Michell Instruments MDM50 User manual

MDM50
Standard Version
User’s Manual
97475 Issue 2
April 2020

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© 2020 Michell Instruments
This document is the property of Michell Instruments Ltd and may not be copied or
otherwise reproduced, communicated in any way to third parties, nor stored in any Data
Processing System without the express written authorization of Michell Instruments Ltd.
MDM50
For Michell Instruments' contact information please go to
www.michell.com

MDM50 LP Version User’s Manual
iv 97475 Issue 2, April 2020
1 INTRODUCTION ................................................................................................1
2 INSTALLATION ..................................................................................................2
3 OPERATION ......................................................................................................3
3.1 General Operation .............................................................................................. 3
3.2 Atmospheric or Line Pressure Measurements ........................................................ 4
3.3 Measuring Dew Points Below -40°C (-40°F) .......................................................... 5
3.4 User Controls ..................................................................................................... 6
3.5 4-20 mA Output Socket Wiring ............................................................................ 7
4 GOOD MEASURING PRACTICE ............................................................................8
5 MAINTENANCE ................................................................................................11
5.1 Calibration ....................................................................................................... 11
5.2 Transmitter / Battery Replacement ..................................................................... 11
5.3 Filter Cartridge Replacement ............................................................................. 13
5.4 Checking the MDM50 Electronics Calibration ....................................................... 13
5.5 List of Spares ................................................................................................... 13
5.6 Troubleshooting ................................................................................................ 14
Contents
Safety ................................................................................................................................ v
Electrical Safety ........................................................................................................... v
Pressure Safety ............................................................................................................ v
Toxic Materials ............................................................................................................. v
Repair and Maintenance ............................................................................................... v
Calibration ................................................................................................................... v
Safety Conformity ........................................................................................................ v
Abbreviations ......................................................................................................................vi
Warnings ............................................................................................................................vi
Appendices
Appendix A Technical Specications ..............................................................................16
Appendix B Quality, Recycling & Warranty Information ...................................................18
B.1 Return Policy ............................................................................... 18
B.2 Calibration Facilities ..................................................................... 19
B.3 Manufacturing Quality ..................................................................19
B.4 FCC (EMC Requirements for North America) .................................. 19
Appendix C Decontamination Declaration ......................................................................21
Figures
Figure 1 Interchangeable Gas Inlet/Outlet Fittings .....................................................4
Figure 2 User Controls .............................................................................................6
Figure 3 Jack Plug Wiring .........................................................................................7
Figure 4 Material Permeability Comparison ................................................................8
Figure 5 2-Wire Connection - View Showing Rear of Connector Terminal Block ...........13

MDM50 LP Version User’s Manual
Michell Instruments v
Safety
The manufacturer has designed this equipment to be safe when operated using the procedures
detailed in this manual. The user must not use this equipment for any other purpose than that
stated. Do not apply values greater than the maximum value stated.
This manual contains operating and safety instructions, which must be followed to ensure the safe
operation and to maintain the equipment in a safe condition. The safety instructions are either
warnings or cautions issued to protect the user and the equipment from injury or damage. Use
qualied personnel and good engineering practice for all procedures in this manual.
Electrical Safety
The instrument is designed to be completely safe when used with options and accessories supplied by
the manufacturer for use with the instrument. The instrument is powered by an internally mounted
rechargeable battery. The input power supply voltage limits for the battery charger supplied with the
instrument are 90 to 264 V AC, 47/63 Hz.
NOTE: No battery charger unit other than that supplied with the instrument should be
used.
NOTE: Do not allow the battery to fully discharge.
Pressure Safety
DO NOT permit pressures greater than the safe working pressure to be applied to the instrument.
The specied safe working pressure is 20 barg (290 psig). Refer to the Technical Specications in
Appendix A.
Toxic Materials
The use of hazardous materials in the construction of this instrument has been minimized. During
normal operation it is not possible for the user to come into contact with any hazardous substance
which might be employed in the construction of the instrument. Care should, however, be exercised
during maintenance and the disposal of certain parts.
Repair and Maintenance
The instrument must be maintained either by the manufacturer or an accredited service agent. Refer
to www.michell.com for details of Michell Instruments’ worldwide oces contact information.
Calibration
The recommended calibration interval for the MDM50 is one year, unless otherwise specied by Michell
Instruments Ltd. The instrument should be returned to the manufacturer, Michell Instruments, or one
of their accredited service agents for re-calibration (go to www.michell.com for contact information).
Safety Conformity
This product meets the essential protection requirements of the relevant EU directives. Further
details of applied standards may be found in the product specication.

