Calibration of Oxygen Bomb Calorimeters

Transcription

1 Calibratin f Oxygen Bmb Calrimeters Bulletin N.101 Prcedures fr standardizatin f Parr xygen bmb calrimeters. Energy Equivalent The calibratin f an xygen bmb calrimeter has traditinally been called the standardizatin. During the standardizatin prcess, a knwn amunt f heat is intrduced int the calrimeter by the cmbustin f benzic acid. This standardizatin prduces the energy equivalent f the calrimeter fr a specific temperature rise. Befre a material with an unknwn heat f cmbustin can be tested in a bmb calrimeter, the energy equivalent (EE r E) r heat capacity f the calrimeter must first be determined. This value represents the sum f the heat capacities f the cmpnents in the calrimeter, ntably the metal bmb, the bucket, the water in the bucket and the jacket. Since the system changes slightly with use, energy equivalents are determined empirically at regular intervals by burning a sample f a standard material with a knwn heat f cmbustin under cntrlled and reprducible perating cnditins. By internatinal agreement in the 1920 s, benzic acid was selected as the chemical standard t be used in calibrating bmb calrimeters. Tday, it remains as the sle chemical primary standard fr this type f calibratin. Desirable Prperties fr Test Substances: Test substances fr bmb cmbustin calrimetry shuld satisfy the fllwing requirements. It must be readily available in pure frm. It must be stable. It must nt be hydrscpic. It must nt be vlatile. It must be easily brught int a frm suitable fr cmbustin. It shuld ffer n unusual difficulties during cmbustin in the bmb. The value fr the heat f cmbustin is characterized with suitable accuracy. While cntrary t sme f the abve requirements, in certain cases the fllwing apply. It is advantageus fr tests invlving materials that are difficult t handle r ignite that the test substance pses similar handling r ignitin difficulties. TechNte /31/08 Page 1

2 It is advantageus fr tests invlving substances whse cmbustin chemistry is cmplex that the test substance yields similar cmbustin prducts under the same experimental cnditins. Fr example, 2,2,4-Trimethyl-pentane (is-ctane) is widely used as a test substance fr xygen bmb cmbustin wrk with vlatile liquids. The amunt f heat intrduced by the reference sample is determined by multiplying the heat f cmbustin f the standard material by the weight f the sample burned. Then, by dividing this value by the temperature rise prduced in the test, we btain a resultant energy equivalent fr this particular calrimeter. Example: Cnsider a standardizatin test in which grams f standard benzic acid (heat f cmbustin cal/g) prduced a temperature rise f C. The energy equivalent (W) f the calrimeter is then calculated as fllws: (0.9914g)(6318.4cal / g) W = = 2374cal / C C Nte: Fr simplicity, the crrectins usually applied fr heats intrduced by the fuse and by acid frmatin are mitted frm the abve example. It is imprtant t nte that the energy equivalent fr any calrimeter is dependent upn a set f perating cnditins, and these cnditins must be reprduced when the fuel sample is tested if the energy equivalent is t remain valid. Fr instance, the additin f ne gram f water in the calrimeter will alter the energy equivalent value by ne calrie per degree Celsius. Less bvius but equally imprtant are the changes resulting frm different bmbs r buckets with unequal masses, different perating temperatures, different thermmeters, r even the biases impsed by different peratrs. After the energy equivalent has been determined, the calrimeter is ready fr testing fuel samples. Samples f knwn weight are burned and the resultant temperature rise is measured and recrded. The amunt f heat btained frm each sample is then determined by multiplying the bserved temperature rise by the energy equivalent f the calrimeter. Then, by dividing this value by the weight f the sample we btain the calrific value (heat f cmbustin) f the sample n a unit weight basis. Cntinuing the abve example: Assume a fuel sample weighing gram prduced a temperature rise f C in a calrimeter with an energy equivalent f 2416 cal/ C. The grss heat f cmbustin (H g ) is determined by multiplying the temperature rise by the energy equivalent, and dividing this prduct by the weight f the sample: ( C)( 2416cal / C) Hg = = 7863cal / g g Nte: Again, fr simplicity, crrectins fr acids and fuse have been mitted. TechNte /31/08 Page 2

