INDUSTRIAL CONTROL INSTRUMENT SETTINGS Lyman H. Allen, Jr. AMERICAN VISCOSE WRWRATIOW. HEADVILLE, PEWWA.
AT TEE
annual meeting of the American Society of Mechanical Engine81~in 1941, a paper m presented by J. G. ZiegIer' wbioh o f f e d a method of arriving at optimum inskumentmthgfrorna&ofdetenninationsmadeofpmcss,cbaracbktica, including dl h g involved. ~ This method had a sound mathelnstical bmkgmnd and m simple enough to permit mmte estsblishment of settings in a few hours, instead of the weeks and sometimes monthe involved in the hit-o~~mim, trisland-error metbods now pacticed, with no e-snrence then thst the optimwn settin@ 6sd berm obtained since the papm wan haed on w n t m h made by the Taylor Instrumemt Compny, question# were rained wn03rning the mlivwml applirability of the data to the adjustment of instnunenta m a n n f a c t d by Other companies Therefore an investigation of the outlined procedure m initiated on a Foxboro wntmbr, Sinca that type 1 ' 118 available to the authbr.
In June, 1941, a Foxborn Model 30 Btebaog tempenturemntmlkhdbssninst.llsdintheAmta~~Bb COvtR Depuhnsnt of the Americsn v i colpmrtion torssulate the stamaan toan UmtoW m2overy oolwnn. Thiaoolumninathirty+te, mi&with a b s y ~ o f 1 2 i L l c b t &I t h u a ~ m l a i c i e n t to recover 140 pounds per minute of wpsr osnt Mebne fmm a 28 per armt aqlmou3aostone mlution when op snting at s r a l b ratio of 1.6 to 1. Hest is supplied through the injeotion of Lne steem through a sp%sr pipe into the 4!&noh-high L w e aection of the oolumn. Vapom M wndeneed in a horisantel multihot can,-.+d and the d t i n g hot o~ndenaoteato a EWI rstio, weir t).pe, split tbw divider Wbiab dividea &he total overhead into product M d r e h x a(rssms. Ody the productstreamiscooled. HotmEnxisreturnedtothe top plate of the column, and the feed solution, prehe~ted through a heat interchange with the mlumn midue nates didmgmg fmm the b, in intmduoed into the column on the tenth plate fmm t& bottom. The tfmperaturs bulb of tbe Foxborn wntml instrument in loretad in the vapor space above the m t h plate from the bottom of the wlurrm, and the tern-ture .O this point is controlled through
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1224
INDUSTRIAL AND ENGINEERING CHEMISTRY
COURTESY. FOXEORO COMPANY
Stabllog Controller, Model 30, Adjustable Proportional Band Instrument with Automatic Reset, Similar to T h a t Used In Investigation
t h e instrument action by positioning an air-operated diaphragm-type throttling valve in the steam line to the sparger pipe. Considerable time was spent in establishing the best settings for the instrument, those which would give a reasonable rate of recovery from upsets in the process resulting from changes in feed rate, feed composition, steam pressure, etc., and still give as close a control as possible of the temperature a t the bulb location. It was found impossible to obtain straight-line control except a t approximately 100 per cent throttling range (Taylor sensitivity of 2.75 pounds per square inch per inch), a setting which would not give a satisfactorily rapid recovery from process upsets. Furthermore, it was possible to detect very little change in the control characteristics obtained, as the reset resistance was progressively increased by cutting in additional spools of reset capillary through the closing of the reset valves provided in the instrument. The problem, therefore, was to calibrate the Foxboro control settings in terms of the Taylor units outlined in Ziegler and Nichols’ paper, to determine the proper controller settings by the method presented, to try these settings in actual operation, and to compare them and the resulting control characteristics with those then being obtained. DEFINITION OF T E R M S
CONTROLEFFECTS.“Proportional response” is by far the most common of control effects, being found in most industrial air-operated controllers. As the name indicates, this effect gives a n instrument air output or valve movement proportional to the amount of pen movement. I n other words, a pen movement of 2“ will give twice the valve movement or air output pressure change resulting from a pen movement of lo. The ratio of valve movement to pen movement is called “instrument sensitivity” (percentage of full chart pen movement required €or full valve
Vol. 35, No. 12
movement is called “throttling range”) and may be fixed or adjustable. An on-off controller is merely one with a fixed high sensitivity. The proportional response instrument has only one definite air output pressure or valve position for any given pen position and, as a result, is unable to compensate for changes in process load which require a change in the rate of flow of the controlled medium and still maintain the original control point. Changes in load with a proportional response instrument result in a pen deviation from the control point. This deviation of pen and pointer with load changes is called “offset”; the degree of offset varies inversely with the instrument sensitivity (directly with the throttling range) and directly with the size of the load change. “Automatic reset” has as its only purpose the elimination of offset. I n all other respects its effect is to create instability of control. Proper reset settings, however, enable this control effect to improve the degree of control, and a slightly decreased instrument sensitivity will eliminate its effect on stability. I n operation, automatic