MMT338 Moisture and Temperature Transmitter for Transformer Oils - Vaisala Online Store

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MMT338 Moisture and Temperature Transmitter for Transformer Oils

The MMT338 enables fast and reliable detection of moisture in mineral based transformer oil. It primarily measures the moisture of the oil as water activity (aw), but in this version the reading is converted into a ppm output calibrated for mineral-based transformer oils.

The probe can be inserted (and removed) directly into the lubrication oil through the supplied ball valve without draining the system.

The transmitter has a keypad and a large graphical display for convenient monitoring of the reading and transmitter setting changes. For example both output signal type (voltage or current) and scaling can easily be changed by the user.

Analog outputs are preset at the factory to:

  • Channel 1: 4…20 mA = 0...100 ppm
  • Channel 2: 4...20 mA = -40...+180°C (-40...+356°F) T

The package includes:

  • Pressure tight probe with 5 m (16.5 ft) cable
  • Calibration certificate (ISO9001 compliant, traceable to NIST)
  • Ball valve set with welding joint
  • ISO1/2" and NPT1/2" pressure fittings for probe installation into pipeline or pressure vessel
  • M20x1.5 cable gland and NPT1/2" conduit fitting included to facilitate wiring
  • User's Guide in English

MMT338 Moisture and Temperature Transmitter for Transformer Oils
MMT338 Moisture and Temperature Transmitter for Transformer Oils
Standard delivery time for single unit: 1 week
About delivery times »
Warranty: 24 months
About warranty »
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Detailed information

Measurement parameter(s)Moisture (ppm)
Measurement range0 ... 1 aw
-40 ... +180 °C (-40 ... +356 °F) T
Measurement accuracy±0.02 (0 ... 0.9 aw)
±0.03 (0.9 ... 1.0 aw)
±0.2 °C (±0.36 °F) T
Operating temperature-40 ... +180 °C (-40 ... +356 °F) probe only
High temperature+180 °C (+356 °F)
Operating pressure0 ... 40 bar (0 ... 580 psi)
Operating voltage10 ... 35 VDC, 24 VAC ±20%
Analog outputs4 ... 20 mA (default setting)
0 ... 20 mA
0 ... 1 V
0 ... 5 V
0 ... 10 V
(both output channels galvanically isolated)
Digital outputsRS-232
Housing classificationIP65 (NEMA 4X)
Links See full specifications on
Download user guides and other product documents on

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