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Quiz & test questions Grade 10 Careers & CTE

Vessenden Instrument Technician: Calibration and Signal Scaling

A grade 10 careers quiz on the water plant instrumentation technician role, covering calibration, 4-20 mA signal scaling, percent error, and alarm setpoints.

Objectives

  1. Students will explain what calibration means and why an online analyzer is checked against a bench sample.
  2. Students will convert between a 4-20 mA transmitter signal and the quantity it represents.
  3. Students will calculate percent error between an online analyzer and a reference measurement.

Vocabulary

Key words: calibration, transmitter, span, setpoint, percent error

Teacher overview

This 35-minute quiz is worth 30 points and checks whether students understand the work of an instrumentation and controls technician at a drinking water plant. It mixes multiple-choice recall, short-answer explanation, and five computation items that move between a transmitter's electrical output and the quantity it measures. The scenario is set at a fictional utility in the town of Vessenden. One point is worth establishing before the quiz. A number on a control screen is a measurement rather than a guaranteed value. Instruments drift as reagents age and optics foul, so a technician checks each analyzer against a bench measurement on a schedule and recalibrates when the two disagree. Items 2 and 6 both rest on that idea.

Vessenden Instrument Technician: Calibration and Signal Scaling — Grade 10 Careers & CTE
Name: ____________________________ Date: ______________

Assessment

  1. (2 points) At a water treatment plant, what does an instrumentation and controls technician mainly do?
    • A) Approves the utility's annual operating budget
    • B) Installs, calibrates, and repairs the sensors and transmitters that feed the control system
    • C) Writes the state drinking water regulations
    • D) Reads residential meters along a route
  2. (2 points) The online turbidimeter already shows a number every second. Why does the technician still carry a grab sample to the bench meter each shift?
    • A) The bench meter is required by the plant's insurance company
    • B) Online analyzers drift as optics foul and reagents age, so the reading must be checked against a reference measurement
    • C) The control system cannot store turbidity data overnight
    • D) Grab samples are the only legal way to measure turbidity
  3. (3 points) Calibration has two parts. Explain the difference between a zero calibration and a span calibration, and state which one uses a known high-value standard.
  4. (4 points) A fictional turbidity transmitter at the Vessenden plant sends 4 mA at 0.00 NTU and 20 mA at 5.00 NTU, and its output is linear between those points. The technician reads 8.8 mA. Write an equation relating the signal to turbidity, then solve it for the turbidity in NTU.
  5. (4 points) The same fictional transmitter still sends 4 mA at 0.00 NTU and 20 mA at 5.00 NTU. Conventional filtration plants must keep finished water at or below 0.3 NTU in 95 percent of monthly samples, so the plant trips its alarm at 0.3 NTU. What signal, in mA, should the technician set the high-turbidity alarm to trip at?
  6. (3 points) During a shift check, the online chlorine analyzer reads 1.05 mg/L while the bench reference reads 1.20 mg/L. Treating the bench value as correct, find the percent error of the online analyzer. Round to the nearest tenth.
  7. (3 points) The plant's chlorine analyzer carries a low alarm at 0.20 mg/L and a high alarm at 4.00 mg/L. Explain what each alarm protects against, naming the regulatory limit behind the high alarm.
  8. (3 points) A fictional plant flow transmitter is scaled so that 4 mA means 0.0 MGD and 20 mA means 8.0 MGD, with a linear output between them. The control screen shows 12 mA. What flow is the transmitter reporting, in MGD?
  9. (3 points) The Vessenden plant reports a flow of 4.0 MGD. Using the rate that one MGD equals 694 gpm, convert that flow to gallons per minute.
  10. (3 points) Name two different routes a person could take into instrumentation work at a water utility, and state one thing each route would require.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: B. The technician installs, calibrates, and repairs the field instruments and transmitters that send data to the control system; the other choices belong to finance, regulators, and the meter shop.
  2. Answer: B. Online analyzers drift over time as optics foul and reagents age, so the technician compares the online value with a bench reference and recalibrates when they disagree.
  3. Answer: A zero calibration sets the instrument's response with a blank or a zero-value standard, and a span calibration sets the response at a known high-value standard near the top of the working range. The span calibration is the one that uses the known high-value standard.
  4. Answer: 1.50 NTU. The equation is signal equals 4 plus 3.2 times turbidity, since 16 mA spans 5.00 NTU and 16 divided by 5 equals 3.2 mA per NTU. Solving 8.8 equals 4 plus 3.2 times turbidity gives 4.8 divided by 3.2, which equals 1.50 NTU.
  5. Answer: 4.96 mA. Using the same 3.2 mA per NTU, the signal is 4 plus 3.2 times 0.30, which equals 4 plus 0.96, or 4.96 mA.
  6. Answer: 12.5 percent. The difference is 1.20 minus 1.05, or 0.15 mg/L, and 0.15 divided by 1.20 equals 0.125, which is 12.5 percent.
  7. Answer: The low alarm warns that the disinfectant residual is falling toward the bottom of the range the system must maintain in distribution, while the high alarm warns that the residual is approaching the maximum residual disinfectant level of 4.0 mg/L set for chlorine. Accept any answer that pairs loss of disinfection with the low alarm and the 4.0 mg/L limit with the high alarm.
  8. Answer: 4.0 MGD. The span is 16 mA for 8.0 MGD, or 2 mA per MGD, so 12 mA is 8 mA above the 4 mA zero, and 8 divided by 2 equals 4.0 MGD.
  9. Answer: 2,776 gpm. Multiplying 4.0 MGD by 694 gpm per MGD gives 2,776 gallons per minute.
  10. Answer: Accept any two distinct routes with a matching requirement. Reasonable answers include a registered apprenticeship with a utility or an electrical local, which requires supervised on-the-job hours; a two-year associate degree or certificate in instrumentation, electronics, or process control technology; military electronics or nuclear-plant training carried into a civilian job; and promotion from an operator or maintenance position while completing coursework. Each route is a legitimate way into the work.

Differentiation

Support

Give students a scaling strip that states the span rule in words, that a 4-20 mA loop carries 16 mA across the instrument's full range, with one worked example on a different fictional instrument. Allow a calculator throughout, and let item 3 be answered with a two-column table comparing zero and span rather than a paragraph.

Extension

Ask students to write the calibration log entry a technician would file after item 6, including the as-found reading, the reference value, the percent error, the action taken, and the date of the next scheduled check. Then have them decide whether that error would change any decision an operator makes, and defend the decision in two sentences.

ELL supports

Pre-teach "calibrate," "transmitter," "span," "setpoint," and "drift" with a labeled diagram of a sensor, a signal wire, and a control screen. Provide a sentence frame for item 7: "The low alarm warns the operator that ___, and the high alarm warns the operator that ___."

Extensions

Have students graph the fictional transmitter from items 4 and 5 as a linear function, with turbidity on the horizontal axis and the signal on the vertical axis, then mark the alarm point they calculated. Ask what the graph would look like if a technician entered the span backwards, and what the operator would see on the screen as a result. Students who want to follow the career further can look up the certification that many utilities recognize for instrumentation staff, compare it with the operator license their state issues, and list what each credential covers.