reset detects pen deviation from the control point, and its action is to cause a slow continuous rate of valve movement in the direction necessary to eliminate this deviation. The rate of valve movement is proportional to the degree of deviation of the pen from the set point; in most industrial instruments this reset rate is adjustable, either in steps through the addition of lengths of capillary tubing to the instrument air circuit, or continuously through the adjustment of a precision needle valve. UNITSOF MEASUREMENT.Proportional response (‘sensitivity” is measured by the air output pressure change produced by an instrument and resulting from a pen movement of one inch on the chart. The units, therefore, are pounds per square inch per inch. Automatic reset produces a continuous rate of instrument air output change proportional to the deviation between pen and set pointer. The deviation may be expressed in terms of proportional response output change; units for the “reset rate”, therefore, are pounds per square inch per minute per pound initial change. “Ultimate sensitivity” is that sensitivity adjustment in a proportional response instrument applied to a particular process which gives a n oscillating record of fixed amplitude. A greater sensitivity will cause oscillations of increasing amplitude, while sensitivity below ultimate will result in oscillations of decreasing amplitude gradually approaching straight-line control. Control effects according to Ziegler’ are summarized as follows: RESPONSE
reset
ACTION
MEASURE
UNIT
Valve movement Pen movement
Sensitivity
Lb./sq.in./in.
x’a11-e velocity Pen movement
Reset rate
Per min.
Optimum controller adjustments‘ are summarized as follows: Proportional: sensitivity = 0.58,
(1)
Proportional plus reset: sensitivity 0.458, reset rate = l.2/PU
(2 1
(3)
where
S, = ultimate sensitivity P, = period of oscillation a t the ultimate sensitivity. INVESTIGATION PROCEDURE
Since the Foxboro instrument involved in this investigation had a proportional band calibrated in per cent throttling range and was provided with four stepwise reset adjustments having no units of calibration, it was first necessary to obtain the missing calibrations and determine the actual conversion between per cent
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1943 throttling range and sensitivity. lined below :
TABLE I. tling Range,
%
10 20 30 40
CONVERSION BETWEEN FOXBORO THROTTLING RANGE AND TAYLOR SENSITIVITY Taylor Sensitivity, Lb./Sq. In./In. Exptl. Calcd. 27.5 26.7 14.6 13.75 9.4 9.16 6.87 6.7
tling Rar, 50 60 80 100
Ta lor Sensitivity, L%./s~.In./In. Calcd. 5.5 4.58 3.48 2.75
Exptl. 5.6 4.7 3.8 3.2
6 . Adjust the throttling range a t 100 per cent and then move the pointer away from the pen a distance sufficient t o give a n air output pressure change of 5 pounds. Measure the distance between the pen and pointer accurately and record this measurement. Repeat this measurement for pen and pointer deviation a t increments of 10 per cent for the entire range of throttling range adjustment available. 7. The instrument sensitivity equivalent t o the various throttling range adjustments can now be calculated thus: air output pressure change, W s q . in. pen and pointer deviation, inches
-
sensitivity, lb./sq.in./ in. of pen travel
The results listed in Table I and plotted in Figure 1 show the conversion obtained between throttling range and instrument sensitivity in t h e Foxboro Model 30 Stabilog temperature controller. A plot of the reciprocal of throttling range against sensitivity gives essentially a straight line and thus confirms the relation between these two units of proportional response measurement. I n the usual case, actual calibration is unnecessary since the conversion may be calculated thus; sensitivity =
I
The procedure followed is out-
1. Place the instrument on remote hand control so that the controlled valve is operated by the pressure-reducing valve in the instrument case, and determine the amount of air pressure required on the controlled valve diaphragm t o stabilize the process a t the desired temperature. 2. Place the rocess on hand control and close the block valves on either side o t t h e automatic controlled yalve. Disconnect the pen from the measuring element, and fix it a t the temperature a t which the rocess is t o be controlled. 3. Plug t f e air output line from the instrument at the case and open all reset valves t o obtain the fastest reset rate ossible. 4. Adjust the pointer to coincide with the pen, a n z s e t the throttling range t o given an air output pressure equal t o t h a t determined in ste l. A sufficiently long waiting period should be allowed at t i i s point t o be sure t h a t t h e instrument is in equilibrium and t h a t there is no reset effect. If a reset change in the output pressure is observed, the pointer should be moved from the pen in t h a t direction and t o a sufficient distance necessary t o stop the reset action. The pointer should then be readjusted t o agree with the pen by means of the micrometer adjusting screw. 5 . Close all reset valves t o obtain the slowest possible reset rate. Disconnect the air piping between the reset capillary unit and the capacity tanks, and blank the connection from the tanks. This operation must be completed as rapidly as possible t o avoid loss of pressure in the reset bellows. As a result of this step, the instrument is now of the proportional response type.
