Automatic Analyzer with Direct Readout for Determining Carbon in Steel L.
L.
LEWIS and M.
J.
NARDOZZI
Applied Research laboratory, United Sfates Steel Corp., Monroeville, Pa.
b An automatic analyzer with direct readout in per cent carbon has been developed for determining carbon in steel. The steel sample i s burned in oxygen and the carbon dioxide formed i s collected and measured for a total analysis time of 80 seconds. Solid state components in the programming and computing system lead to high reliability in a plant environment. Instrument stability i s excellent and the need for calibration checks i s infrequent. The linear operating range of the instrument is from 0.01 to 1.2% carbon, and the range can be extended down to 0.001 and up to 3.0%. Oxygen of the quality used for plant operation in a steel mill can be used for the analysis. Experiments have shown that the analyzer can be used without modification for determining oxygen in steel by the carrier gas method.
C
conductivity analyzers for determining carbon in steel have been developed by Lewis and Nardozzi ( I ) , Walker and Kuo (Z), and others in which the sample is burned in oxygen and the carbon dioxide that is formed is swept through a molecular sieve trap and collected. Advantages of this method are that high sensitivity is obtained by concentrating the carbon dioxide from the oxygen stream on the trap before measurement and that, argon and nitrogen (both present as impurities in the oxygen) do not interfere because they are not retained on the trap. Our previously reported work (1) showed that the relation between peak height or peak area and quantity of carbon dioxide was not linear over a broad concentration range of carbon dioxide, so calibration curves were needed for quantitative determinations. The work reported here describes how the analyzer was converted into a rapid and fully automatic instrument by providing a linear relationship between the carbon dioxide present and the peak area. OMBUSTION-THERMAL
EXPERIMENTAL
The equipment consists of a combustion apparatus ( I ) , an analyzer chassis, a control chassis, and a direct readout system. 1214
e
ANALYTICAL CHEMISTRY
Figure 1 .
> >> >>> R NV
r
S V MST 1
Gas handling system Furnace flow Measuring carrier flow Reference carrier flow Flow regulator Needle valve Trap Solenoid Vent Molecular sieve trap Leco purifying train
Combustion Apparatus. A Leco (Laboratory Equipment Co.) induction furnace, Type 521, is used with a Leco Jet combustion tube, No. 550-122. Analyzer Chassis. The analyzer chassis houses the detector, the gas handling system, purification traps, and the molecular sieve trap. The detector is a two-cell, thermistor type (No. 9677, Gow-Mac Instrument Co.) operated a t a current of 6 ma. and a temperature of 40' C. The gas handling system, Figure 1, is composed of three oxygen streams (furnace, reference, and measuring) ; five three-port solenoid values (SI to S6, Skinner Precision Industries, type V5); four needle valves (WV, to NV4, Ideal Aerosmith, No. 52-2-13); and two constant-differential-type gas flow regulators (R, and Rp, Moore Products Co.). Two steps are required to get the carbon dioxide into the detector for measurement. The carbon dioxide is
first adsorbed from the furnace-gas stream as follows: The oxygen flow from the furnace passes through Ss, Sa,the molecular sieve adsorption trap, Sp,and then to the atmosphere through a vent, V4; meanwhile the oxygen of the measuring carrier flow stream passes directly through SI,S4,and the measuring side of the detector. In the second step, the flush or desorption step, the oxygen from the measuring carrier flow stream passes, in order through SI and Sz, the heated molecular sieve trap, SB,and S4,thus carrying the desorbed carbon dioxide to the measuring side of the detector. During the desorption, the furnace stream passes through SS and is vented to the atmosphere through V3. Oxygen is supplied to the analyzer at a regulated pressure of 10 p.s.i. 1 Present address: Chemistry Deparb ment, General Motors Corp., Warren, Mich.
