University of Nigeria Virtual Library

Serial No

Author 1

MBAJIORGU, Constantine C.

Author 2

WILKIE, Ken I.

Author 3

Title Automation of an Evaporation Pan for Water Level Control and Digital

Recording

Keywords

Description Automation of an Evaporation Pan for Water Level Control and Digital

Recording

Category Engineering

Publisher Proceedings Nigerian Society of

Agricultural Engineers

Publication Date 1995

Signature

Nigerian Society

. . I

SEVENTEEN ANNGAL CONVERENCE PAPERS RWIEWED, fiRllSED AND ACCEPTED FOR PUBLICATION

I

!, !

Conference Therne: Engineering A Sustainable Agricuiturai Procluction In Nigeria

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Corrfetsnce Held At: Tixmias idibiy e F:rancis Auciltorium, t C'ederal University of Technology, Akure,

, I , . ' . ' 1 . . . I . % . - . I ,, : ) 8 : i :: . . .Ondo State. Niyitria

AUTOMATION OF AN EVAPORATiON FAN FOR WATER LEVEL CONTROL AND DIGITAL RECORDING

eY

Constantine C. Mbajioryul And Ken 1 . Wilkie

ABSTFi ACT I

Evapotranspiration (E'T) data are required in the development and opwation simulation models

of watershed hydrology, and also in the traditional areas of ~rriyatton scheduling, systems design and

water management. Estimating ER form pon evapora:iofi is an indirect method by which he effects of .

atmosphere variables are integrated. The method has been fomd to give equally accurate estimates

as othar, methods rec(uiring.conaidernbly mora input data, given some standa;dization of pan shape,

environmental setting and operation. Apart trom human error and labour costs associated with manual

reaging of pan evaporation,,inaccuracy arises due to low or high water levels in the pan. This work C '

Is an attempt to facilitate and improve pa evaporation measurement through pan water-level control

and ,d~gital recording, thereby incorporating water accour\ting capability with automation. Dhtinctive

features of the devices include r-i~itialization of pan water following encroachment of the operating

limits, level measurements at present tima intervals; steppar-motor dr~ven water sewing probe for level

measurement, pump for level control; nnd, axternal mounting of level sensing unit over a stilling well.

laboratory and field testing of the device s h c ~ ~ e d a good am! promising performance.

INTWODUCTION . .

Automation can be built into a device, machine or process by a control mechanism whereby

variables of interest are treated in a certain manner, or cauaed to ns?,ume certain values, at the proper

time in order to provide, facilitate or maintain a desired corweniance or performance. Thus a data

acquisition system can be automated by introducing control, not only in the measurement process but

also in the factor3 that affect the measurement proce.3~. Ihe reference water level normally assumed

in a class A evaporated pan corresp~ndr, to the 190mm depth. According to Phene and Campbell

(1975), variation in water level of mace than +-'LSmm from the reterences level introduces errors in

evaporation measurement. Such errors can result trom a wave motion on the water surface, due to the

And action on n t-,gh water level, or from air turbulence above n low level also due to wind action. The

automation problc-rr is to maintain level fluctuat~on within t-25mm of the reference level, and to

measure and reco 113 water levels inn dally or shorter time intervals. At present, a micrometer hook

gmge 1s commonly used to measure time intervals. At present, a micrometer hook guage is commonly

used to measure p i !I water levels manually.

The LVDFloat and yr essure transducer methods were introduced in previously reported automation

of the

evaporation pan. Th x e were for measurement purposes and did not ~ncorporate water level control

in the pan. Phene and Campbell (?(-i7Sj ::T;Pc! the linecl: varizble diferentiul transformer (LVDT) method,

with a stainless steel float rrxmted in ;I :i!;11111c; VJC-ll, to en:wre and mmsure water levels. Mckinnon

and Trent (1985) used the pressure trocsducer method, by which s!atic pressure of water in the pan

is converted to an electrical signal, for level measurement.

2. D e s c r i p ~ ~ n - ~ f A ~ ~ o m _ a ~ q ~ 3yg!em

2.1 tl_ar_dwlr_e

'The hardware used to monitor the water balance o f the Class A evaporation pan la s h o w

schematically Fig. 1. The levd of water in the pan is measured using a bidirectional linear actuator

(actuator 701-AM, AMSI Corp. 01 Sm~:htown N.Y.). This device is a modified Airpax stepper motor,

wlth an internally threaded rotor fi!ted to a lead crew shaft, Ey stepping the motor, the thread shaft

moves in linear increments of 0.001 mch per pulse. The stepper motor is mounted over a stllllng well

such that the threuded shaft is veitical. I he still in^ well, which is external to 4mm internal-dlarneter, and

serves to dampen out oscillations of the vmter surface in the pan for level measurement. Measurement

is by stepping the threaded shaft from a fixed reference position downward until a probe , affixed to

the lower, end of the shaft, makes electrical contact with the water surface. The steps count to the water

surface in the distance in thousandths of an inch from the reference posltion. The difference in steps

count between successive mwsurernents is the loss/goin of water from/in the pan.

