Vaisala Moisture in Oil Probe MMP8

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Moisture in Oil Probe MMP8 for Continuous Online Measurement​

Vaisala HUMICAP® Moisture in Oil Probe MMP8 enables fast and reliable measurement of moisture in oil. It uses the proven Vaisala HUMICAP® sensor that was developed for demanding moisture measurements in transformer and lubrication oils, hydraulic fluids, and other liquids.

MMP8 measures dissolved moisture in oil in terms of water activity (aw), relative saturation (%RS), and temperature (T). It also calculates ppm concentration for specific oils.

MMP8 is part of the Indigo family and is compatible with Indigo 500 and Indigo 200 series transmitters. It can be used with the Vaisala Insight PC software for data visualization, easy probe configuration and self-calibration.


  • Continuous online measurement of moisture in oil and temperature
  • Measurement temperature range -70 ... +180 °C (-94 ... +356 °F)
  • Measurement accuracy up to ±0.02 aw / 2 %RS
  • Temperature measurement accuracy up to 0.1 °C (0.18 °F)
  • Proven Vaisala HUMICAP® sensor, used for over 15 years in oil applications
  • Modbus RTU over RS-485 for flexible connectivity
  • Includes traceable calibration certificate: five points for humidity, one point for temperature
  • Compatible with Indigo series transmitters
  • Sensor has excellent sensitivity in the dry end of the range

Key Benefits

Accurate and Reliable Measurement
MMP8 incorporates the latest-generation Vaisala HUMICAP® 180L2 sensor, which is the result of over 15 years of field experience. It was developed for demanding moisture measurement in transformer and lubrication oils and other liquids.

The sensor's excellent chemical tolerance provides accurate and reliable measurement over a wide measurement range, with outstanding sensitivity in the dry end of the range – typically needed in transformer applications.
Measure the Margin to Water Saturation
MMP8 measures moisture in oil in terms of water activity (aw), relative saturation (%RS), and temperature (T). 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, the average mass concentration of water in oil. Vaisala has this conversion readily available for mineral transformer oil. For other oils, the oil-specific conversion coefficients can be calculated if the water solubility of the oil is known and the solubility characteristic remains constant. This allows the continuous measurement of ppm concentration in power transformer condition monitoring.
Easy Installation
MMP8 can be installed with the ball-valve installation kit even in processes where the probe needs to be installed or removed while the process is running. The probe installation depth is adjustable and pressure fitting options are ISO 1/2" and NPT 1/2". MMP8 is delivered with a manual pressing handle that allows the probe to be pushed against process pressure.

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