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A common problem in water reclamation plants is selecting properinstrumentation equipment. Plant personnel will try to choose equipment thatmonitors accurately with minimal maintenance. Users see equipment accura-cy presented in many ways, however, and its important to understand thedifferences.

If an instrument has an accuracy claim of 0.5% of full scale, forinstance, you should recognize that the actual accuracy diminishes as theoperating conditions fall below the full-scale setting. Sometimes,though, you will hear claims that a meter offers 0.5% of reading overfull range. Although the difference may sound insignificant, it could

be very costly to the owner.Imagine, for instance, that a paddle-wheel flowmeter claims

to have an accuracy of 0.5%. Suppose, further, it is a percentof full range, and the full range is 50 feet per second (ft/sec).

If the flow range you will use it in is 6 ft/sec, which is com-mon in treatment plants, the actual accuracy is much differ-

ent than you might expect:

0.005 x 50 ft/sec = 0.25 ft/sec

If you apply this accuracy against a flow rateof 6 ft/sec, you see that the actual accuracy is:

0.25/6 ft/sec = 0.0417, or 4.17%

Comparing a magnetic flowmeterwith an accuracy of 0.5% of reading toa Doppler flowmeter with an accuracyof 0.5% of full range yields a similar

result.A common problem occurs when

a city or municipality uses two differ-ent types of flowmeters. Imagine onemeter is a highly accurate magneticflowmeter located in a meter vault tomonitor the plants effluent flow, andthe other is a Doppler meter monitor-ing the influent flow; this metersaccuracy diminishes as the flowate drops.

Case histories have shown that the

plant appears to be either generatingwastewater, because the effluent is more than

the influent, or something is evaporating the

By John Davis

Get out your

calculator and

read the fine

print: over full

range vs. of

full range.

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wastewater. We know in both cases that neitherof these conditions really exists. What is reallyhappening is that the Doppler meter is notmatching the accuracy of the magnetic meter.The difference between 0.5% of 12 million gal-

lons a day (Mgd) and 4.17% of 12 Mgd is sub-stantial:

4.17% 0.5% x 12 Mgd = 0.44 Mgd, or305 gal/min

Matters are made even worse if the Dopplermeter is used for pacing chemical feed into thewastewater with the same inaccuracies, resultingin either overdosing or underdosing. Water treat-ment plants have low, average daily, and highpeak demand flows, and further, low and average

daily flows occur more frequently. This demon-strates the importance of being cautious in choos-ing meter types for those flow variables.

Many types of flowmeters suffer in perfor-mance as the flows decrease and approach thelower end of their viable flow range; therefore,pacing during low flow periods may be highlysuspect. Chemicals are becoming more costly,analytical instruments for measuring the effectsof these chemicals are becoming costly, and cor-rosion due to underdosing or overdosing waste- water can be costly to equipment. All of these

may contribute to effluent that is a danger towildlife and, in extended cases, human life.

Repeatability Another tool in evaluating equipment is

repeatability, defined as the quantity that charac-terizes the ability of an instrument to give identi-cal indications or responses for repeated applica-tions of the same value of the quantity measuredunder the same conditions of use. In the past, when equipment operated on motion balance,where equipment used linkages and temperature

compensation values, repeatability was critical.Today, however, a number of field instru-

ments work on force balance techniques, such as

piezoelectric crystals, capacitance, and straingauges. These all work on the principle that ifyou put a force on an instrument, there should beno motion, though an electric signal is generatedon the output of that instrument. There are still

flow, level, and chemical measuring devices thatdo not work on the force balance principle, andfor these types, looking at the repeatability of thatpiece of equipment is still important. A steadywidening of the repeatability is an indication thatsomething is going wrong with the instrument.

Although some might believe good repeatabil-ity is a measure of accuracy, that is incorrect. Tounderstand the difference between accuracy andrepeatability, imagine an archer shooting at aconventional archery target. Suppose one archerhits the bulls-eye consistently. Because he was

always accurate, the shots were repeatable. Nowimagine an archer that hits the target but missesthe bulls-eye consistently. Although the archerhas good repeatability, the archer was not accu-rate. Good repeatability does not guarantee accu-racy. If you do not see a proper accuracy state-ment on equipment but only a repeatability state-ment, be cautious.

Rangeability and uncertaintyOne of the most common problems with

instrumentation equipment is the exaggeration

of its range. How many times have you heard ameter can read flow rates at velocities of 1100ft/sec, giving the impression that you can readflows accurately through that total velocity range?

