a silicon diaphragm. Pressure applied to the diaphragm causes a change in the balance of the bridge which is proportional to the applied pressure. The bridge balance change is not linear and is very sensitive to temperature. However, since these effects are quite stable with time and through repetitive changes of condition, the sensors can be very accurate in measuring pressure provided they are carefully characterized. During manufacture, Fluke pressure modules are characterized by reading temperature and pressure at more than 100 points. A least-squares regression is used to calculate the coefficients of a polynomial expression for pressure. The coefficients, unique to that pressure module, are stored in the module’s memory. Each module has its own microprocessor, allowing it to run the measurement circuitry and to communicate digitally with a calibrator. When connected to the calibrator, the modules coefficients are uploaded from the pressure module to the calibrator. Then, as pressure measurements are made, raw sensor values for pressure and temperature are digitally loaded to the calibrator, where the raw sensor values and coefficients are manipulated to derive and display the pressure reading. This innovative technique provides several benefits:

1. Digital communication eliminates errors due to poor connections and electrical interference.
2. The modules are inherently temperature-compensated from 0°C to 50°C (32°F to 122°F).
3. The modules are fully interchangeable because all measurements are completed in the pressure module itself and then communicated to the calibrator in digitized form. Modules are calibrated independently of the calibrator, and can be used with any 700 Series calibrator. Each module has its own serial number to facilitate traceability.

Sensor protection in isolated modules

Many of these modules incorporate a stainless steel diaphragm to isolate the sensor. With these modules, any medium that is compatible with stainless steel can be used on the high side of the module.

Rugged construction

A urethane overmolding protects against shock if a module is accidentally dropped and also seals against dirt, dust, and moisture. Pressure connections are 1/4? NPT. A BSP/ISO adapter is also provided on all but the P29, P30 and P31. Convenient setup A one-meter cable between the pressure module and calibrator reduces the length of connecting tubing to the pressure source. The remote pressure head also provides an extra margin of safety and convenience by removing the calibrator and operator from the pressure source.

Originally published by Fluke Calibration at Fluke Calibration Training Center

 

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