MDM50 LP Version User’s Manual
vi 97475 Issue 2, April 2020
Abbreviations
The following abbreviations are used in this manual:
AC alternating current
atm pressure unit (atmosphere)
barg pressure unit (=100 kP or 0.987 atm) gauge
°C degrees Celsius
°F degrees Fahrenheit
dp dew point
ft feet
kg kilogram
lb pound
mA milliampere
m meter
mm millimeter
MPa megapascal
Nl/min normal liters per minute
psig pound(s) per square inch (gauge)
scfh standard cubic feet per hour
μm micrometer
Ω ohm
V Volts
Warnings
The following general warnings listed below are applicable to this instrument. They are
repeated in the text in the appropriate locations.
!
Where this hazard warning symbol appears in the following
sections, it is used to indicate areas where potentially hazardous
operations need to be carried out.

MDM50 LP Version User’s Manual
Michell Instruments 1
INTRODUCTION
1 INTRODUCTION
The MDM50 comprises a fast responding polymer sensor tted into a sample block with
an integrated lter cartridge, and Legris push ttings or optional Swagelok® tube ttings
on the gas inlet and outlet. The measured dew point from the sensor is shown on the
clear red LED display on the front panel of the instrument.
The MDM50 can be supplied with either °C or °F dew-point measurement units.
A 4-20 mA analog output is provided for connection to a chart recorder, data-logger or
computer system, so dew-point trends can be analyzed over time.
The electronics are housed in a rugged Peli case, providing NEMA 6 protection when the
lid is closed. The case is supplied with a lifetime guarantee.
The MDM50 is powered by a rechargeable NiMH battery pack, giving between 12 and
16 hours of measurement time from a full charge. The instrument is delivered complete
with a universal battery charger stored in the lid. It takes 16 hours to fully charge the
battery pack, during which time the instrument can be switched on or o. The charger
is suitable to connect power to the instrument indenitely, but it is recommended to
allow the battery pack to go through a full charge-discharge cycle at least once per
month.
MDM50 polymer moisture sensors are subject to a 9-point calibration, where their
performance is characterized against a fundamental reference hygrometer. This process,
and subsequent quality testing, ensures that all sensors behave optimally before they
are used in the eld. Each MDM50 is supplied with a calibration certicate traceable to
national standards (NIST) from Michell Instruments' accredited laboratory.

MDM50 LP Version User’s Manual
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INSTALLATION
2 INSTALLATION
On delivery, please check that all the following standard components are present in the
packing box:
• MDM50
• 2m of 6mm O/D PTFE tube
• Battery charger
• Mains lead
• 3 pole ¼” jack plug (inside lid) - for analog output connections
• Certicate of calibration
Before using the MDM50 for the rst time it is recommended that the battery pack is
charged for a minimum of 12 hours.

MDM50 LP Version User’s Manual
Michell Instruments 3
OPERATION
3 OPERATION
!
Maximum operating pressure is 20 barg (290 psig).
It is important that the gas ttings are correctly tightened, and
any tubing is secure in the push ttings before use.
Failure to do so will aect the instrument's pressure rating.
3.1 General Operation
To operate the MDM50, follow these instructions:
1. Ensure that all gas ttings are fully tightened before use.
2. Gas under test should be restricted to ow rates between 1 and 5 Nl/min
(2 and 10.6 scfh) and pressures from atmospheric to 2 MPa (20 barg (290
psig)).
3. Connect the sample gas supply line to the instrument Gas In port.
4. Connect the sample gas vent line to the instrument Gas Out port.
5. Switch the instrument on. The display will begin to change as the sensor
responds to the applied dew point.
6. Allow the gas to ow until the display shows a stable reading. Typically
this would be around 5 to 15 minutes for spot checks at dew points of
-40°C (-40°F) and above.
7. Switch the unit o and disconnect the sample lines.
NOTE: Filters are essential for potentially dirty/contaminated gases – the
tted lter should be checked before and after use and replaced regularly - as
required.

MDM50 LP Version User’s Manual
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OPERATION
3.2 Atmospheric or Line Pressure Measurements
The MDM50 is supplied with two labelled, interchangeable, gas inlet/outlet ttings.
Number Description
14mm free ow tting
2 0.4mm restricted tting

Gas Out Gas In
Filter Fitted
1
2

Gas Out Gas In
Filter Fitted
2
1
ATMOSPHERIC
LINE PRESSURE
Figure 1 Interchangeable Gas Inlet/Outlet Fittings
In order to perform measurements at line pressure, the restricted tting should be on
the gas outlet port.
To perform measurements at atmospheric pressure, the restricted tting should be on
the gas inlet port.
!The Gas In port is ALWAYS on the right hand side.

