Instrument ParametersThe 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. Theresulting discrepancy is stated as a deviation from the true value, and is normallyspecified 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 inreadings 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 -40C to 100C.

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 accuracyis given as 2.5 percent of a reading. What is the deviation at the 50 and 100 lbreadings, and what is the %FSD accuracy?

Deviation at 50 lb = (50 x 2.5/100) lb = 1.25 lbDeviation at 100 lb = (100 x 2.5/100) lb = 2.5 lbMaximum 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 numbernot 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 normalatmospheric pressure (14.7 psi).

Atmospheric pressure is the pressure on the earths surface due to the weight of the gases in the earths 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.

Tekanan Atmosfir1 bar a = 0 bar g(pendekatan)Vacun sempurna(0 bar a)Gauge PressureAbsolute Presure

Beberapa jenis alat ukur tekanan meliputi :- Manometer- Bourdon TubesC Type BourdonSpiral Type BourdonHelical Type Bourdon- Bellows- Diaphragm- Strain GaugesManometerAlat 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 USecara mendasar prinsip kerja dari peralatan ini hampir sama denganbarometer. 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 UWith 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 ElemenDigunakan untuk indikasi lokal, transmisi sinyal tekanan dan aplikasi- applikasi 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 terhadappengaruh tekanan yang berfluktuasi dan mudah adaptasi untuk penggunaantekanan 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 %.

BellowsAlat ukur tekanan jenis ini, biasanya digunakan untuk mengukur tekananabsolut. Jika dibandingkan dengan tabung bourdon, alat ini memiliki sensitifita yangjauh lebih besar. Selain itu bellows juga memiliki jangka waktu pemakaian yang lamadan akurasi yang bagus yaitu % dari span. Material penyusun terdiri dari brass,phosphor brounze, beryllium-cooper, stainless steel of monel dengan efek histerisiyang kecil. Prinsip kerjanya: bila fluida bertekanan memasuki ruangan bellows, akanmenimbulkan defleksi pada pegas elastis yang dipasang didalamnya. Bila pegas inidihubungkan dengan sebuah jarum penunjuk, maka besarnya defleksi pegas iniakan sebanding dengan pergerakan jarum penujuk yang menyatakan besarnyatekanan 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 % 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 ofair 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 hencedepends 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.

InstalasiA 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.

SensorLimits of ApplicationAccuracyDynamicsAdvantagesDisadvantagesbourdon, "C"up to 100 MPa1-5% of full span--low cost with reasonable accuracy-wide limits of application-hysteresis-affected by shock and vibrationspiralup to 100 MPa0.5% of full span-helicalup to 100 MPa0.5-1% of full span-bellowstypically vacuum to 500 kPa0.5% of full span--low cost-differential pressure-smaller pressure range of application-temperature compensation neededdiaphragmup to 60 kPa0.5-1.5% of full span--very small span possible-usually limited to low pressures (i.e. below 8 kPa)capacitance/ inductanceup to 30 kPa0.2% of full span---resistive/strain gaugeup to 100 MPa0.1-1% of full spanfast-large range of pressures-piezoelectric-0.5% of full spanvery fast-fast dynamics-sensitive to temperature changes