Moisture in oil probe MMP8

for continuous online measurement

Smart and robust. That’s Vaisala MMP8. Powered by the Vaisala HUMICAP® sensor technology, it has become a virtual standard in its field. Developed for demanding moisture measurements in transformer and lubrication oils, hydraulic fluids, and other liquids, HUMICAP® has continuously proven its worth over 20 years.

New: two probe lengths

The MMP8 is available in two probe lengths: 

  standard 262 mm 

  longer 448 mm

Versatile and capable. That’s MMP8. You can now measure dissolved moisture in oil in terms of water activity (aw), relative saturation (%RS), and temperature (T). It also calculates ppm concentration for transformer oils.

Flexible connectivity. That’s MMP8. Part of the Indigo Family and fully compatible with e.g. the flagship Indigo520 transmitter, the Indigo300, the handheld Indigo80, and our Vaisala Insight software for data visualization, easy probe configuration and self-calibration.

DNV - Type Approved Product

 *DNV GL certificate applies to 262-mm long MMP8.


  • Continuous online measurement
  • Measurement range -70 ... +180 °C (-94 ... +356 °F)
  • Measurement accuracy up to ±0.02 aw / 2 %RS
  • Temperature accuracy up to 0.1 °C (0.18 °F)
  • Excellent sensitivity in the dry end of the range
  • Includes traceable calibration certificate (five points for humidity, one for temperature)
  • Modbus RTU over RS-485 for flexible connectivity
  • Compatible with the Indigo series transmitters

Key Benefits

Robust & reliable out of the box

Reliability. That’s MMP8. Its latest-generation Vaisala HUMICAP® 180L2 sensor, is the result of over 20 years of field experience. Its excellent chemical tolerance provides accurate and reliable measurement over a wide measurement range, with outstanding sensitivity in the dry end — making it ideal for transformer applications.

Measure the margin to water saturation

Water activity or relative saturation directly indicate whether there is a risk of free-water formation. This data is relevant in lubrication oil applications where detecting water ingress and preventing free water formation is crucial. The measurement is independent of oil type and age.

In addition to water activity, MMP8 can output ppm water content. This conversion is readily available for mineral transformer oil and other oil-specific conversion coefficients can be calculated if the water solubility of the oil is known and the solubility characteristics remain constant. 

Easy installation

The MMP8 can be installed (and removed) even while a process is running — thanks to its ball-valve installation kit. Its probe installation depth is adjustable and pressure fitting options are ISO 1/2" and NPT 1/2". The MMP8 is delivered with a manual pressing handle that allows the probe to be pushed against process pressure.

Superb connectivity

Standalone. The MMP8’s smart measurement and digital Modbus RTU over RS-485 connection within the probe fully support standalone use.

Indigo-connected. The MMP8 is a part of the Vaisala Indigo family, which offers a robust display for data visualization, easy access to probe configuration, and more extensive options for connectivity, supply voltage and wiring.

Easily configurable. Whether using the MMP8 on its own or with an Indigo transmitter, the Vaisala Insight software offers easy field calibration, probe configuration, and device analytics.

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