MDM50 LP Version User’s Manual
Michell Instruments 5
OPERATION
3.3 Measuring Dew Points Below -40°C (-40°F)
Due to the signicantly lower levels of moisture present at dew points of this level, and
the increased amount of time to dry the system out, the response times of the sensor
will be signicantly increased. The table below oers an approximate guide to the
times taken for the instrument to stabilize at a given dew point (from a starting point
of 10°Cdp (50°Fdp) ambient):
Target Dew Point T100 Response Time
(mins)
°C °F
-30 -22 5
-40 -40 15
-50 -58 30
-60 -76 45

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OPERATION
3.4 User Controls
Figure 2 User Controls
1Digital Display
Indicates the measured dew point in °C from -60 to +20 (-76 to
+68°F).
Under certain conditions the following error messages may be
displayed:
ErrL = Transmitter under range
ErrH = Transmitter over range
Err I = Sensor fault or transmitter disconnected
2 Power Switch Switches the MDM50 ON or OFF.
3Fuse
This 1A quick blow fuse provides protection for the display
electronics in the event of a fault with the charger or battery pack.
Another protection fuse is located on the display PCB.
4NiMH Battery
Pack
Located in the top right hand corner of the instrument, it can be
accessed by removing two screws (5). The battery pack can be
recharged using the supplied charger, via the battery-charging
socket. See Section 5.6 for troubleshooting information.
The battery pack will charge if the instrument is switched ON or
OFF.
6Analog Output
Socket
The MDM50 features an analog output socket that provides
a linear 4-20 mA current loop, scaled to -60 to +60°C (-76 to
+140°F) dew point. This allowed the instrument to be connected
to a chart recorder, data logger or PC to enable dew-point trends
to be analyzed over time. For wiring information see Section 3.5.
7Gas Fittings
The MDM50 can be used for measurements at line pressure (up to
20 barg (290 psig)) or atmospheric pressure. Legris push ttings
are supplied for use with 6mm OD PTFE tubing or, optionally,
Swagelok® tube ttings for use with 6mm or ¼" OD stainless steel
tube. There is a 32mm particulate lter (99.5% removal of 0.1
micron particles) tted as standard under the Gas In port position.

MDM50 LP Version User’s Manual
Michell Instruments 7
OPERATION
3.5 4-20 mA Output Socket Wiring
The MDM50 provides a linear 4-20 mA output scaled from -60 to +60°C (-76 to +140°F).
The socket accepts a 3 pole ¼” jack plug (supplied) and should be wired as shown
below:
-VE
-VE
+VE
+VE


 
Figure 3 Jack Plug Wiring

MDM50 LP Version User’s Manual
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GOOD MEASUREMENT PRACTICE
4 GOOD MEASURING PRACTICE
Ensuring reliable and accurate moisture measurements requires the correct sampling
techniques, and a basic understanding of how water vapor behaves. This section aims
to explain the common mistakes and how to avoid them.
Sampling Materials – Permeation and Diusion
All materials are permeable to water vapor since water molecules are extremely small
compared to the structure of solids, even including the crystalline structure of metals.
The graph above demonstrates this eect by showing the increase in dew point
temperature seen when passing very dry gas through tubing of dierent materials,
where the exterior of the tubing is in the ambient environment.
- 20
- 30
- 40
- 50
- 60
- 70
12345
Time (hours)
Dew point (ºC)
nylon
copper
polyethylene
PTFE
nickel
stainless steel
Figure 4 Material Permeability Comparison
What this demonstrates is the dramatic eect that dierent tubing materials have on
the humidity levels of a gas passed through them. Many materials contain moisture
as part of their structure and when these are used as tubing for a dry gas the gas
will absorb some of the moisture. Always avoid using organic materials (e.g. rubber),
materials containing salts and anything which has small pores which can easily trap
moisture (e.g. nylon).
As well as trapping moisture, porous sampling materials will also allow moisture vapor
to ingress into the sample line from outside. This eect is called diusion and occurs
when the partial water vapor pressure exerted on the outside of a sample tube is
higher than on the inside. Remember that water molecules are very small so in this
case the term ‘porous’ applies to materials that would be considered impermeable in
an everyday sense – such as polyethylene or PTFE. Stainless steel and other metals
can be considered as practically impermeable and it is surface nish of pipework that
becomes the dominant factor. Electropolished stainless steel gives the best results over
the shortest time period.
Take into consideration the gas you are measuring, and then choose materials appropriate
to the results you need. The eects of diusion or moisture trapped in materials are
more signicant when measuring very dry gases than when measuring a sample with a
high level of humidity.

