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MFG-MECH15-04 · Lesson 5 · 14.5 clock hours · lab

Pressure and Vacuum Sensors

Objective. Students install a pressure transducer in a pneumatic circuit, calibrate its analog output at zero and span against a reference gauge, and wire the signal to a PLC analog input module.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Pressure and Vacuum Sensors in Mechatronics Systems

Pressure sensors are among the most common analog instruments in pneumatic and hydraulic systems. In a manufacturing or marine automation environment, a pressure transducer converts mechanical pressure into a proportional electrical signal—most often a 4–20 mA current loop—that a PLC can read, scale, and display. This lesson walks through the principles, installation, calibration, and PLC integration of a 0–150 psig pressure transducer.

How a Pressure Transducer Works

A pressure transducer contains a sensing element—typically a piezoresistive strain gauge or a ceramic capacitive diaphragm—bonded to a small metal diaphragm. When pressure acts on the diaphragm, it deflects slightly, changing the electrical resistance or capacitance of the sensing element. Internal signal-conditioning electronics convert that change into a standardized output. For industrial use, the 4–20 mA two-wire current loop is the dominant standard because current signals are immune to voltage drop over long cable runs and resist electrical noise.

At zero pressure (0 psig), the transmitter outputs exactly 4 mA. At full-scale pressure (150 psig in this lab), it outputs exactly 20 mA. Any pressure in between produces a proportional current. A transducer is called a transmitter when all conditioning electronics are integrated into the housing.

Vacuum sensors work on the same principle but are ranged below atmospheric pressure. A compound gauge reads both vacuum (negative gauge pressure, down to −14.7 psig / 0 psia) and positive pressure.

Accuracy, Linearity, and Span

A sensor datasheet specifies accuracy as a percentage of full scale (FS). For example, ±1% FS on a 0–150 psig transducer means the reading can be off by ±1.5 psi at any point in the range. Linearity describes how closely the output-versus-pressure curve follows a straight line. When you plot your five calibration points on a graph and they fall on or near a straight line from (0 psig, 4 mA) to (150 psig, 20 mA), the transducer is performing within its linearity specification.

Span is the difference between the upper and lower range values (150 − 0 = 150 psi). Zero is the lower range value (0 psig = 4 mA). These two concepts are central to the scaling formula used in the PLC.

Safety First: OSHA 1910.147 Lockout/Tagout

Before touching any pneumatic fitting or component on the training panel, you must control all hazardous energy. Compressed air stores significant energy: a sudden release can drive a fitting like a projectile, cause crush injuries from a moving actuator, or produce an air-injection wound.

Follow these steps from the energy-control procedure sheet:

1. Notify affected personnel that the pneumatic panel is being de-energized.

2. Shut off the main air-supply valve (the energy-isolating device).

3. Apply your personal padlock and tag to the valve.

4. Bleed residual pressure using the panel's dump valve or manual bleed point.

5. Verify zero pressure on the reference gauge before loosening any fitting. The gauge must read 0 psig.

6. After the transducer is installed and all fittings are tight, remove your lock and tag, restore air slowly, and check for leaks with a leak-detection solution before applying full pressure.

PPE for pneumatic work includes safety glasses and, when working above 30 psig, a face shield. Never point a pressurized line at yourself or others.

Installing the Transducer

With the system locked out and at zero pressure:

Calibrating the Transducer

Calibration compares the transducer output against a known reference. Your reference here is a precision test gauge whose accuracy is traceable to a higher-level standard.

1. Set the regulator to 0 psig. Confirm on the reference gauge. Clamp your calibrated loop calibrator (set to read mA) in series with the signal loop and record the mA reading.

2. Increase pressure to each setpoint (25, 50, 100, 150 psig), hold steady, and record the mA output.

3. Plot pressure (x-axis) versus mA output (y-axis). The points should fall on a straight line from (0 psig, 4 mA) to (150 psig, 20 mA).

4. Calculate the expected mA at each point and compare. Any point more than ±2% FS away from the ideal line (±0.32 mA on a 16 mA span) indicates a problem with the transducer or the calibration setup.

