Transcript Slide 1

Instrument Parameters
The accuracy of an instrument or device is the difference between the
indicated value and the actual value. Accuracy is determined by
comparing an indicated reading to that of a known standard.
Accuracy depends on linearity, hysteresis, offset, drift, and sensitivity. The
resulting discrepancy is stated as a ± deviation from the true value, and is normally
specified as a percentage of full-scale reading or deflection (%FSD). Accuracy can also
be expressed as the percentage of span, percentage of reading,
or an absolute value.
Example 1 A pressure gauge ranges from 0 to 50 psi, the worst-case spread in
readings is ± 4.35 psi. What is the %FSD accuracy?
%FSD = ± (4.35 psi/50 psi) x 100 = ± 8.7
The range of an instrument specifies the lowest and highest readings it can measure, i.e., a
thermometer whose scale goes from -40 C to 100 C has a range from -40ｰC to 100ｰC.
The span of an instrument is its range from the minimum to maximum scale value, i.e., a
thermometer whose scale goes from -40 C to 100 C has a span of 140 C. When the accuracy is
expressed as the percentage of span, it is the deviation from true expressed as a percentage of the
span.
Reading accuracy is the deviation from true at the point the reading is being taken and is
expressed as a percentage, i.e., if a deviation of ± 4.35 psi in Example 1 was measured at 28.5 psi,
the reading accuracy would be (4.35/28.5) x 100 = ±15.26% of reading.
Example 2 In the data sheet of a scale capable of weighing up to 200 lb, the accuracy
is given as ± 2.5 percent of a reading. What is the deviation at the 50 and 100 lb
readings, and what is the %FSD accuracy?
Deviation at 50 lb = ± (50 x 2.5/100) lb = ± 1.25 lb
Deviation at 100 lb = ± (100 x 2.5/100) lb = ± 2.5 lb
Maximum deviation occurs at FSD, that is, ± 5 lb or ± 2.5% FSD
The absolute accuracy of an instrument is the deviation from true as a number
not as a percentage, i.e., if a voltmeter has an absolute accuracy of ±3 V in the 100-volt
range, the deviation is ± 3 V at all the scale readings, e.g., 10 ± 3 V,
70 ± 3 V and so on.
Precision refers to the limits within which a signal can be read and
may be somewhat subjective. In the analog instrument shown in
Figure, the scale is graduated in divisions of 0.2 psi, the position of
the needle could be estimated to within 0.02 psi, and hence, the
precision of the instrument is 0.02 psi. With a digital scale the last
digit may change in steps of 0.01 psi so that the precision is 0.01 psi.
Offset is the reading of an instrument with zero input.
Drift is the change in the reading of an instrument of a fixed
variable with time.
Hysteresis is the difference in readings obtained when an
instrument approaches a signal from opposite directions, i.e., if an
instrument reads a midscale value going from zero it can give a
different reading from the value after making a full-scale reading.
This is due to stresses induced into the material of the instrument
by changing its shape in going from zero to full-scale deflection.
Hysteresis is illustrated in Figure 
Vacuum is a pressure measurement made between total vacuum and normal
atmospheric pressure (14.7 psi).
Atmospheric pressure is the pressure on the earth’s surface due to the weight of the gases
in the earth’s atmosphere and is normally expressed at sea level as 14.7 psi or 101.36
kPa. It is however, dependant on atmospheric conditions. The pressure decreases
above sea level and at an elevation of 5000 ft drops to about 12.2 psi (84.122 kPa).
Absolute pressure is the pressure measured with respect to a vacuum and is expressed in
pounds per square inch absolute (psia).
Gauge pressure is the pressure measured with respect to atmospheric pressure and is
normally expressed in pounds per square inch gauge (psig). Figure shows graphically
the relation between atmospheric, gauge, and absolute pressures.
Differential pressure is the pressure measured with respect to another pressure and is
expressed as the difference between the two values. This would represent two points
in a pressure or flow system and is referred to as the delta p or .p. Figure 5.2b shows
two situations, where differential pressure exists across a barrier and between two
points in a flow system.
