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This small pure gas transmitter is a rugged, self-contained hygrometer designed specifically for the measurement of trace moisture content in ultra high purity gases.

Simple to use and install, the Pura is available in two-wire, three-wire and hazardous area versions and is fully configurable to meet specific needs.

Pure Gas Dew-Point Transmitter

In many high purity gas applications, such as the semiconductor industry, the residual moisture content of the gas is critical to the satisfactory operation of a process.

Historically trace moisture measurement has been problematic, demanding the use of complex moisture analysers or expensive analytical techniques. Now Michell has brought to market a simple, economical and very effective solution for on-line measurement of dew-point temperatures down to -120°C (-184°F) (equivalent to less than 1 part per billion).

The Pura transmitter benefits from Michell’s experience and expertise in the production and calibration of impedance dewpoint sensors. Incorporation of industry standard materials and manufacturing processes gives the first low cost transmitter suitable for large-scale integration into a semiconductor fabrication plant or high purity gas line.

Simple To Use

This self-contained transmitter module has been designed to fit seamlessly into your pure gas process and give you the measurement you need - continuously and reliably.

Pura’s sensor housing is fabricated from cold drawn stainless steel with an internal 0.25 Ra μm electro-polished finish for minimal moisture adsorption and cleaned to oxygen standards. The transmitter is delivered fully calibrated and ready to use. The calibrated 4-20 mA output can be connected to a Michell process indicator or centralised control system.

The unit is designed to have minimal internal volume. This ensures the fastest possible response speed in commissioning and also when a moisture event occurs. The sensor containment seal is rated to 10-9 torr whilst the whole system will handle pressure right up to the VCR coupling rated maximum of 240 Barg (3480 psi).

Customisable Information

While Pura 2-wire is shipped ready to use, calibrated at 10°C dew point intervals across its measurement range against transfer standards traceable to NIST and NPL, it is also user customisable.

The 4-20 mA output can be user set over any part of the operating range, with a minimum output span of 1°C. Also, the factory pre-set alarm signals, providing over-range, under-range and sensor fault conditions, can also be easily reset to suit your own application and system needs. Adjustment of all these parameters is achieved through a simple PC-based user interface, available as a download at www.michell.com

Easy Installation and Operation

Pura is simple to install. The ¼” male VCR gas connection ports are set at a pitch of 120mm (4.72 in) to fi t into a standard Mass Flow Controller footprint. The whole unit is only 150mm (5.9 in) in height and weighs less than 500g (1.1 lb). Pura is a two or three wire transmitter, providing ultimate fl exibility in operation and powered by any regulated 12 to 28 V DC source (max 25 mA).

Optional Monitor

If the application calls for the dew point or moisture content to be displayed then the Pura can be supplied as a hygrometer, with an Online (standard) or Advanced Online panel mounted display. Our range of monitors are simple to connect and also acts as a power supply for the Pura Transmitter.

Service Exchange/Recalibration Program

Michell offers two services for customers who want minimum downtime and equipment traceability:
Sensor Exchange – customers place an order for a guaranteed, reconditioned sensor. When this arrives, they exchange it for the installed sensor which is returned to Michell, resulting in zero process downtime.
Recalibration – customers return their installed sensors to Michell, where they are inspected, checked and recalibrated before being returned. This provides on-going sensor traceabilty for the process.

Other Options

Mechanical options:

  • Premium - Clean room cleaned, inert gas fi lled, double bagged, 1/4” VCR.
  • OEM - Clean room cleaned, single bagged.
  • Sensor - 1/2” VCR connections.

Electrical Options:

  • PUR-TX-2W - 2 wire 4 to 20mA
  • PUR-TX-3W - 3 wire 4 to 20mA
  • PUR-IS-2W - 2 wire hazardous area 4 to 20mA

Display Options:

  • Advanced Online - 5 digit display, user configurable
  • Online - 4 Digit display

Sensor Technology Ceramic Impedance Sensor

The Pura uses Impedance technology, based on Michell’s advanced ceramic sensor. The operation of this sensor depends on the dielectric property of water molecules absorbing onto an active porous insulating layer sandwiched between two layers of conductive material deposited on a ceramic substrate. Water has a very high dielectric compared to the dielectric of the active layer and the background of the carrier gas so it can be detected easily.

The active layer is very thin – less than one micron and the porous top conductor that allows water molecules to penetrate into the active layer is less than 0.1 micron thick. This allows the sensor to respond very rapidly to changes in the moisture surrounding it both when moisture decreases (drying) and increases in the sensor environment.


Range: -120 to -40°C dew point (-184 to -40°F)
Accruacy: ±1 to ±4°C range dependant (±1.8 to ±7.2°F)
Output: 4-20 mA or RS-485
Alarms: Up to 4 user confi gurable set points
Pressure Range: 10-9 torr to 3500 psig