PLC Scaling and HMI Display

The PLC analog input module converts the 4–20 mA signal into a raw integer count (often 0–32,767 or 4,000–20,000 depending on the module). To display pressure in engineering units (psig), apply the standard 4–20 mA scaling formula:

Pressure (psig) = [(mA − 4) / 16] × Span + Zero

Substituting this lab's range: Pressure = [(mA − 4) / 16] × 150

In ladder logic, you implement this with math instructions (SUB, DIV, MUL, ADD) operating on the raw analog input word, producing a scaled tag displayed on the HMI. Verify the HMI value against the reference gauge at two test pressures to confirm the scaling is correct.

Key terms10

Pressure transducerA device that converts mechanical pressure into a proportional electrical signal, typically 4–20 mA, using an internal sensing element and signal-conditioning electronics.
4–20 mA current loopA two-wire industrial signaling standard in which 4 mA represents the zero (low) end of the measurement range and 20 mA represents full scale; current signals resist noise and voltage-drop errors over long cable runs.
SpanThe difference between the upper and lower limits of a sensor's measurement range; for a 0–150 psig transducer the span is 150 psi.
Zero (lower range value)The minimum measured value corresponding to the minimum output signal; for a 0–150 psig 4–20 mA transducer, 0 psig corresponds to 4 mA.
LinearityHow closely a sensor's output-versus-input curve follows a perfect straight line across the full measurement range; expressed as a percentage of full scale.
Accuracy (% of full scale)The maximum allowable error of a sensor expressed as a percentage of the full-scale range; e.g., ±1% FS on a 150 psig transducer equals ±1.5 psi.
Loop calibratorA portable instrument that can source or measure a 4–20 mA signal, used to verify transmitter output during calibration without disconnecting the transmitter from the process.
LOTO (Lockout/Tagout)The energy-control procedure required by OSHA 29 CFR 1910.147; involves isolating all hazardous energy sources, applying personal padlocks and tags, and verifying zero energy before working on equipment.
Analog input moduleA PLC hardware module that converts a continuous analog signal (such as 4–20 mA or 0–10 V) into a digital integer value the processor can use in a control program.
Engineering units scalingThe mathematical conversion of a raw PLC analog count or mA value into meaningful physical units (e.g., psig, °F, gpm) using the linear 4–20 mA scaling formula.

Worked examples4

1. A 0–150 psig pressure transducer with a 4–20 mA output reads 12.0 mA on the loop calibrator. What is the corresponding pressure in psig?
  1. Identify the known values: mA = 12.0, span = 150 psig, zero = 0 psig.
  2. Apply the scaling formula: Pressure = [(mA − 4) / 16] × span + zero.
  3. Subtract 4 from the mA reading: 12.0 − 4 = 8.0 mA above the live-zero.
  4. Divide by the full span in mA: 8.0 / 16 = 0.50 (this is the fractional position in the range).
  5. Multiply by the pressure span: 0.50 × 150 = 75.0 psig.
  6. Add the zero value: 75.0 + 0 = 75.0 psig.
Answer: The pressure is 75.0 psig.
2. You apply 50 psig to the transducer. What mA output should the ideal transducer produce? Is a reading of 9.5 mA within the ±2% full-scale accuracy requirement?
  1. Calculate the ideal mA output. Use the inverse formula: mA = [(pressure − zero) / span] × 16 + 4.
  2. Substitute values: mA = [(50 − 0) / 150] × 16 + 4.
  3. Calculate the fraction: 50 / 150 = 0.3333.
  4. Multiply by 16: 0.3333 × 16 = 5.333 mA above live-zero.
  5. Add 4: 5.333 + 4 = 9.333 mA — this is the ideal output.
  6. Calculate the allowable error: ±2% of full scale in mA units = 0.02 × 16 = ±0.32 mA.
  7. Find the acceptable range: 9.333 − 0.32 = 9.013 mA to 9.333 + 0.32 = 9.653 mA.
  8. Compare the reading 9.5 mA to the range 9.013–9.653 mA: 9.5 is within the range.
Answer: The ideal output is approximately 9.33 mA. A reading of 9.5 mA is within the ±2% FS tolerance (acceptable range: 9.01–9.65 mA).
3. A PLC analog input module converts 4–20 mA to a raw integer count ranging from 0 to 32,000 (counts). The module reads a raw count of 16,000. What pressure in psig should be displayed on the HMI for a 0–150 psig transducer?
  1. First convert the raw count to a fractional position: 16,000 / 32,000 = 0.50.
  2. A raw count of 0 corresponds to 4 mA and a raw count of 32,000 corresponds to 20 mA, so this fractional position is equivalent to 4 + (0.50 × 16) = 4 + 8 = 12.0 mA.
  3. Apply the pressure scaling formula: Pressure = [(mA − 4) / 16] × span.
  4. Pressure = [(12.0 − 4) / 16] × 150 = [8 / 16] × 150 = 0.50 × 150 = 75.0 psig.
  5. The HMI should display 75.0 psig.
Answer: The HMI should display 75.0 psig.
4. Before the lab begins, you must verify that residual pressure has been fully bled. The reference gauge reads 3 psig after the dump valve is opened. Is this acceptable to begin installing fittings? What must you do?
  1. Per OSHA 29 CFR 1910.147, all stored hazardous energy must be released or restrained before work begins—the standard does not allow a 'close enough' approach.
  2. 3 psig is not zero; residual pressure could still cause a fitting to eject or an actuator to move unexpectedly.
  3. Investigate the source of residual pressure: check that the dump valve is fully open and that no downstream check valve is trapping air.
  4. Continue bleeding until the gauge reads 0 psig and remains at 0 psig.
  5. Only after confirming 0 psig on the reference gauge may you proceed with installing fittings.
Answer: No, 3 psig is not acceptable. Continue bleeding until the reference gauge reads 0 psig and stays at 0 psig before touching any fittings.