I 1
FURNACE ON
TRAP IN FURNACE STREAM TRAP IN 'DETECTOR STREAM
HEATER ON
FAN ON
READOUT ON
0
20
40 60 TI ME, seconds
80
Figure 2. Programming time and sequence chart
Keedle valves N V , and N V z (Figure 1) are adjusted to provide oxygen flow rates of 200 and 500 ml. per minute, respectively, through the reference and measuring sides of the detector. The oxygen flow rate through the furnace is 1200 ml. per minute. When the instrument is not being used, the detector voltage and oxygen are turned off, and needle valves N V 3 and N V 4 are then closed to protect the traps from atmospheric contamination. For rapid determinations, a relatively high flow rate of 500 ml. per minute is required through the measuring side of the detector because of the internal volume of flow regulator Rz. This high flow rate results in dilution of carbon dioxide in the oxygen stream and some loss of sensitivity. For the best sensitivity, Rz can be bypassed and the flow rate reduced to about 80 ml. per minute. With RZout of the system, linearity in integrated voltage from the detector signal will be limited to samples with less than about 0.3% carbon. The analyzer chassis has traps T1 and Tz (Figure 1) to purify the furnace stream of water and sulfur oxides. Water and carbon dioxide are adsorbed from the measuring carrier stream by traps T3 and T4. The trap on which the COZis adsorbed is a quartz U trap (7 mm. o.d., 4 mm. id., and 60 cm. long) containing 7 grams of l/ls-inch pellets of molecular sieve, Type 5A (Linde Co.). The trap is wound externally with Nichrome ribbon (Driver-Harris Co.) for resistance heating. The voltage applied to the resistance ribbon is adjusted so that the ribbon becomes red a t the end of a heating cycle of 20 seconds. Longer heating periods a t lower trap t e m p e m tures have been used successfully. If overheated, the molecular sieve will fuse. With proper heating, however, traps have been used for more than 10,000 cycles without sign of deterioration of the molecular sieve. Control Chassis. The control chassis contains the electrical com-
ponents and associated controls for the detector, the timing cycle (both automatic and manual), and the molecular sieve trap heater. The timing cycle consists of: furnace purge, during which time the air trapped during sample loading may be purged from the furnace tube if desired; furnace burn, during which time the sample is burned and COZ is adsorbed on the molecular sieve trap; flush, during which time the molecular sieve trap is heated and the COz is desorbed; and read, which activates the readout system just before the COZ passes through the detector. A programming time and sequence chart for the an-
alytical cycle is given in Figure 2. Individual adjustable solid-state timers (Syracuse Electronics Corp., Type TER) control each part of the cycle and provide a versatile and completely automatic timing program. Manual on-off switches are provided for individual activation of any part of the timing cycle for maintenance purposes. Direct Readout System. A schematic diagram of the direct readout computing system is shown in Figure 3. The input signal from the detector is amplified by a value of Rz/R1 by operational amplifier AI. Variable resistor Rz is used as a linear weight compensator to automatically correct for samples of varying weight; the value of the feedback resistor RZ was selected to give a sample weight range of 0.85 to 1.15 grams. The amplified signal coming from the detector is integrated by operational amplifier Az. The potential across the capacitor C1 is measured across resistor R4 by the digital voltmeter (DVM). By adjusting R4 in the calibration procedure, the desired voltage is read on the DVM for direct readout. Component values are chosen so that 0.1volt output is equivalent to 0.01% carbon, 1-volt to 0.1% carbon, etc. When the carbon dioxide from a sample is to be measured, the read part of the program is activated by the opening of relay RL1. The short across C1 is removed as RL2 opens and the integration period begins. When the integration time is ended, RL1 and RLz close and the capacitor is discharged. The voltage reading is stored on the DVM, however, by a reading retention circuit that is activated through PI. Toggle switch S1 is closed
Figure 3. Direct readout wiring diagram RLi
RLa E
A s1
c1 P1
DVM Ri
RZ Rs
Rc RK
Read delay timer Relay, DPDT, Potter 8 Brumfleld No. KRPl 1 A G Detector signal Operational amplifler Switch, spring loaded, DPDT, Arrow-Hart 8 Hegeman, No. 21 8 5 8 - X 0.5 rnf., ZOO-VDC, Mallory, PVC-205 Phone jack Digital voltmeter 1 .O kohm., 1 -W, Dale Products, No. RS-1 B 1 2 0 kohm., 5-W, 3 turn, 0.25% linear tolerance, Clorortat, No. 5 9 - 1 4-JA 15 Meg, 1 -W, International Resistance Co., No. DCF 200 kohm. 5-W, 10 turn, 0.25% linear tolerance, Borg, No. 2 2 0 1 - 6 1 kohm., 1 -W, Dale Products, No. RS-1 B VOL. 38, NO. 9, AUGUST 1966
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momentarily at the beginning of a determination to check the balance of the detector bridge by means of the display on the DVM. As SI closes, the reading on the DVM retained from the previous determination is cancelled automatically. The stabilized operational amplifiers used are from Philbrick Researches, Inc. Both the tube type (Model UPA-2) and the solid-state type (Model SP 656) have proved to be satisfactory for this application. The digital voltmeter is the Model 2200 made by the United Systems Corp. The quality of the readout system was studied by making measurements with known input voltages over carefully measured time periods. The reproducibility was determined to be within =k0.2701which is satisfactory. Operating Procedure. The instrument is calibrated with 1-gram samples of a standard sample, preferably one that contains carbon in the concentration range of interest. After the crucible that contains the standard and 1 gram of copper accelerator is placed in the furnace, switch SI (Figure 3) is closed and the detector bridge circuit is balanced. The automatic program is started, and when the read portion commences, RL1 is kept open by means of a switch so that the integrated voltage will be stored in Ct. After the program is finished, R4 is adjusted so that the carbon content of the standard appears on the DVM.
