WATLOW 998D 998A Digital/Analog Dual-Channel Base Unit
Watlow 998D is a high-performance, digital dual-channel temperature and process controller. Known for its vertical 1/8 DIN form factor,
it is a legacy workhorse in the industrial automation sector, particularly valued in the semiconductor and heat-treating industries for its high-accuracy PID logic.
1. Introduction
The 998D Digital Dual-Channel Base Unit is designed to manage two independent process loops within a single compact chassis.
By consolidating two controllers into one unit, it significantly reduces panel space and wiring complexity.
This unit features dual four-digit LED displays that provide real-time feedback for both loops simultaneously.
Although now discontinued by the manufacturer, it remains a critical component in many existing industrial systems due to its reliability and precise control.
2. Technical Parameter Table
| Feature | Specification |
| Control Loops | 2 Independent PID Channels |
| Form Factor | 1/8 DIN Vertical ($55\text{mm} \times 102\text{mm}$) |
| Input Types | Universal (T/C J, K, T, N, C, R, S, B; RTD; 0-10V, 4-20mA) |
| Accuracy | $\pm 0.1\%$ of Span |
| Sampling Rate | 5Hz (200ms) per channel |
| Power Supply | 100-240 VAC/VDC (Model 8) or 24-28 VAC/VDC (Model 6) |
| Outputs | Up to 6 total (Switched DC, Mechanical Relay, SSR, or Analog) |
| Display | Dual 4-digit LEDs (Process Value / Setpoint) |
| Communication | EIA-232 or EIA-485 Modbus RTU (Standard) |
3. Product Advantages and Features
Dual-Loop Synchronization: Capable of controlling two separate heating/cooling zones independently or in a coordinated fashion (e.g., differential control).
High Calibration Precision: Offers a calibration accuracy of 0.1%, which is essential for laboratory-grade or semiconductor applications.
Isolated Circuitry: Opto-isolated inputs and outputs prevent ground loops and electrical noise from interfering with sensitive process readings.
Vertical Space Efficiency: The vertical orientation is ideal for narrow panel layouts where horizontal width is limited.
Retransmission Ability: Can be configured to retransmit the process value or setpoint to a remote recorder or PLC.
4. Application Cases
Environmental Test Chambers: Controlling both temperature and humidity loops in a single unit.
Semiconductor Quartz Heaters: Managing the inner and outer heating elements of a wafer processing pedestal.
Industrial Kilns: Syncing multiple heat zones to ensure uniform temperature gradients during firing.
Plastic Extrusion: Independent control of the barrel and die-head temperatures in a compact dashboard.
5. Other Models in the Same Series
The 998D belongs to the larger Anafaze-legacy family of multi-loop controllers:
Series 999: The Horizontal 1/8 DIN equivalent of the 998.
Series 996: Single-channel predecessor.
Series 997: The dual-channel base unit for chassis-mount applications.
Series 988: A legacy 1/8 DIN family with similar interface logic but older hardware.
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6. Installation and Maintenance
Installation
Cutout: Prepare a vertical 1/8 DIN opening ($45\text{mm} \times 92\text{mm}$).
Wiring: Use shielded twisted-pair wire for all sensor inputs. Ensure AC power lines are routed away from low-voltage signal cables.
Grounding: Connect the controller’s ground terminal to a solid industrial earth ground to ensure display stability and communication reliability.
Maintenance
Memory Check: The 998D stores parameters in non-volatile memory. If the unit loses setpoints after a power cycle, the internal battery or capacitor may require servicing.
Cleaning: Use a damp, soft cloth to clean the front membrane keypad. Avoid harsh solvents that can cloud the LED windows.
Recalibration: Annual verification of the A/D converter is recommended using a certified process calibrator to maintain the 0.1% accuracy standard.
7. Unique Product Description
“The Watlow 998D is the vertical sentinel of the industrial panel.
It masters the art of dual-channel control, allowing two separate processes to be viewed and adjusted from a single high-contrast interface.
It is a legacy masterpiece of efficiency, providing laboratory-grade PID precision in a rugged, space-saving format that has defined reliability for a generation of thermal engineers.