The measurement arid control system is operated by an 83C31 embedded microcontroller,

which is apt of Intel's MCS.51 family o f micrcprocessors. A minimal 80C31, a 10 MHz crystal. 8- bit

latch (741S373) and an 8k EPROM (2'7C6.1). provides 16 digital 1/0 lines. two 8/16 bit timerfcountera,

and up to 8k of autoexecutable c3de (Gupta, 1989). This three chip control procedures. The stepper

motor which drives the threaded shaft towards and away form the water surtace is controlled through

a pair of driver integrated circuits. Sensing switches - the "home" switch and the water contact probe - are connected directly to the digital rnput lines of the 80C31. Two minimum and maximum level limlts

are exceeded, are controlled u+nq solid state relays connected to two digital output lines of the 80C31.

An Omnidat Easv Logger is used to record the pan water loss/gain at 10 minutes interval. The scan

relay svdch of thk. data logger con~ectec: to an input digital line of the 80C31, Indicates when the dutn

logger is scclnnin .I data channels and provides an event counter to t?:, 80C31 for sequencing of the

mmsurement cycls.

2.2 $of.1_w;1re

The logic ,I )LV of the measurement a,rd control softwore is illustrated in Fig. 2. The key

process is the meccuement routine. In measurement operation, the motor is first stepped upwards

to screw the t h r a d r-l shott against !he "home" position, when a bracket mounted on top of the shaft

closes the "home" :.v,i?ch. The rxotor is further stepped upwards to insure that the brackets Is flrmly

against a stop. This -; the zero a u n ! position. Then the motor is stopped and the steps continued,

such that the threadel shaft travds downward until the water

I I . - -- I.'

sensing probe indicates contact wi:n I k ~ r ' , 7 : ~ - ; zl r ta : :~ (bntnct with the water surface is sensed via

a digital input line on the BCC31. 'Ths ;~LIIT~IH~ of ;lei-;:: !ram toe zero cour:t position is recorded, and

the motor is reversed and steppxl l i p a d s ii~i!/1 ' ~ : G , ? : P :;v;':;tch :? arjn;n closw The diiference

between recorded counts from the curisfit x r f :he P~PvI:;;;:: measurements is the par, water loss!gain

over the measurement time interval. - h r ; 01fferencc.r is scalecl and an o\~tput frequency is producd

using the timers on the 80C31. The relc?tii)r~~h~p is pro(-JLJCPC~ clmn!] the timers on the 80C31. The

relationship between change in water and frequency is as toilows:

1.008 inches rain ----'> 371 hertz

0.768 " " . - - > 3 2 3 "

0.513 " " " $!.)I "

0.250 " " " 5 3 "

Zero lcsslgain inches evap - - > 1533 hertz

The BOG31 sense and cairnts the closure of the ?con really switch on the data logger. This

switch Indicates that the doto logger is rwclording data. In this particular installation, tipping bucket

rain guages are monltored every 2 rninv~es and the waporat~d pan is to be monitored every 10

minutes along with several other ser,sors. Recame the dntn loqcler I:; limited in commcrnicnting, the

00c31 output frequency signs1 on every scim relay wi:ch o los~: r~ . and the signal is changed every

fifth witch closure correspoding t znew ineosuiemsr,: (! e every 10 min~~tes). The length of switch

closure is proporticnal to the ndrnbsr of ePann;"-l; beirig ;cannet!, nzd frequency is output tor a period

of 10 "andawh ich is approximately 3 times lor;cyx :ha:; the n;nx;muir; scan period. Following each

water level measurement. the level is mmpnrad to lin;i!s ?r,tabli.;hd c n the first measurement which

indicate the lavsl at which vmter st;oald be add& ts ::: removxj from the pan. If either limit is

exceeded thb appropriate pump is oc?;vnted for i.l :i.;sr! p~;;od of time, 1::) retiirn the level to near the

mid-ranga value. Another measurement irnmat1io:eiy !:,!lowf; !o prov~d:! n basis for the next water

balancci,g!cu\ation.