Vacun sempurna
(0 bar a)
Absolute Presure
Gauge Pressure
Tekanan Atmosfir
1 bar a = 0 bar g
(pendekatan)
Beberapa jenis alat ukur tekanan meliputi :
- Manometer
- Bourdon Tubes
C Type – Bourdon
Spiral Type – Bourdon
Helical Type – Bourdon
- Bellows
- Diaphragm
- Strain Gauges
Manometer
Alat ini bekerja dengan menggunakan prinsip perbedaan ketinggian
antara dua buah permukaan cairan didalam tabung manometer. Secara
matematis perbedaan ketinggian ini dapat dikonversikan dalam satuan
tekanan dengan terlebih dahulu melalui perhitungan matematis yang
dinyatakan dalam persamaan (1). Fluida yang berada didalam tabung
manometer berfungsi sebagai sensor tekanan sederhana yang ekonomis,
handal dan akurat.
Manometer Bentuk U
Secara mendasar prinsip kerja dari peralatan ini hampir sama dengan
barometer. Perbedaan tinggi kedua permukan cairan menunjukkan
besarnya tekanan absolut yang diukur. Cairan yang digunakan pada
manometer ini bisa berupa merkuri ataupun air (H2O). Alat ini Memiliki
range pengukuran > 150 inchi H2O. Tekanan maksimum operasi > 400 psig.
Simple U
With leg connection
Tabung Bourdon (Bourdon Tube)
Tabung bourdon merupakan alat ukur tekanan yang paling sering digunakan di
industri. Hal ini karena bentuknya yang sederhana dan kasar. Range ukur alat ini bisa
mencapai dari 0 – 100.000 Psig. Alat ini terdiri dari tabung silinder yang membentuk
huruf C, spiral atau Helical. serta dengan luas penampang yang tidak berbentuk
lingkaran. Tabung bourdon ini biasanya terbuat dari pospor, baja ataupun perak.
Prinsip kerjanya: bila sebuah fluida bertekanan memasuki tabung ini, maka hal ini
akan merubah bentuk tabung ini , misalnya dari oval menjadi lingkaran. Hasil dari
perubahan ini adalah pergerakan pointer pada papan skala. Bila besarnya pergerakan
ini sebanding dengan tekanan maka hasil pengukurannya dapat diketahui melaui
besarnya pergerakan jarum penunjuk ini.position sensor
As the Bourdon tube may be damaged by
high temperatures, it is common practice
on steam systems to install the gauge at the
end of a syphon tube. The syphon tube is
filled with water which transmits the
pressure of the working fluid to the
Bourdon tube, enabling the gauge to be
located some distance from the actual point
where the pressure is being measured. The
two most common forms of syphon tube
are the 'U' and ring types. The ring tube is
used on horizontal pipelines where there is
sufficient space above the pipe, and the 'U'
type is used when mounting the gauge on a
vertical pipeline, or on horizontal pipelines
where there is not sufficient space for a ring
type siphon
Type-Bourdon Elemen
Digunakan untuk indikasi lokal, transmisi sinyal tekanan dan aplikasiapplikasi pengendalian.
 Tabung berbentuk seperti busur lingkaran (arc 2500) dan digunakan
dalam istem transmisi sinyal baik pneumatik maupun elektrik,
 Akurasi pengukuran bervariasi mulai dari ± 0,5 - ± 2 %.
Spiral Type-Bourdon
- Digunakan untuk tekanan akibat gerakan
akhir yang bebas.
-Tidak dapat memberikan tekanan yang cukup
besar terhadap gerakan yang
dibutuhkan.
- Akurasi pengukuran normal ± 0,5%.
Helical Type – Bourdon
- Keuntungan memiliki kapasitasi range yang cukup tinggi, stabil terhadap
pengaruh tekanan yang berfluktuasi dan mudah adaptasi untuk penggunaan
tekanan tinggi.
- Jumlah koil-koil paling sedikit memiliki 3 coil untuk range etekanan rendah
dan > 16 coil untuk range tekanan tinggi.
- Akurasi pengukuran bervariasi mulai dari ± 0,5 - ± 1 %.