What often goes unmentioned is that the par-ticular meters accuracy has a 10:1 turndownratio. This means that a meter sized to measure arange of 030 Mgd has a true accuracy over thefull range 330 Mgd. Below 3 Mgd, the meteraccuracy diminishes.

Additionally, different types of meters havedifferent turndown ratios over their full range. It

is common for a Venturi tube, for example, tohave two transmitters measuring the flow. This isbecause a Venturi tube with one transmitter mea-

Chemicals are

becoming more

costly, analytical

instruments for

measuring the effects

of these chemicals

are becoming costly,

and corrosion due

to underdosing or

overdosing waste-

water can be costly

to equipment.

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43In Tech www.isa.org August 2001

specifications

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sures accurately with a 6:1 turndown ratio overthe full range. So if we look at a range of030Mgd, the meters accuracy diminishesbelow 5 Mgd.

The range over which the instrumentmeets the stated linearity of uncertainty

requirements is its rangeability. Un-certainty is the range of values withinwhich the true value lies with a specified

probability. Uncertainty of 1% at95% confidence means the instru-

ment will give the user a range of1% for 95 readings out of 100.

Another common erroroccurs during the equipment

sizing. In the water reclama-tion industry, it is a com-

mon practice to assume

that solids in waste-water will settle out

around a velocityof 2 ft/sec. Amagnetic flow-meter readsaccurately ifthe minimumvelocity isabove 2 ft/sec, but be-low this, set-

tling is likelyto occurand who canthen say whatthe accuracyreally is?

Typically, de-signers size plants

to handle increasedflow capacities for

20 years. For this reason,designers often oversize pipes

for early life-cycle flow, and there iscorresponding settlement inside the pipe. This

settling can also occur in the inner liner of themeters. Because these meters are velocity-sensingdevices with an assumed constant cross section,they will give a false reading if the inner linerbecomes coated with sludge.

A solution may be to reduce the size of themeter to increase velocity by utilizing a pipereducer on the inlet side and a pipe expansionsection on the discharge side of the meter. If pos-sible, avoid connecting the reducer and expander

directly onto the meter. Manufacturers recom-mend that when you reduce the pipe, theflowmeter has a minimum of six to 10 pipe diam-

eters upstream from an elbow or valve and at leasttwo pipe diameters downstream of a pipe elbowor valve. This provides a less distorted flow pro-file for the meter to read.

Be certain you can afford to lose the pressurehead when you reduce the meter. Maximum

velocities should not exceed 15 ft/sec. By main-taining a minimum scouring effect inside thepipe, your sludge buildup inside pipes and anyin-line equipment will diminish, helping avoidmeasurement errors and costly maintenancedowntime.

Misconceptions and truthsSome people will ask for the accuracy of a cer-

tain flowmeter, level, or pressure-measuringdevice and, upon hearing a low number, thinkthat everything involved with the flowmeter will

be of the same accuracy. However, the meteraccuracy is not the accuracy for the entire flowsystem. A mathematical equation known as theroot mean square (RMS) correctly determinesthe accuracy of the complete system. Considerthe case of a magnetic flowmeter that recordsflow locally, sending an analog signal to an oper-ators workstation via a programmable logic con-troller (PLC).

You must look at each components accuracy:a magnetic flowmeter (0.5%), a magneticflowmeter transmitter (0.5%), a wire connec-

tion to the recorder (0.01%), a wire connectionto a local control panel terminal block (0.01%),and the I/O card of the PLC (0.4%). Each com-ponent in the system has its own measurementerrors and uncertainties, which contribute to theoverall accuracy of the complete system. In realcases, there could be more components attachedto a control system.

To use the RMS method, first squareeach number, yielding 0.000025, 0.000025,0.00000001, 0.00000001, and 0.000016.Second, add the numbers. Then find the square

root of the sum. The entire system has an accu-racy of approximately 0.813% instead of0.5%. This accuracy equation works for anyindividual chemical, pressure, level, tempera-ture, or flow loop.

Remember, too, that no two flowmeters orinstruments will have exactly the same accuracy.For this reason, the accuracy statement shouldindicate a component.

When choosing an instrumentation controlstrategy, look at all the manufacturers equip-ment literature regarding accuracy; consider the

range, repeatability, turndown ratio, and pipingconstraints; choose similar equipment types;and utilize the RMS equation. IT

August 2001 www.isa.org InTech44

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Behind the byline

John Davis is a senior member

of ISA who operates Davis

Instrumentation Services. Hisaddress is nira47@aol.com.