Table 1.
Sample NBS 55e NBS 166b NBS 125a NBS lOle NBS 129b NBS 20.f NBS 30e NBS 50b NBS 153a NBS 51b
CARBON DIOXIDE IN OXYGEN, percent
Figure 4.
Detector response to carbon dioxide in oxygen
Samples are analyzed by placing the mcelerator and a weighed quantity of the sample in the furnace, adjusting Rz for the sample weight, balancing the bridge circuit, and starting the automatic program. Experience has shown that water and carbon dioxide may be adsorbed on the molecular sieve held in the trap when the instrument is not in use and
Determination of Carbon in Standard Steel Samples
Analyzer results
Carbon, certificate value, %
Carbon av., %
Std. dev.
Rel. std. dev.
0.011 0.019 0.032 0.054 0.094 0.380 0.505 0.728 0.902 1.21
0.0117 0.0212 0.0323 0.0541 0.0935 0.381 0.508 0.739 0.903 1.19
0.00074 0.0017 0.00076 0.0014 0.00070 0.0018 0.0019 0.0087 0.0031 0.011
6.3 8.0 2.3 2.6 0.8 0.5 0.4 1.2 0.3 0.9
RESULTS AND DISCUSSION
Determination of Carbon in Samples Containing More than 1.2% Carbon
Table II.
Carbon, reported value,
Carbon detd., yo
1.77
Std. dev. 0.0036
Rel. std. dev.
1.82
. --
Av .
1.77 1.78 1.77 1.77 2.05 2.02 2.06 2.02
2.03
0.025
1.23
2.71
0.018
0.66
0.20
1.11
2.16
2.02
3.18
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2.72 2.70 2.69 2.73 2.71
ANALYTICAL CHEMISTRY
that the instrument should therefore be run through two cycles to rid the trap of these contaminants before calibration. The considerable quantity of water that must be present in a newly installed trap is best removed by heating the trap after switching it into the furnace gas stream. The resistance ribbon on the trap is heated to a red color and then the heater is turned off and the air blower is turned on; this is repeated, permitting the trap to cool each time, until the absence of a negative signal on the DVM indicates that the water has been removed.
The desired range of the analyzer with a direct readout required a linear response over a concentration range of 0.001 to 3.5% carbon. Achieving this was made difficult by changes in the gas flow through the detector brought about by heating the trap and switching the trap in and out of the gas stream. When the trap was heated (from ambient temperature to about 600' C. in 15 seconds), sudden pressure increases resulted from desorbing the carbon dioxide and heating the oxygen and carbon dioxide. (The carbon from 1 gram of sample with 2.5% carbon produces 47 ml. of COS,STP.) Early experimental results showed linearity with up to 0.370 carbon. When the carbon content exceeded 0.3%, pressure increases caused faster gas flow through the detector and low results. Nevertheless, results were reproducible and satisfactory determinations over the whole range could be made by using a calibration curve (1). Linearity was increased to 1.2% carbon (1 gram of sample) by placing a constantdifferential-type flow regulator between the trap and the detector (Rzin Figure 1). With this regulator, constant gas flow was maintained through
Table 111. Data on a Low-Carbon Standard with and without Crucible Preheating
Carbon, certificate value, yc 0.0044
Crucible condition As-received
0.0044
Preheated
0.0044
Preheated
Carbon
detd., % 0,0087" 0.0081 0.0096 0.0066" 0.0058 0.0066 0.0067 0.0060 0.0061 0.0062 0.0063 0.0046* 0.0046 0.0048 0.0044 0.0046
a Instrument standardized on a sample containing 0.380ojO C. Instrument standardized on a sample containing 0.00407c C.