Bellows
Alat ukur tekanan jenis ini, biasanya digunakan untuk mengukur tekanan
absolut. Jika dibandingkan dengan tabung bourdon, alat ini memiliki sensitifita yang
jauh lebih besar. Selain itu bellows juga memiliki jangka waktu pemakaian yang lama
dan akurasi yang bagus yaitu ½ % dari span. Material penyusun terdiri dari brass,
phosphor brounze, beryllium-cooper, stainless steel of monel dengan efek histerisi
yang kecil. Prinsip kerjanya: bila fluida bertekanan memasuki ruangan bellows, akan
menimbulkan defleksi pada pegas elastis yang dipasang didalamnya. Bila pegas ini
dihubungkan dengan sebuah jarum penunjuk, maka besarnya defleksi pegas ini
akan sebanding dengan pergerakan jarum penujuk yang menyatakan besarnya
tekanan fluida yang diukur. Konstruksi bellow dinyatakan dalam gambar 10 berikut
Diaphragm
- Prinsip kerja elemen diaphragm adalah mirip seperti bellows.
- Merupakan keping flat datar yang fleksibel. (corrugated surface).
- Terdiri dari disc tunggal atau dua diaphragm yang terhubung satu sama lain dengan
konfigurasi berbeda yang digunakan untuk bentuk capsuler elements.
- Kondisi pengukuran tekanan absolute, capsuler memiliki peranan penting.
- Range akurasi pengukuran dari ± 0,5 - ± 1 ¼ % dari span penuh.
- Aplikasinya biasa digunakan untuk range tekanan rendah kecuali untuk diaphragm
bagian bawah digunakan untuk tekanan 15.000 psig.
The nozzle – flapper system is widely used in D.P. cells. The form shown
below converts differential pressure (e.g. from a differential pressure flow
meter) into a standard pneumatic signal. This is widely used in the control of
air operated pipeline valves.
The bellows respond to the differential pressure and moves the lever. This
moves the flapper towards or away from the nozzle. The air supply passes
through a restrictor and leaks out of the nozzle. The output pressure hence
depends on how close the flapper is to the end of the nozzle. The range of the
instrument is adjusted by moving the pivot and the zero position is adjusted by
moving the relative position of the flapper and nozzle. This system is used in a
variety of forms. Instead of bellows, a bourdon tube might be used and this is
operated by an expansion type temperature sensor to produce a temperature pneumatic signal converter.
Strain gauges can be bonded to
the surface of a pressure capsule
or to a force bar positioned by
the measuring element. Shown in
Figure is a strain gauge that is
bonded to a force beam inside
the DP capsule. The change in
the process pressure will cause a
resistive change in the strain
gauges, which is then used to
produce a 4-20 mA signal.
Instalasi
A DP transmitter is used to measure the gas
pressure (in gauge scale) inside a vessel. In this
case, the low-pressure side of the transmitter is
vented to atmosphere and the high-pressure side
is connected to the vessel through an isolating
valve. The isolating valve facilitates the removal
of the transmitter. The output of the DP
transmitter is proportional to the gauge pressure
of the gas, i.e., 4 mA when pressure is 20 kPa
and 20 mA when pressure is 30 kPa.
Sensor
Limits of
Application
Accuracy
Dynami
cs
bourdon,
"C"
up to 100 MPa
1-5% of full
span
-
spiral
up to 100 MPa
0.5% of full
span
-
helical
up to 100 MPa
0.5-1% of full
span
-
Disadvantages
-low cost with
reasonable accuracy
-wide limits of
application
-hysteresis
-affected by shock and
vibration
-
-low cost
-differential pressure
-smaller pressure range of
application
-temperature
compensation needed
-very small span
possible
-usually limited to low
pressures (i.e. below 8
kPa)
typically vacuum
to 500 kPa
0.5% of full
span
diaphragm
up to 60 kPa
0.5-1.5% of full
span
-
capacitanc
e/
inductance
up to 30 kPa
0.2% of full
span
-
resistive/st
rain gauge
up to 100 MPa
0.1-1% of full
span
fast
piezoelectr
ic
-
0.5% of full
span
bellows
Advantages
-large range of
pressures
very fast -fast dynamics
-
-sensitive to temperature
changes