3. S-Y &m-~vg!ycJ@n

;'I P!sed cn 'laboratory test d a k for :he syst~rn. :I iecjrssalon of steps count, C, on output

8 ~ n r 0 , , , r a c u a ~ ' j 8 ' GaJa :ae f31\e\JJi;;g :e\cticn ciJi',!-;'s;-; R:' c r ~ \ f 4 I ~ O , ~ I . 'l W '/:

C = 0 ~ 4 7 % 4 - o . c T G ~ ~ T o ~ ( ~ ) - 0 02we=(10g(f)ei(p2) . . , . . , . . . , , . , . . . . . . . . . . .... . ,.... .. . . . . . . . . , . . ~ . . . . . .... .... (1)

where hg is loga;:thm to the base 19 and I is frequency in heft;. A residuals plot for the regression

h v m h Fig. 3 :0 h-,ve both co;;s:ar,cy of w n i vorimce and independence of terms.. For eqclation

(I), a step m ~ n t o' 512 ccrrsspor~ds :a a I:equercy of 791, Hz, giving the fotlov;ing relationship for

evaluating pan watw loss!gain:

E = U C * ,,, , 2 zcunt):I 000)*%5 4 ( 2 )

p = {(S'J;;: - 5: 2j;: fi!>oX 25.4 (3)

where both evapora:icn, E, x i 1 iai~fall, I;, nie in mm

A simple OJSlC p;ograir; v~zs ciaveloped bassd on the above relotionships for evaluatlng

Thus the system can be deemed to perfor;;: ntlequatel:,: i f f ~ r c:. s:endiiy varying woter Ikvekdnd hence

output frequency, a fairly constar;: v n ! x ;f cvoporation n;V,or rn;nt'afr ornount is measured. A sample

of results obtained for this laboratory test gisaedure 1s pfesented it-) Table 1.

DISCUSSION From laboratory test data, the system seems capable of a measuremer~t accuracy of the order

of 0.05mm. Also the system responds promptly ard adequately to keep water level variation within

operating limits in the laboralory. 'The system is a minimt.im hirdware design, capable of being

interfacad to a wide variety of data loggws of with addition cf PAM of being self contained. It requires

+5 vdc for the 80C31 chip and stepper motor, and 113 vac for the punips vhich may be optional for

remote application. Flg 4 . Shows afield installatior; of the system, after successful testing in prince

Edward Island, Canada

5. CONCLUSION At this stage, a variati~n of the ailtomated evaporation pan has been developed and tested

successfully, The device has a potential lor research measurements and meteorological station

appllmtion. It is a ~ m ~ p l e t e equipped fs: water balance and management studies, being capable of

both rainfall and evaporation measurements with accuracy.

REFERENCES

Gupta, S.K. 1989, MC3t h s e d Miriim~l Tant:oller. Micro roritrol .Journal, Nov/Dec.page 19.

Mbajlorgu, C.C. '1088. Liquid Level masursn;er;t arld conlrol fm the Auiornntion of Pan Evaporation

Measurement. A€ 6300 Course Projest Repoi:, Dept. Of Agric. Engineering, TUNS, Halifax N.S.

Wnc~cm. J.M. d A. T ra~ t . 1935. Aatmation of a Class A Evaporation Pan. Trans. Ot ASAE 28(1):

7 e e + n .

Phme, C.J. and R.E. Campbell. 1975. Automting Pan Evaporation Measurements for Irrigation

Ch-ttrol. Agricultural Meteomlogical, 15: 181 -1 91

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TA9: E 1 . S A k I P E l.P,l<:)?.?~7 3Rk' TEST DATA ' I ' : I .. . -- . - - -. ..-.. -~ .-------

Sample t . , %,., .,.A , . ,- .-. \ ' ' y Evaporation rainfall n u m b a (hstz'j (mm) (mm) - - .- . - . - - - - -. - - - - - --

1 94 IWI 3.67 195 l O 9 i 3.87 196 1091 3.67 197 1 C35 3.02 196 1091 3.87 199 1 085 3.82

1W1 3.87 LWO I

201 1080 3.57 202 1085 .: 3.82 203 1080 3.57 204 1047 3.26 205 1m 3.57 206 1097 ' 3.72 ',: .

t .

20 7 I W I 3.87 ,.

?OR 1091 3.67 209 I f391 3.67 31 0 1CX31 3.87 21 1 1085 3.82 21 2 1 085 3.02 21 3 1085 3.62

. I . .

21 4 1102 3.77 1080 ,3.57 . , 21 5

21 6 ? 068' . 3.46 .. , . . ' I

21 7 1 080 3.57 3.31 2 10 1052

21 9 : 006 2.58 , ,

220 1 085 3.02 22 1 1.102 3,77 , . .

222 1080 3.57 \ .

223 l&ll 3.67 224 1120 3.92 . .

225 lot38 3.46 2% 1108 3.02 227 7091 3.87 228 1102 3.77 228 1080 3.57

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