Selecting 24 VDC Air Temperature Sensors for PLC Control

After the fix, the PLC receives a stable temperature signal across the 62–82 °F control band, including locations with approximately 20 MPH plenum airflow. The cleanest low-quantity solution is usually an assembled 24 VDC transmitter with an output that directly matches one analog input. A bare thermistor remains viable when component cost matters more than engineering, calibration, and replacement effort.

Signal architecture and selection boundary

The term sensor here means the temperature-sensitive element. A transmitter includes signal conditioning and presents a defined analog output. Separate these functions before choosing hardware: an NTC thermistor, Pt100, or Ni1000 is a resistance element, while 4–20 mA, 1–5 VDC, and 10 mV/°F are conditioned signals.

The PLC has two analog inputs, with the second potentially allocated to an insolation sensor for comfort compensation. Count channels before selecting a sensing element: an external converter can add cost and wiring without solving the limited-input problem.

Check 1: Analog-input compatibility

Read the PLC analog-input specification and identify whether each channel accepts voltage, current, or direct resistance. A resistance element cannot be wired to an ordinary voltage input without excitation and conversion.

1. Check 1A: Expect a documented input type and full-scale range. If the channel accepts 4–20 mA or 1–5 VDC, select the matching transmitter and proceed to Check 2.
2. Check 1B: If the channel is voltage-only, compare its range and resolution with the proposed signal. A 10 mV/°F sensor produces 0.62 V at 62 °F and 0.82 V at 82 °F. The usable span is therefore only 0.20 V. If the input cannot resolve that span with adequate margin, use a transmitter with a larger conditioned span.
3. Check 1C: If the channel has a direct RTD input, verify that its supported element matches the proposed Pt100 or other resistance sensor. A PLC without an RTD module needs an external transmitter or a different sensor architecture.

Never select a resistance value merely because it is larger. Input compatibility, excitation current, linearization, wiring resistance, and measurement span jointly determine performance.

Check 2: Thermistor divider range

For a voltage input, a thermistor can form a divider. With a series resistor connected to +24 VDC, the 10 kΩ at 25 °C NTC connected to common, and the analog input connected to their junction:

Vout = 24 V × RNTC / (Rseries + RNTC)

At 25 °C, a 22 kΩ series resistor produces 7.50 V; a 33 kΩ resistor produces approximately 5.58 V. These calculated readings are wiring checks, not proof that either divider suits the PLC. Compare the maximum divider voltage over the full required temperature range with the channel limit before connection.

1. Check 2A: With the sensor disconnected from the PLC, expect approximately the calculated room-temperature divider voltage when the NTC is near 25 °C. A reading near 0 V indicates a short, reversed measurement point, or missing supply. A reading near 24 V indicates an open thermistor or missing common.
2. Check 2B: Warm the NTC and expect its resistance to decrease. In the stated divider orientation, Vout must also decrease. If it rises, the divider orientation or scaling direction differs from the defined circuit.
3. Check 2C: Calculate temperature from the thermistor resistance curve supplied for the selected part. A single 10 kΩ at 25 °C point does not define the nonlinear resistance-to-temperature relationship.

Check 3: Airflow and installation error

A dry, non-self-heated sensor approaches the surrounding air temperature. Air velocity primarily changes response time; it does not create meteorological wind chill in the sensing element. Wind chill describes heat loss from a body warmer than the air. A wet sensor can instead approach a wet-bulb condition through evaporation, while radiation from duct walls or heaters can bias the reading.

1. Check 3A: Compare the sensor reading with the fan stopped and running after both readings stabilize. Expect no material steady-state shift for a dry sensor with negligible self-heating and unchanged air temperature.
2. Check 3B: If the running-fan reading shifts, inspect for sensor self-heating, moisture, radiant heat, poor immersion, or air leakage around the probe. Increased airflow removes self-generated heat more effectively and can expose an excitation-current error.
3. Check 3C: For a location near 20 MPH, use an assembled probe or transmitter explicitly rated by its manufacturer for that airflow and mounting method. A statement such as “not for use in fast moving air” is a selection stop, even when the underlying temperature element could measure moving air.

Resolving-branch installation procedure

For intermittent, low-quantity builds, choose an assembled 24 VDC probe/transmitter when its output matches the PLC. This removes recurring potting, lead protection, linearization, and field-replacement work.

1. Record the PLC input type, range, resolution, common arrangement, and available analog channels.
2. Select one output architecture: 4–20 mA, 1–5 VDC, or a linear 10 mV/°F signal. Do not combine assumptions from different interfaces.
3. Confirm that the probe assembly is intended for the actual medium, airflow, mounting depth, and required temperature range.
4. Wire the 24 VDC supply and signal according to the transmitter and PLC terminal definitions. Keep the signal reference consistent with the input’s common-mode requirements.
5. Configure two-point PLC scaling from the transmitter’s documented low and high endpoints. For a 10 mV/°F device, use Temperature (°F) = measured volts × 100.
6. Apply any calibration correction as a documented offset or two-point adjustment. Do not hide correction values inside unrelated control logic.

Numbered acceptance readings

1. Check 1: Measure the transmitter supply at its terminals. Expect a stable value within the device’s documented supply range; the nominal system supply is 24 VDC.
2. Check 2: Read the raw PLC input and the signal at the terminals. Expect corresponding values without clipping, negative overrange, or full-scale saturation.
3. Check 3: Compare the PLC temperature with a reference at two points spanning the normal operating band. Expect the error to remain within the project tolerance at both points.
4. Check 4: Cycle the fan from stopped to the approximately 20 MPH operating condition after stabilization. Expect the dry sensor’s final reading to track air temperature without a persistent airflow-dependent offset.
5. Check 5: Disconnect one sensor lead. Expect the PLC logic to identify an invalid or out-of-range input rather than command heating or cooling from a plausible substituted temperature.

Frequently asked questions