DL421 pH Transmitter

Honeywell's DirectLine transmitter modules are the next generation family of analytical transmitters.  Unique architecture in the DirectLine transmitters combines the latest in microelectronics technology with the proven performance of Honeywell's unique sensing technologies; providing unequaled savings during installation, startup operation, and maintenance.  For pH, the DirectLine transmitter modules can be mounted integrally or remotely with the Meredian II, Durafet II and Durafet III pH electrodes.

The DirectLine architecture consists of an electronics transmitter module and a sensor which may be mounted integral to the transmitter or remotely eliminating the need and added cost of a separate analyzer or transmitter. The transmitter modules are intrinsically safe, in a weatherproof, corrosion-resistant sealed composite enclosure.

Completing the system is a unique Honeywell sensor connected to the sensor side of the transmitter and a 4-20 mA weatherproof output cable on the power/output side. The transmitter is designed for outside use in harsh environments where moisture, dust, and chemical attack may be present.  For submersion or special mounting applications, a remote mounting option is available which connects the electronics module to the pH electrode via a cable. The module can be mounted on a 2" pipe, wall or DIN rail.

Signal output of the DirectLine transmitter is a standard 4-20mA dc output proportional to pH. This signal may be connected to any host monitor or control device that accepts 4-20 mA loop powered inputs including:

  • Single loop Honeywell or competitive controllers
  • Honeywell's HC900 modular control systems
  • PLCs with analog inputs
  • DCS systems
  • Honeywell's Trendview data acquisitions systems



  • Plug-in modular design allows probe to be safely removed and replaced without cycling power on the electronics module.
  •  ½" NPT conduit Connection gives increased protection and noise immunity of output cable.
  • Local display and keypad facilitates quick set-up, calibration, and operational activities.
  • Electronics and electrode diagnostics reduces troubleshooting times.
  • Manual and Auto buffer recognition calibration options that reduce calibration time.
  • Playing card-sized guide facilitates simple, correct, and consistent calibration and configuration.
  • IP 68 Durafet III watertight sensor connection for reliability in submersion applications.
  • Global approvals:
    - UL General Purpose
    - CSA General Purpose
    -  IP66 Enclosure
    - NEMA4X Enclosure
    - CE
    - FM Class I, Div 1 (Intrinsically Safe)
    - FM Class I, Div 2 (non-incendive field wiring).



DL421 Brochure


DL421 Datasheet