The Q68RD3-G is Mitsubishi Electric’s 8-channel high-resolution resistance thermometer input module for the MELSEC-Q programmable controller series, doubling the channel count of the Q64RDN while delivering enhanced 0.01°C measurement resolution for precision multi-zone temperature monitoring, quality control data acquisition, and thermal mapping applications where eight simultaneous high-accuracy RTD temperature measurements are required from a single base unit slot. As the highest-channel-count RTD input module in the standard MELSEC-Q range, the Q68RD3-G is the preferred specification for large-scale temperature monitoring systems and applications where slot economy in the base unit is essential alongside a high RTD channel count. Available now at Atlantech Drives with worldwide express delivery and 12-month warranty — complete our quote form for pricing and availability.
What Is the Q68RD3-G?
The Q68RD3-G is a MELSEC-Q series intelligent function module providing eight channels of RTD temperature measurement at 0.01°C resolution — ten times finer than the Q64RDN’s 0.1°C resolution — making it suitable for precision temperature mapping, differential temperature measurement, and quality-critical monitoring applications where small temperature differences between measurement points must be reliably detected. Like the Q64RDN, the Q68RD3-G is a measurement-only module without integrated PID control output: measured temperatures are stored in the module’s buffer memory and transferred to the CPU via FROM instructions or auto-refresh for use in custom control logic, data logging, or process monitoring programs. The module supports 3-wire Pt100, Pt1000, and JPt100 RTD connections with built-in lead resistance compensation on all eight channels. The «G» suffix denotes a second-generation design with improved noise immunity and enhanced accuracy compared to the earlier Q68RD3 model.
Key Technical Specifications
- Model: Q68RD3-G
- Input Channels: 8
- Module Type: RTD input measurement only (no analogue output)
- Input Type: Pt100 (3-wire), Pt1000 (3-wire), JPt100 (3-wire)
- Temperature Range: -200°C to +850°C
- Input Resolution: 0.01°C
- Input Accuracy: ±0.2°C at 25°C ambient (±0.5°C over full operating temperature range)
- Lead Resistance Compensation: Built-in 3-wire compensation per channel
- Conversion Speed: 40 ms per channel (all 8 channels updated every 320 ms)
- Isolation: Optical, each channel independently isolated from internal bus
- Current Consumption (5 VDC internal bus): 0.50 A
- External Power Supply: 24 VDC ±10%, 0.30 A (required)
- Operating Temperature: 0°C to 55°C
- Weight: Approx. 0.25 kg
Architecture & Design Overview
The Q68RD3-G achieves its 0.01°C resolution through a higher-precision analogue-to-digital conversion architecture compared to the Q64RDN, using a sigma-delta ADC topology that provides 18-bit effective resolution on the RTD resistance measurement before conversion to temperature engineering units. The module’s signal conditioning circuit applies a precision constant current excitation to each RTD in sequence, measuring the resulting voltage drop with the high-resolution ADC and applying Callendar-Van Dusen equation coefficients to convert the measured resistance to a calibrated temperature value. The «G» generation improvement over the original Q68RD3 includes enhanced common-mode noise rejection (CMRR), which reduces susceptibility to 50/60 Hz interference from adjacent power conductors in industrial cable trays — a critical improvement for installations where RTD cables run parallel to motor drive output cables or other high-frequency noise sources. The module also implements a background self-calibration routine that periodically measures an internal precision reference resistor and adjusts the ADC gain correction factor to compensate for drift caused by ambient temperature changes within the module itself, maintaining the rated ±0.2°C accuracy over the full 0°C to 55°C operating temperature range without requiring external calibration.
Expert Tips
When using the Q68RD3-G for differential temperature measurement — such as measuring the temperature difference between inlet and outlet of a heat exchanger to calculate heat transfer rate — take advantage of the module’s 0.01°C resolution by computing the differential directly from the buffer memory values rather than comparing individually rounded temperature readings. A 0.01°C resolution on each channel provides a differential temperature resolution of approximately 0.02°C (root-sum-square of two independent 0.01°C measurements), which is adequate for detecting heat exchanger fouling at an early stage before significant performance degradation occurs. For eight-channel installations with long RTD cable runs (over 30 metres), use shielded twisted-pair RTD cable with the shield connected to earth at the Q68RD3-G terminal block end only — do not earth the shield at both ends, as this creates a ground loop that can couple power-frequency interference into the measurement circuit and degrade accuracy below the rated ±0.2°C specification. When commissioning a new Q68RD3-G installation, verify measurement accuracy on at least one channel using a calibrated reference thermometer in a temperature-controlled bath at a known temperature within the process operating range — this confirms that the RTD sensor, extension cable, and module are all functioning correctly before the system enters production service. Store the verified reference measurement and the GX Works2 project file containing the module’s channel configuration parameters in the plant’s equipment calibration records for future reference during maintenance activities.
Frequently Asked Questions
Q: Can the Q68RD3-G measure temperatures below -200°C using special low-temperature RTD sensors?
A: No. The Q68RD3-G’s measurement range is -200°C to +850°C, corresponding to the valid resistance range of standard Pt100, Pt1000, and JPt100 RTDs. Temperatures below -200°C require specialised cryogenic sensors (such as Pt-Co or rhodium-iron resistance thermometers) that are outside the Q68RD3-G’s supported sensor range. For cryogenic temperature measurement below -200°C in a MELSEC-Q system, external signal conditioning transmitters converting the cryogenic sensor output to a 4–20 mA or 0–10 VDC signal, fed into the Q64AD analogue input module, are the standard approach.
Q: What is the conversion cycle time for the Q68RD3-G when all 8 channels are enabled, and is this adequate for process control feedback?
A: With all eight channels enabled, the Q68RD3-G updates all channel values every 320 ms (8 channels × 40 ms per channel). This 320 ms total update rate is adequate for the vast majority of industrial temperature monitoring and slow process control applications, where thermal time constants are typically measured in seconds to minutes. For processes with faster thermal dynamics (such as thin-film heating elements or laser-heated substrates), where a sub-100 ms temperature feedback rate is required, the Q64TCRTN or Q64TCRTBWN temperature control modules (which execute PID loops at 500 ms internally without CPU scan dependency) or an external high-speed temperature transmitter feeding an analogue input module should be considered instead.
Q: Does the Q68RD3-G support channel-by-channel enable/disable to extend the conversion cycle time for active channels when fewer than 8 channels are in use?
A: Yes. Unused channels can be disabled via buffer memory parameter settings in GX Works2, and the module only converts enabled channels — reducing the total conversion cycle time proportionally. With four channels enabled, the conversion cycle reduces to 160 ms; with two channels, to 80 ms. Disabling unused channels is recommended not only for improved update rate but also to prevent disconnection detection alarms from triggering on channels with no sensor connected.
Do you need more references from this brand? Explore our full catalog of Mitsubishi here: https://atlantechdrives.com/manufacturers/mitsubishi/