lb. air change for full valve travel x 100 % ' throttling range X chart width in inches
( I n the procedure described here, the proportional response mechanism was first calibrated and then the ultimate sensitivity and period determined followed by calibration of the reset mechanism. If desirabie, however, calibration of the proportional response and reset mechanisms may be completed first and followed by the determination of ultimate sensitivity and period.) 8. Determine the throttling range adjustment necessary t o produce exactly a one pound output air pressure change for a pen deviation from the pointer of one major chart division. 9. Adjust the instrument for a wide throttling ranp;e, replace the connection between the pen and the measuring element, reconnect the output air line t o the controlled valve diaphragm, and place the instrument in control of the process. Aft8r the pen
1225
lO%+l
f
SEN
Figure 1.
Conversion between Throttling Sensitivity
Range and
is lined out at the desired temperature, create an upset by moving the pointer away from the pen for a few minutes, and then return it t o its ori 'nal position. Observe the stability of control obtained a6 ingcated by the rate of return of the pen t o the control point and the number of cycles produced before even control is obtained. Repeat this procedure for progressively nm-rower throttling range settings until a cyclic chart record of constant and small amplitude is obtained, or until the waves die very The throttling range adjustment a t this gradually t o stabilit point is noted and t%e instrument allowed t o control with this setting for approximately four or five cycles. 10. From the throttling range setting roducing an even cyclic operation (step 9) and the data o b t a i n e f i n step 7 (Table I and Figure l), the equivalent sensitivity is calculated. This value is the ultimate sensitivity, S,, for !he instrument ,when controlling the particular process under consideration.
.
TABLE 11. RESETRATESFOR FOXBORO MODEL30 TEMPERATURE
CONTROLLER WITH ORIGINALARRANQEMENT AND ONE POUND INITIAL CHANGE I
PROPORTIONAL RESPONSE HYDRON
RESET
( ->
x
-
I +,CAPILLARY
HYDRON
CAPACITY TANK
i
TANK 8 CAPILLARY
Resistance Open
Pressure Change Lb./Sq.Ik. 9 t o 11 11 to 9
-TimeMin. 14 17
Sec. 65 11
B
to 11
11 to 9
16 18
21 21
C
9 to 11 11 to 9
15 18
45 14
D
9 to 11 11 to 9
15 18
42 24
A
9
Reset Rate Lb./Min./L&.
;:"0
} 0.125
1226
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
11. The period of oscilIation, P,, at the ultimate sensitivity is calculated by counting the number of complete cycles occurring over a given interval of time and then dividing the time in minutes by the number of complete cycles. 12. The optimum controller settings may now be caIculated from steps 10 and 11 and formulas 1, 2, and 3. 13. I n order to calibrate the reset adjustments available in the instrument, the process is again placed on hand control, the pen disconnected from the measuring element, and the air output line plugged at the instrument case. Reconnect the reset air piping between the reset capillary unit and the capacity tanks, and set the throttling range a t the value determined in step 8. Close all reset valves to give the slowest possible reset rate and check the throttling range setting to be sure that exactly a onepound output air pressure change is obtained at a pen deviation of one major chart division from the pointer. This operation must be done rapidly to minimize the effects of air output change created by the reset action. Adjust the air output pressure to approximately 7 pounds. This may be done by moving the pointer away from the pen in the direction necessary to give the required air change. Wait for this air pressure change to OCCUY, and then return the set pointer to the pen.
