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Calibration and documentation for process manufacturing_ Costs, benefits and feasibility Страница: 2

Fluke 754 [2/5] How do field instruments work and what kind of calibration do they require

2 Fluke Corporation Calibration and documentation for process manufacturing: Costs, beneﬁts and feasibility

In so-called “validated” industries (such as

the pharmaceutical industry), any changes to a

process line, including repair or replacement of

a process instrument or a ﬁnal control element,

must be re-validated and traced to documenta-

tion before the process line can be put back

into service. Poor calibration documentation can

make this validation process time consuming and

expensive and put the manufacturer at risk of

ﬁnes by the regulating government agency.

Savings

Calibration and documentation are usually

considered expenses, and the higher efﬁciency

resulting from good calibration practices may be

hard to distinguish. Consider, then, the known

costs. On the one hand: loss of an entire batch

due to quality issues. On the other: legal costs and

lost revenue from accidents, which at the reﬁnery

mentioned earlier has exceeded $100 million. If

disaster strikes, good calibration records can be

a part of a facility’s defense in the event of legal

action (just as poor records can put an organiza-

tion in a less defensible legal position).

How do field instruments work,

and what kind of calibration do

they require?

Most field instruments are made up of two parts:

a primary element and a transmitter.

• Primary elements include flow tubes, orifice

plates, pressure sensors, wet chemistry sen-

sors such as pH, ORP, and conductivity probes,

level gauges of all types, temperature probes,

and others. Primary elements typically produce

a signal—usually voltage, current, or resis-

tance—that is proportional to the variable they

are designed to measure, such as level, flow,

temperature, pressure, or chemistry. Primary

elements are connected to the input of field

transmitters.

• Field transmitters include pressure, tem-

perature and flow devices. They process the

signal generated by the primary element, first

characterizing it in linear format and applying

engineering unit coefficients to it, before then

transmitting it in analog (usually 4-20mAdc)

or digital format (usually some variety of

fieldbus).

Note: When a field instrument is manufactured, both the

primary element and the transmitter (or the actuator, if a

control valve) are calibrated at the factory and the calibra-

tion information is supplied with the unit. This calibration

data is often lost. Entering this information into centralized

calibration records when the device is put into service

should be part of standard work, and not just for effi-

ciency’s sake. Centralizing calibration information ensures

knowledge stays with the facility even as teams change.

Analog devices

Analog devices—often called “4 to 20 milli-

amp loop” devices—are so called because they

transmit a signal that is an electrical “analog”

representation of a measured physical quantity

(temperature, for example). They transmit an

electric current that is proportional (analogous)

to the magnitude of a measured physical quan-

tity, with 4 milliamps of current representing the

minimum scaled value and 20 milliamps repre-

senting the maximum scaled value. This relatively

simple technology has low sensitivity to electri-

cal “noise” and has been used for many years.

Although many system aspects are now digital,

analog devices are still in active use throughout

the process manufacturing world. A 2010 survey

in Control Global magazine found that 30 % of

plants surveyed continued to use analog instru-

ments and current loops. Because analog circuits

such as current loops drift over time, they require

regular calibration.

Digital devices

Digital devices convert a measured physical value

into a digital signal. Many different digital encod-

ing methods are used in the process industry,

including Foundation, Profibus, and HART.

There is a widespread belief that ﬁeldbus

(digital) ﬁeld devices do not require calibration.

This is not true.

Although a fieldbus signal (whether Foundation,

Profibus, or connected HART) provides diagnostic

information, it does not provide information about

the accuracy of the device, nor does it verify that

the device is reporting the process accurately

and precisely. For example, a Foundation fieldbus

differential pressure transmitter can report diag-

nostic information about the transmitter, but it

cannot report on the physical condition of the ori-

fice plate across which it is measuring pressure.

Consequently, even if the electronics are operat-

ing perfectly, the flow reading transmitted may be

inaccurate. Thus, calibration is required even for

digital devices.

Fieldbus systems do not have an analog output

that technicians can use to verify the accuracy of

instrument transmissions to the control system.

Without an easily readable output, facilities must

either install a readout display at the device or

perform calibrations with one technician at the

device and the other in the control room. Both

options increase calibration costs.

Control valves

Control valves have actuators that also require

calibration to adjust for wear and the effects of

stiction. Often these valves must be given a partial

stroke test if they haven’t been actuated regularly.

Fluke 754 [2/5] How do field instruments work and what kind of calibration do they require

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