the detector regardless of increases in upstream pressure. Most steels of commercial interest contain less than 1.2% carbon, so this upper limit in linearity is not a serious problem. The 1.2% carbon limit in linearity is evidently due to nonlinearity in detector response when the gas mixture being analyzed contains more than 30 volume yo carbon dioxide, Figure 4. The data for Figure 4 were collected under the conditions used for analysis, namely, a detector temperature of 40' C., a detector current of 6 ma., and a gas flow rate of 50 ml. per minute. Results obtained on standard steel samples with a carbon content range of the greatest commercial importance (0.01 to 1.21y0 carbon) are shown in Table I. These data were obtained after calibratr ing the instrument with National Bureau of Standards (NBS) sample 20f, which contains 0.380% carbon. (The data for NBS sample 20f were obtained over a 2-hour period, during which all data were collected and during which time no instrument recalibration was necessary.) Each sample was analyzed five times. Data are given as the standard deviation from the average, and the relative standard deviation is calculated as the standard deviation from the average divided by the average. The data are of satisfactory precision and accuracy. An attempt to increase the linearity further was made by placing a stream splitter between the furnace and analyzer to vent a reproducible fraction of the furnace gases. However, rapidly fluctuating pressures in the furnace gas stream caused by irregular usage of oxygen during combustion led to uneven splitting, as evidenced by an un-
satisfactory reproducibility (within *3%). The use of flow and pressure regulators with a stream splitter did not improve the reproducibility of the splitting significantly. Although the linearity of the instrument was not extended beyond 1.2% carbon (1-gram sample), steels with higher percentages of carbon may be analyzed by using fractional gram samples. Of course, the instrument would not be direct-reading and the values shown on the DVM would have to be corrected. Alternatively, the customary 1-gram sample could be used with a calibration curve. The data in Table I1 show that in this carbon range detector response is reproducible, but not linear. Direct readout for samples containing less than 0.01% carbon is not straightforward because blanks arising from the ceramic crucible and the copper accelerator become relatively significant. Data obtained on NBS 131a (0.004470 carbon) are given in Table I11 to illustrate the problem. When the analyzer was calibrated on NBS 20f (o.380y0 C) and the crucibles were used in the as-received condition, an average value of O.OOSS~Ocarbon was determined for NBS 131a. The value was reduced to 0.0063 by preheating the crucibles a t 1000' C. Thus the blank can be reduced from about 0.004 to 0.002% carbon by preheating the crucibles. Direct readout in the low carbon range can be obtained by calibrating the instrument with a standard sample that contains a carbon content in the range of interest and preheating the crucibles as shown by the final data of Table 111. Alternatively, the offset voltage in the operational amplifier used as the integrator may be adjusted to correct for the blank so that the instrument is direct reading over a concentration range of 0.001 to 1.2% carbon; this latter method has been preferred. Instrument sensitivity is high enough to detect carbon dioxide a t the 1-pg. level. However, special efforts would be required to reduce the blank before samples with less than 0.001% carbon could be analyzed with confidence. Water has a t times interfered even though attempts are made to dry all gases completely. Both water and carbon dioxide are adsorbed on the molecular sieve a t ambient temperature, and they are both desorbed in the 200' to 350' C. temperature range. However, interference from water is seldom a problem because of the small quantity present and because any water vapor present is desorbed and swept through the system to the detector more slowly than the carbon dioxide, so that the water peak occurs after the carbon dioxide peak. Water vapor passing through the detector gives a signal op-
posite in polarity to that of carbon dioxide and causes the integrated voltage signal to decrease. The analyzer is therefore programmed to stop the integration function as soon as the carbon dioxide has passed through the detector so that any water vapor present does not interfere. This instrument can also be used without modification to analyze for oxygen in steel by the carrier gas fusion method. In this method, the sample is burned in a carbon crucible in a helium atmosphere, and the carbon monoxide formed by the reaction of the oxygen of the sample and carbon of the crucible is converted to carbon dioxide by passing it over iodine pentoxide. The carbon dioxide is then collected on the molecular sieve trap and measured in a helium stream. The necessary change of carrier gas from oxygen to helium requires 1 hour for temperature equilibration of the detector block. Preliminary results with carbon dioxide in a helium stream showed a linearity of response with 4 to 700 parts per million of oxygen (1-gram sample) and a reproducibility within 1%. Direct readout for such analysis would be difficult because the oxygen blank is high. Therefore, use of a calibration curve is necessary. Because an oxygen determination may require as much as a 5-minute fusion, depending on the sample size, it is necessary to check the balance of the detector bridge after the fusion and just prior to measuring the carbon dioxide. Two carbon analyzers have been constructed. An estimated total of 60,000 carbon determinations has been made with no serious operational or maintenance difficulties on either instrument. Instrument stability is excellent, and the solid-state components in the programming and computing system have led to satisfactory reliability in a plant environment. Oxygen of the quality used for production operations in a steel mill has been used without difficulty because nitrogen and argon, common contaminants of oxygen, are not adsorbed on the molecular sieve trap and therefore do not interfere with the analysis. ACKNOWLEDGMENT
The authors thank P. P. Eismont, J. J. Giglio, and S. W. Damian for their keen interest and thorough evaluation of the instrument. LITERATURE CITED
(1) Lewis, L. L., Nardozzi, M. J., ANAL. CHEM.36, 1329 (1964). (2) Walker, J. M., Kuo, C. W., Zbid., 35, 2017 (1963).
RECEIVEDfor review March 2, 1966. Accepted May 18, 1966. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1966. VOL. 38, NO. 9, AUGUST 1966
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