Vol. 35, No. 12
14. Make a defin i t e reset rate adjustment and then move the pointer one major chart division toward the outer edge of the chart. The output pressure will immediately change 1 pound and then maintain a slow but c o n t i n u o u s rate of change resulting from the effects of automatic reset. Start a stop watch when the output pressure passes the next even pound graduation on the output air pressure gage and obtain the time required for an additional 3 or 4 p o u n d s . L e t the pressure change slightly more than 2 a d d i t i o n a l pounds from the end of the t i m i n g period, and then move the pointer to a position one scale d i v i s i o n from the pen toward the center of t h e c h a r t . A l l o w the reset rate to become established and then obtain the time required for an air output pressure change in the reverse direction, Compute the average of these two times and record i t as the time for t h a t particular reset adjustment. Repeat for a check, make another reset rate adjustment, and obtain the time for this new setting. If the times become to long move the pointer a distance sufficient t o give an initial change of 2 pounds or time over a range of only 2 instead of 3 or 4 pounds in air pressure change. C a1cula t e the reset rates for the various adjustments as follows: lb. change in pressure during timed period = av. time in minutes X initial pressure change reset rate = lb./sq.in./min./lb. initial pressure change R E S E T RATES
Before the work was initiated, the degree of control being obtained from the Foxboro temperature controller was similar to that shown on the actual recording chart record (Figure 2 ) . The instrument settings of 60 per cent throttling range equivalent t o a Taylor sensitivity of 4.6 pounds/sq. in./in. and a reset rate later determined by calibration to be equal to 0.125 pound/min./ pound had been established after a considerable period of trial and error experiments. It is true that the variations in the controlled temperature record could have been decreased to some degree by widening the throttling range or reducing the sensitivity. This was, however, a step in the wrong direction since it would result in too slow a rate of recovery from the process upsets. Very little change in the appearance of the chart record was
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1943
1227
TABLE111. RESET RATES FOR FOXBORO 30 TEMPERATURETABLE IV. RESETRATESFOR FOXBORO MODEL30 TEMPERACONTROLLER WITH N o TANKS AND ONE POUND INITIALCHANGE TURE CONTROLLER, FIRSTREVISEDARRANGEMENT WITH ONE POUNDINITIALCHANGE I I I
:i
xt I
I
t._.
B
A
C
Resistance Open
Pressure Change, Lb./Sq.In.
--TimeMin.
Sea.
Reaet Rate Lb./Min./Lb.
A
9 to 13 to 13 9
....
45 67
54:;: } 4.77
E
9 to 13 t o 13 9
1
25 12
E:: } 3.07
G
9 tto l3 o 13 9
11
23 57
z;::
D
9 to 13 13 to 9
1 2
46 42
} }
All closed
13 9 to to 13
3 2
36 16
i,gt5} 1 . 3 8
(4)
5.4 lb./in. sensitivity = 51% throttling range
(5)
60/9.5 = 6.33 min. per cycle
(6)
1.2/6.33 = 0.19 lb./min./lb.
6
Sec. 46 23
B
9 to to 11 11 9
7 6
17 9
C
o 11 119 tto 9
16 0
D
9 to 11 11 to 9
7 10
1.87
So = 0.458, = 0.45 X 12.0 = 5.4 lb./in.
=
-TimeMin. 5
2.47
obtained through the use of the four different reset rate settings available in the instrument. Figure 3 shows the record of the determination of ultimate sensitivity, S,, for the instrument and process under consideration. It will be noted that the ultimate sensitivity, S,, was obtained at a throttling range setting of approximately 23 per cent, equivalent to a Taylor sensitivity of approximately 12.0 pounds/sq. in&. The chart record on the right-hand side of Figure 3 illustrates the type of control that would have been received on a proportional response type of instrument operating with a sensitivity setting of half the ultimate. It is obvious from this record that process load changes are constantly occurring in the equipment, which points to the necessity for the automatic reset control effect if an even controlled temperature record is to be obtained. The record of the ultimate sensitivity shows that there are approximately 91/, oscillating cycles per hour, equivalent to a time of 6l/9 minutes per cycle. Calculations follow for determining the optimum operating sensitivity 8, and reset rate RR required, based on the determina%ionsfor ultimate sensitivity and period of oscillation:
RR
A
Pressure Change, Lb./Sq.In. 9 to 11 11 to 9
Resistance Open
Reset Rate,
Lb./Min./Lb.