The math in this lesson2

Learners check whether a reading falls within ±2 percent of full-scale accuracy.
7.RP.3F-LE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Pre-lab: apply OSHA 1910.147 LOTO to the pneumatic training panel, bleed residual pressure, verify zero pressure with a gauge before installing fittings. · Installation and calibration lab: students install a 0–150 psig pressure transducer, apply known pressures with a regulator (0, 25, 50, 100, 150 psig) from the reference gauge, record the mA output at each point, and plot output vs. pressure to confirm linearity within datasheet spec. · PLC scaling exercise: using the 4–20 mA scaling formula [(mA − 4) / 16 × span + zero], students write and test a PLC ladder rung that converts the raw analog input word to engineering units displayed on an HMI screen.
Materials: Pneumatic training panel, 0–150 psig pressure transducer (4–20 mA), precision reference gauge, regulator, 4–20 mA loop calibrator, PLC analog input module, HMI, multimeter, fittings, OSHA 1910.147 energy-control procedure sheet.
Check: Calibration data table reviewed by instructor; all five pressure points must read within ±2% of full scale; PLC HMI displays correct engineering-unit value verified by instructor at two test pressures.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

PMMI Mechatronics – Fluid Power / Industrial Electricity
1. A 4–20 mA pressure transducer is installed in a pneumatic circuit rated 0–100 psig. What mA output should it produce at exactly 25 psig?
Siemens SMSCP L1 – Electro-pneumatic control; PMMI Mechatronics – Safety
2. Which OSHA standard requires workers to lock out the air-supply valve and verify zero pressure before installing a pressure transducer on a pneumatic panel?
PMMI Mechatronics – Fluid Power; Siemens SMSCP L1
3. A technician plots five calibration points for a 0-150 psig transmitter with a 4-20 mA output. At 100 psig the expected output is 14.67 mA, and the reading is 14.80 mA. Is this within a ±2% full-scale tolerance?
PMMI Mechatronics – Industrial Electricity 1; Siemens SMSCP L1 – Electrical Components
4. On a two-wire 4–20 mA loop, what provides power to the transmitter?
PMMI Mechatronics – Industrial Electricity 2; Siemens SMSCP L1 – Electrical Components
5. What is the primary advantage of using a 4–20 mA current signal rather than a 0–5 V voltage signal for transmitting pressure data over a long cable run?
PMMI Mechatronics – Fluid Power; Safety – LOTO
6. After installing a pressure transducer and restoring air to the system, a technician detects a slow hiss at the transducer fitting. What is the correct immediate action?
PMMI Mechatronics – Programmable Logic Controllers; Siemens SMSCP L1 – PLCs
7. In a PLC ladder program, the analog input module delivers a raw count of 8,000 out of a 0–32,000 count range for a 0–150 psig transducer (4–20 mA). What pressure should the HMI display?
Siemens SMSCP L1 – Electro-pneumatic control; PMMI Mechatronics – Fluid Power
8. Which statement best describes the term 'span' as it applies to a pressure transducer?
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