3 Standard Test Methds Standard Test Methds have been written, tested, and published by a number f internatinal standards rganizatins including the American Sciety fr Testing and Materials (ASTM), Internatinal Organizatin fr Standardizatin (ISO), British Standards Institutin (BS), and Deutsches Institute fr Nurmung (DIN). Each f these methds cntains prcedures fr bth the initial standardizatin and the cntinuing verificatin f that standardizatin. Labratries shuld calibrate (standardize) their calrimeters in accrdance with the details spelled ut in the test methd they are using. Recmmended Standardizatin Prcedure If a standard methd is nt being used, the fllwing standardizatin prcedure is recmmended. Initial standardizatin: 1) Determine the average energy equivalent as the average f 10 tests. 2) T be acceptable, the relative standard deviatin f this series f tests shall be 0.17% r less fr 6400, 6300, and 6200 Calrimeters, 0.34% r less fr 6100 Calrimeters, 0.50% r less fr 1341 Calrimeters and 0.67% r less fr 6725 Calrimeters. (Review TechNte 100 Precisin and Accuracy fr additinal infrmatin). 3) If these criteria are nt met, the surce f the prblem must be fund and crrected and the entire series repeated until an acceptable energy equivalent is determined. The calibratin f any instrument shuld reflect as clsely as practicable, the current state f the measuring system. T this end, the Parr 6000 Series Calrimeters supprt the calculatin TechNte /31/08 Page 3

4 and autmatic maintenance f a rlling average f the energy equivalent. A calibratin run limit f n less than five tests is recmmended when using a rlling average methd. It is imprtant that the E value NOT be prtected in rder fr the instrument t autmatically update the E value when new final standardizatin tests becme available. The rlling average methd keeps the E value current, by analyzing a sample f benzic acid run every tenth test in standardizatin mde. The fllwing type f test reprt is issued when a standardizatin test is perfrmed: Nt nly is an E value prvided and incrprated int the rlling average, but a grss heat value is back calculated using the current average instrument calibratin factr. This grss heat value can be quickly cmpared against the accepted value f Btu/lb in rder t assess the degree f verall cntrl n the measurement prcess. In rder fr the standardizatin test t be incrprated in t the rlling average, the test must be final, nt preliminary. The Operating Instructin Manuals fr each calrimeter prvide details n fixed crrectins which will create a final test if a mre rigrus determinatin f the crrectins is nt required. Peridic Heat Capacity Checks The calrimeter heat capacity value (energy equivalent) shall be checked n a regular basis. Renewed determinatin f the heat capacity is required whenever significant alteratins have been made t the instrument r t the test cnditins. The ptimum frequency fr checking the heat capacity r instrument calibratin will depend n the stability f the measurement system and the risk invlved when the system departs frm statistical cntrl. Since all data btained during the perid last-knwn-in-cntrl t first-knwn-ut-f-cntrl are suspect, such intervals may need t be minimized. TechNte /31/08 Page 4

5 There are several empirical appraches t making a decisin n hw frequently the instrument calibratin shuld be checked. The experience f the labratry may indicate the expected frequency f ccurrence f truble, in which case reference sample measurements, at least three in number, shuld be equally spaced within such an interval. Anther apprach is the "length f run" cncept. In this, recgnizable breaks in the prductin (f data) prcess are identified which culd cause significant changes in precisin r bias. Such breaks culd include: change f wrk shift rest perids change, mdificatin, r adjustment f apparatus use f new calibratin standards significantly lng dwn-times use f a new lt f reagents At least three reference samples shuld be measured during any f these perids when the perids are cnsidered t be ptentially significant. Peridic checks f the instrument calibratin are a risk-reducing prcedure. Hwever, if it invlves mre than ten percent f a labratry's measurement effrt, either the quality cntrl prcess may need imprvement r t much effrt is being exerted in this directin. If less than five percent f effrt is devted t such measurements, the labratry may be taking t high a risk f prducing unacceptable data, r may nt even knw the quality f the data it is prducing. The abve statements are made with a labratry making a significant number f high-quality rutine measurements in mind. If a labratry's prgram invlves ccasinal r ne-f-a-kind measurements, the amunt f quality assurance effrt required, including the number f measurements f reference materials t be made may be significantly mre than that indicated abve. TechNte Bulletin N. 100 Precisin and Accuracy discusses the determinatin f acceptable results frm a calrimeter in detail and shuld be reviewed. PARR INSTRUMENT COMPANY 211 Fifty-Third Street Mline, Illinis USA Phne 309/ Fax 309/ parr@parrinst.cm TechNte /31/08 Page 5