0 347 01314
51 38
::Ag g:;:
} 0’331 } 0 299 } 0.240
22 22
0 272 0.193
} 0.232
The first attempt consisted of the removal of the two capacity tanks furnished with the instrument; results of the calibration of this arrangement are shown in Table 111. It is obvious that the reset rates obtained through the use of the revised reset air piping arrangement are much too fast. Table I V presents the reset rate calibrations for a reset piping arrangement consisting of the Foxboro capillary reset unit with
(7)
The reset rates for the original arrangement of the automatic reset adjustments furnished with the instrument were then determined and are listed in Table 11. There is little change in the reset rate for the four possible adjustments, which explains the observation that there was very little change in the controlled temperature record obtained, regardless of the reset rate adjustment employed. Also, it was impossible to obtain the optimum reset rate calculated according to Equations 6 and 7. I t was then decided to rearrange the capillary spools and capacity tanks in an attempt t o obtain the calculated reset rates.
OOURTEOY, TAYLOR INSTRUMENT OOMPANIEO
Adjustable Sensitivity Type of Fulscope Controller with Automatic Reset
1228
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 35, No. 12 optimum reset rate. In this system the Foxboro c a p i l l a r y reset unit was followed in order by a 10-foot spool of 0.021inch-diameter capillary and a number 5 capacity tank. Figure 5 gives the controlled temperature record for a throttling range setting of 51 per cent equivalent to a Taylor sensitivity of 5.4 poundslsq. in./in. (calculated optimum s e n s i t i v i t y ) . The record a t the left is that obtained with a reset rate of 0.232 p o u n d / min./pound in accordance with the last setting given in Table IV. The record on the righthand side of Figure 5 was obtained with a reset rate of 0.181 p ound/min./pound, which is a p p r o x i mately equal to the calculated value of the optimum reset r a t e as g i v e n i n Equations 6 and 7. This record represents that obtained with the calculated o p t i m u m settings, and without doubt gives the best controlled temperature of all settings tried. ADJUSTMENT O F T A Y L O R INSTRUMENTS
a No. 5 capacity tank and a 20-foot spool of 0.021-inch diameter capillary inserted between the capillary reset unit and the reset bellows in the order given. Although the reset rates obtained with this arrangement were still faster than the optimum rate calculated in Equations 6 and 7, the instrument was placed in operation with this reset arrangement to determine the degree of control which might be obtained. Figure 4 shows the actual controlled temperature record obtained with each of the reset rates given in Table I V and with the instrument adjusted to a throttling range of 55 per cent, equivalent to a Taylor sensitivity of 5 pounds/sq. in./in. It is interesting to note that a reset rate of 0.331 pound/minute/pound produced a cyclic chart record while a reset rate of 0.232 pound/minute/pound gave a controlled temperature record with the minimum amplitude of fluctuation. Table V gives the calibration of a reset arrangement with which i t was possible to obtain approximately the calculated
Since the outlined procedure presented in the body of this paper applies only to the Foxboro Model 30 instrument, it was thought that a similar outlined procedure for Taylor instruments might be of value. An outline follows for both the Taylor proportional response controller and the Taylor proportional response and reset controller:
PROPORTIONAL RESPONSE. 1. With the process on hand control and the instrument recording but not controlling, adjust the hand-operated valve to bring the pen to the desired control point. 2. Turn the pointer to the position of maximum output pressure, and guess the output pressure required to give the controlled valve a flow handling capacity equal to that of the hand-operated valve. Put this air pressure on the diaphragm of the controllcd valve by regulating the reducing valve within the instrument case. 3. Ad'ust the proportional response dial for the. lowest sensitivity adjustment, and set the pointer t o coincide wlth the pen. 4. Close the hand-operated valve and open the block valves around the automatic controlled valve. If the temperature as recorded by the pen tends to rise or fall away from the pointer, make the necessary correction in the air pressure on the valve
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
December, 1943
1229
