Calibrating pressure transmitters and switches in potentially explosive atmospheres

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Calibrating pressure transmitters and switches in potentially explosive atmospheres (700.29 KB)

Pressure calibration can be a difficult challenge in the best of environments. Doing so in an area that has the potential for explosion takes the degree of difficulty to another level—one where the technician needs proper training and equipment. This article will demonstrate the practical application of pressure calibrators designed to perform calibrations in potentially explosive environments.

What is “intrinsically safe?”

Intrinsic safety is a protection method employed in potentially explosive atmospheres. Devices certified as “intrinsically safe” are designed to be unable to release sufficient energy, by either thermal or electrical means, to cause ignition of flammable material (gas, dust/ particulates).

Intrinsically safe standards apply to all equipment that can create one or more of a range of defined potential explosion sources:

• Electrical sparks
• Electrical arcs
• Flames
• Hot surfaces
• Static electricity
• Electromagnetic radiation
• Chemical reactions
• Mechanical impact
• Mechanical friction
• Compression ignition
• Acoustic energy
• Ionizing radiation

Intrinsic safety is particularly important for technicians working in industries like petrochemical and pharmaceutical, around bulk materials such as grain, mining, or any environment where explosive gases are present.

The importance of safety in these environments can’t be stressed enough. It takes a very small amount of energy to cause an ignition; e.g., a mixture of hydrogen in air requires only 2O uJ of energy. The proper practices and tools will minimize the inherent risk involved in working around these hazards.

Intrinsically safe pressure calibration

To conduct pressure calibrations in potentially explosive environments, you need a pressure calibrator that is certified as intrinsically safe. Intrinsically safe pressure calibrators, such as the Fluke 718Ex, must be certified in accordance with the European ATEX (“Atmosphères Explosibles,” French for explosive atmospheres) directive ( II 1 G Ex ia IIC T4) for use in Europe and NEC 500; I.S. Class 1, Division 1 areas Group A-D T4 for use in the U.S.

In addition to an intrinsically safe pressure calibrator, strict adherence to calibration procedures is recommended, including:

Lock out: Make sure the system is shut down and other workers are notified that a potentially dangerous operation will be taking place.

Tape off area: Tape the work area off to prevent workers from entering with potentially dangerous electrical devices (cell phones, handheld computers, non-intrinsically safe tools). Purge or vent the systems: Safely purge or vent the system to remove any gases that may remain.

Use a gas detector: In an environment where explosive gas may be present, the use of a gas detector is a prudent step before starting a pressure calibration. Gas “sniffers” are available for a wide variety of applications and from handheld to larger, carted models.

Calibrate: Perform your calibration using an intrinsically safe pressure calibrator.

Clean up and reactivate: At the conclusion of calibration, reverse the process and reactivate the system.

Pressure calibration

The 718Ex Pressure Calibrator has the ability to apply pressure to a pressure transmitter or pressure switch while simultaneously measuring the mA signal or switch contacts.

Typical Pressure Applications

How to calibrate a P/I Transmitter in a classified location

It is important to be familiar with intrinsically safe work practices before performing any maintenance task in a classified area where explosive gasses may be present. Be sure to take proper precautions and refer to the 718Ex users manual and Fluke 718Ex CCD before proceeding.

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

1. Depressurize and isolate the transmitter. If opening the transmitter exposes voltages beyond the Instrinsic Safe (I.S.) rating use a continuous monitoring gas detector to monitor for explosive gases.
2. Plumb the transmitter to the 1/8 inch NPT pressure port of the 718Ex. Connect the test leads per the figure above.
3. Turn the calibrator on. Vent the calibrator and press zero to zero the pressure displayed. Close the vent control.
4. Press the UNITS key until PSI shows in the display.
5. With the 718Ex’s bleed valve open to atmosphere, press the ZERO key. Close the bleed valve. Set the pressure / vacuum valve to +, for positive pressure.
6. 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.
7. Press the HOLD key, and record the psi and mA readings. Press the HOLD key to resume reading.
8. 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.
9. Repeat steps 5 through 7 at mid-range, around 9 psi, to check linearity at mid-span.
10. 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 718Ex. Continue to monitor for explosive gas until the transmitter is returned to service and all non-I.S. voltages or energy sources are isolated from exposure to the classified environment.

Measuring less than 9 inches in length and weighing just over two pounds, the rugged 718Ex is easy to carry into the field. The 718Ex is offered in both 30 psi and 100 psi models. Media compatibility is dry air and non-corrosive gasses. A built-in pump generates pressure or vacuum. Min, Max, and Hold functions are available. The 718Ex can also measure pressure using any of the Fluke 700PEx Pressure Modules, to cover applications up to 3,000 psi. The 718Ex comes complete with protective holster, test leads, test clips, Users Manual, and one 9-volt battery (installed).

How to use the 718Ex to calibrate a pressure switch in a classified area

It is important to be familiar with intrinsically safe work practices before performing any maintenance task in a classified area where explosive gasses may be present. Be sure to take proper precautions and refer to the 718Ex users manual and Fluke 718Ex CCD before proceeding.

1. Depressurize and isolate the pressure switch from the process. If opening the switch exposes voltages or energy that is not intrinsically safe, use a continuously monitoring gas detector to monitor for the presence of explosive gasses.
2. Plumb the 718Ex and make connections as per the illustration.
3. Turn on the 718Ex 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 718Ex.
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.
10. Continue to monitor for explosive gasses until the switch is returned to service and all non-IS voltages or energy sources are isolated from exposure to the classified environment.

Specifications

Pressure specifications (internal sensor)

18 °C to 28 °C, one year

Operating modes

Input/output

Battery

Current specifications

18 °C to 28 °C, one year

Zeroing modes

Environmental and safety requirements

Operating temperature	-10 °C to 55 °C
Non-operating temperature	-40 °C to 60 °C
Relative humidity (%RH operating without condensation)	95 % (10 to 30 °C) 75 % (30 to 40 °C) 45 % (40 to 50 °C) 35 % (50 to 55 °C)
Dimensions (LxWxD)	216 mm x 94 mm x 66 mm (8.50 in x 3.72 in x 2.60 in)
Weight	35 oz (992 g)
Vibration	Random, 2g, 5-500 Hz
Shock	1 Meter Drop test
Safety	I.S. Class I Div 1 Groups A-D T4AEx ia IIC T4 II 1 G Ex ia IIC T4 KEMA 04 ATEX 1061 Ta = -10 °C to +55 °C
EMC	EN61326 2002-02, Criteria C

Pressure Modules

The I.S. family of pressure modules

A family of 8 Intrinsically Safe pressure modules covers the most common pressure calibrations from 0-10” H2O (0-2.5 kPa) to 0-3,000 psi (0-20,000 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 P29Ex high pressure Ex module.

Quick and easy measurements

Fluke 700PEx 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.

Pressure module performance

Fluke 700PEx 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 (see table below).

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 P29Ex.

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.

700PEx:

I.S. Class I Div 1 Groups A-D T4 AEx ia IIC T4

II 1 G Ex ia IIC T4 Ta = 0 °C to +50 °C

Pressure module specifications
(All specifications in % of full span. Specifications reflect a confidence interval of 95 %.)

Pressure Accessories

Pressure Terminology