Michell Instruments Pura Specifications

Pura

Dew-Point Transmitters

Pure Gas Moisture Transmitter

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.

Highlights

• Measurement range down to -120°C (-184°F)
dew point (1ppbV)

• Stable and repeatable measurement

• Analog and digital output

• Simple installation

• VCR process connections

• Optional display / alarm unit

• Compact design

• Fast response

• Optional hazardous area approval

• Service exchange program

www.michell.com

Applications

• Pure gases

• Semiconductor manufacture

• Fibre optic production

• Electronic component manufacture

• Optical coating processes

• Industrial gas production and distribution

Dew-Point Transmitters

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
dew point intervals across its measurement range against
transfer standards traceable to NIST and NPL, it is also user­customisable. The 4-20 mA output can be user set over any part
of the operating range, with a minimum output span of
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

10°C

1°C.

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 fit 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 flexibility 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 filled, 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

www.michell.com

©

h

h

h

h

h

h h h

Dew-Point Transmitters

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.

g Hygroscopic Mono-layer

g Base Electrode

Ceramic Substrate

g

Moisture
Molecules

Dimensions

PREMIUM & OEM SENSOR

96mm

(3.7”)

91mm

Setup

(3.6”)

Pura

Advanced

Online

h

ª

h

43mm

(1.8”)

47mm

(1.9”)

16mm

(0.6”)

117mm

h h

(4.6”)

42mm

(1.7”)

h

6mm

(0.2”)

40mm

(1.6”)

h

88mm

(3.5”)

www.michell.com

Michell ceramic sensor tile layers

ADVANCED ONLINE (AOL)

48mm

(1.9”)

97mm

(3.8”)

94mm
(3.7”)

Pura

Online

h

h

Hygr

ometer

46mm

(1.8”)

84mm
(3.3”)

h

44mm
(1.7”)

91mm

(3.6”)

ONLINE (OL)

Monitors

Advanced Online
Monitor: PUR-AOL

Online Monitor: PUR­OL

Measurement Units

°F, °Cdp; ppm

V

and ppbV with user
selectable range and
resolution

Simple display defined by
2-wire sensor input

Secondary
measurement variable

Pressure via external
transducer 0-30
MPa (300 variable
barg/4351 psi).
Completely user
configurable for any
industry standard
transducer. Automatic
compensation for
ppmV and

ppb

V

ppmV with no

line
pressure correction input
available with PUR-TX-2W
& PUR-IS-2W

Power Supply

Universal 85 to 265
V AC; 95 to 370 V DC

100 to 240 V AC (+10,

-15%)
50/60 Hz

Ingress Protection

IP54 (NEMA 12);
IP66 (NEMA 4X) for
front panel only with
optional cover

IP65 front panel (IP20
rear panel)

Operating Temperature

0 to +50°C
(-4 to +122°F)

0 to +50°C
(+32 to +122°F)

Relays

2 off 10 A/240 V
c/o fully user
configurable:
can accommodate
full range for
selected engineering
unit + sensor fault
alarm functions

Alarm 1 relay:
single pole make contact,
rating 3 A @ 250 V AC
Alarm 2 relay:
changeover contacts,
rating 5 A @ 250 V AC

Display Resolution

5 Digit

4 Digit

Outputs: Analog

Digital

4-20 mA fully user
configurable and
scaleable for range

RS232: provides 2­way

communication
– full data access to
user and total
availability of all set­up options

0-20 or 4-20 mA fully user
configurable and scaleable
for range

RS232: provides 2-way
communication – full data
access to user and total
availability of all set-up
options

Options

Power Supply

18 to 36 V AC;
9 to 60 V DC

24 V DC

Relays

1 or 2 further 5 A
relays (max 4 in
total) n/o type fully
user configurable:
can accommodate
full range + full unit
choice + sensor fault
alarm functions

1 further optional relay ­refer to factory

Outputs: Analog

Digital

0-10 V fully user
configurable and
scaleable for range
RS485

0 to 10 V output using
500 Ω shunt resistant

Dew-Point Transmitters

Sensor

Sensor Type Michell Advanced Ceramic Moisture Sensor

Range -120 to -40°Cdp (calibrated from -100 to -40°C)

(-184°F to -40°Fdp calibrated from -148 to -40°F)

Accuracy ±1°C from -40 up to -60°Cdp (-40 up to -76°Fdp)

±2°C from -60 up to -100°Cdp (-76 up to -148°Fdp)
±4°C from -100 to -120°Cdp (estimated)
(-148 to -184°Fdp)

Operating Pressure Minimum 10

-7

Pa (10

-9

torr);

Maximum 24 MPa (240 barg/3481psi)

Operating Temperature -40 to +60°C (-40 to +140°F)

Flow Rate 1 to 5 Nl/m (2.11-10.59 scfh)

Electronics/Electrical

Power Nominal 24 V DC, 10-28 V DC maximum current

25 mA 3- or 2-wire configuration

Output PUR-TX-2W: 4 to 20mA 2 wire

PUR-TX-3W: 4 to 20mA 3 wire
PUR-AOL-SEN only: Digital String (RS485)

Cable with Display PUR-OL & PUR-IS-OL: 2m (standard) (6.56 ft)

optional 5 or

10m

(16.4 or 32.8 ft)

PUR-AOL: 2m (standard) (6.56 ft)

optional 5 or

10m

(16.4 or 32.8 ft)

Mechanical

Sensor Block Cold drawn stainless steel, 0.1 - 0.2 Ra μm
(Premium & OEM) electro-polished internal finish

Pressure Connections Premium and OEM versions: ¼” male

VCR connections

Pura Sensor version: ½” male VCR connection

Installation Profile Premium and OEM versions: 120mm pitch

(4.72 in), drop in across gas port face seals. Zero
pipe clearance required. 2 off M4 x 0.7, 7mm
(0.03, 0.28 in) deep mounting holes in block,
diagonally opposed at 20 x 20mm pitch
(0.79 x 0.79

in)
Pura Sensor version: fits into user’s ½” VCR
configured port

Overall Dimensions Premium and OEM versions: 120 x 35 x

150mm (wxdxh) (4.72 x 1.38 x 5.9 in)
Pura Sensor version: 132 x ø35mm (5.2 x ø1.38 in)

Weight Premium and OEM versions: 450 g (0.99 lb)

Pura Sensor version: 180 g (0.4 lb)

Packaging Pura Premium: Double bagged and sealed in

UHP inert

gas

Pura OEM and Pura Sensor: Single bagged in

1000 gauge

polythene
All options: shipped individually in profiled
sponge protection and carton
Sensor version supplied with protective guard
over sensor surface for transportation and
handling

Service Exchange Available on the Pura Sensor (SEN) ½”

VCR

connection

Technical Specifications

Michell Instruments Ltd, Rotronic Instruments Corp. 135 Engineers Road, Suite 150, Hauppauge

NY 11788, Tel: [phone], Email: [e-mail], Web: www.michell.com/us

Michell Instruments adopts a continuous development programme which sometimes necessitates specification changes without notice.
Issue no: Pura_97172_V5.1_US_0718

© 2018 Michell Instruments