diaphragm required to return the pen t o the pointer by adjusting the large knurled wheel in the upper right-hand corner of the instrument case. 5 . Adjust the instrument sensitivity upward in steps, creating a disturbance in the process for each adjustment by moving the pointer away from the pen for a minute or two and then returning it t o the original set oint. This procedure will finally produce an even cyclic controied temperature record of constant amplitude. 6. The sensitivity adjustment producing this record is the ultimate sensitivity. The optimum sensitivity will be half the ultimate. PROPORTIONAL RESPONSE WITH AUTOMATIC RESET. 1. Set the instrument sensitivity at high and turn the pointer t o the position of maximum output air pressure. 2. Ad'ust the reducing valve in the instrument t o give a controlled valve position estimated t o be sufficient t o control the process. 3. Open the block valves on either side of the controlled valve and close the hand-operated valve. 4. Adjust the reducing valve in the instrument to stabilize the process. 5. Adjust the sensitivity dial for the lowest possible sensitivity and open the reset rate valve to graduation 5. Wait one minute and then set the reset rate valve to 0. 6. Set the supply air pressure t o the instrument a t 20 pounds and adjust the pointer t o agree with the pen. 7. Adjust the instrument sensitivity as in the case of the proortional response instrument until the ultimate sensitivity is found. Allow the instrument t o control at the ultimate sensitivity for approximately four or five complete cycles, and determine the time required for one complete oscillating cycle. 8. Set the sensitivity dial at 45 per cent of the ultimate sensitivity. 9. Set the reset dial a t 1.2 over the time for one complete cycle a t ultimate sensitivity. CONCLUSIONS
The work described here confirms the data presented in the original paper by Ziegler and Nichoh. The method for obtaining optimum settings proved t o be straightforward and simple, and offered a more rapid method of obtaining optimum settings than the trial-and-error, hit-or-miss method of instrument adjustment commonly used. Furthermore, the settings obtained
TABLE V.
COURTELY. VULCAN COPPER & SUPPLY COMPANY
Aloohol Recovery Column, Showing Foxboro Stabilflo Steam-Control Valve with Integral-Mounted Vernier Va lvactor
RESET RATESFOR FOXBORO MODEL30 TEMPERAin this manner should be consistsntly closer t o the actual optimum CONTROLLER, SECOND REVISEDARRANGEMENT WITH ONE POUND INITIAL CHANGE than those obtained by trial and error. I n this particular case the Ziegler-Nichols procedure for obtaining optimum settings led to a revision of the reset arrangement in the instrument itself, without which it would have been impossible t o obtain the calculated optimum settings and the degree of control finally achieved. Naturally, the Ziegler-Nichols procedure has greater value in the establishment of optimum settings for Taylor instruments than for those of other manufacturers; the reason lies in the fact that the Taylor instruments are calibrated in terms of the units presented in the Ziegler-Nichols paper, whereas instruments of other manufacturers must be calibrated in these units in the field by the one whose duty it is to make the instmment settings. The acceptance by various instrument manufacturers of a universal system of units for the calibration of the various control effects which may be obtained with their instruments would be Pressure valuable in the establishment of optimum settings in accordance Resistance Change, -TimeReset Rate with the general procedure outlined in the Ziegler-Nichols paper. Open Lb./Sq. In. Min. Sea. Lb./Min./Li. It is also felt that the preceding data disproves the contention 9 to 11 7 23 A } 0.238 1 1 to 9 9 58 of one instrument manufacturer that, once the optimum reset 9 t o 11 10 2 rate is established, slower rates of reset may be employed withB 11 t o 9 12 25 out damage to the control record. Evidence presented here not 9 to 11 53 0.134 only confirms the fact t h a t a too rapid reset rate produces a cyclic c 11 t o 9 22 l4 45 0.088 } 0.111 chart record, but also indicates t h a t too slow a reset rate will 9 t o 11 22 0.109 D produce an erratic chart record, the irregularities of which can be 50 11 t o 9 20 0.098 } 0.1°3 decreased only by reducing the instrument sensitivity. Often this 19 30 0 102 A11 closed 19 12 is a step in the wrong direction, especially if the fastest possible 0:104 } '"03 rate of recovery from process upsets is required.
TURE
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