MDM50 LP Version User’s Manual
Michell Instruments 9
GOOD MEASUREMENT PRACTICE
Temperature and Pressure eects
As the temperature or pressure of the environment uctuates, water molecules are
adsorbed and desorbed from the internal surfaces of the sample tubing, causing small
uctuations in the measured dew point.
Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved
solid to the surface of a material, creating a lm. The rate of adsorption is increased at
higher pressures and lower temperatures.
Desorption is the release of a substance from or through the surface of a material. In
constant environmental conditions, an adsorbed substance will remain on a surface
almost indenitely. However, as the temperature rises, so does the likelihood of
desorption occurring.
Ensuring the temperature of the sampling components is kept at consistent levels is
important to prevent temperature uctuation (i.e. through diurnal changes) continually
varying the rates of adsorption and desorption. This eect will manifest through a
measured value which increases during the day (as desorption peaks), then decreasing
at night as more moisture is adsorbed into the sampling equipment.
If temperatures drop below the sample dew point, water may condense in sample
tubing and aect the accuracy of measurements.
Maintaining the temperature of the sample system tubing above the dew point of the
sample is vital to prevent condensation. Any condensation invalidates the sampling
process as it reduces the water vapor content of the gas being measured. Condensed
liquid can also alter the humidity elsewhere by dripping or running to other locations
where it may re-evaporate.
Although ambient pressure does not change drastically in a single location, the gas
sample pressure does need to be kept constant to avoid inconsistencies introduced
by adsorption or desorption. The integrity of all connections is also an important
consideration, especially when sampling low dew points at an elevated pressure. If
a small leak occurs in a high-pressure line, gas will leak out; however, vortices at the
leak point and a negative vapor pressure dierential will also allow water vapor to
contaminate the ow.
Theoretically ow rate has no direct eect on the measured moisture content, but
in practice it can have unanticipated eects on response speed and accuracy. An
inadequate ow rate may:

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GOOD MEASUREMENT PRACTICE
• Accentuate adsorption and desorption eects on the gas passing through
the sampling system.
• Allow pockets of wet gas to remain undisturbed in a complex sampling
system, which will then gradually be released into the sample ow.
• Increase the chance of contamination from back diusion. Ambient air
that is wetter than the sample can ow from the exhaust back into the
system. A longer exhaust tube can help alleviate this problem.
• Slow the response of the sensor to changes in moisture content.
An excessively high ow rate can:
• Introduce back pressure, causing slower response times and unpredictable
changes in dew point
• Result in a reduction in depression capabilities in chilled mirror instruments
by having a cooling eect on the mirror. This is most apparent with gases
that have a high thermal conductivity such as hydrogen and helium.
System design for fastest response times
The more complicated the sample system, the more areas there are for trapped moisture
to hide. The key pitfalls to look out for here are the length of the sample tubing and
dead volumes.
The sample point should always be as close as possible to the critical measurement
point to obtain a truly representative measurement. The length of the sample line to the
sensor or instrument should be as short as possible. Interconnection points and valves
trap moisture, so using the simplest sampling arrangement possible will reduce the time
it takes for the sample system to dry out when purged with dry gas.
Over a long tubing run, water will inevitably migrate into any line, and the eects of
adsorption and desorption will become more apparent.
Dead volumes (areas which are not in a direct ow path) in sample lines, hold onto
water molecules which are slowly released into the passing gas. This results in increased
purge and response times, and wetter than expected readings. Hygroscopic materials
in lters, valves (e.g. rubber from pressure regulators) or any other parts of the system
can also trap moisture.
Plan your sampling system to ensure that the sample tap point and the measurement
point are as close as possible to avoid long runs of tubing and dead volumes.
Filtration
All trace moisture measurement instruments and sensors are by their nature sensitive
devices. Many processes contain dust, dirt or liquid droplets. Particulate lters are used
for removing dirt, rust, scale and any other solids that may be in a sample stream.
For protection against liquids, a coalescing or membrane lter should be used. The
membrane provides protection from liquid droplets and can even stop ow to the
analyser completely when a large slug of liquid is encountered, saving the sensor from
potentially irreparable damage.

MDM50 LP Version User’s Manual
Michell Instruments 11
MAINTENANCE
5 MAINTENANCE
Routine maintenance of the MDM50 is conned to regular re-calibration of the internal,
removable SF82 transmitter and replacement of the lter cartridge.
5.1 Calibration
The calibration of the internal transmitter is traceable to national standards. For this
reason it should only be calibrated in an accredited, e.g. UK United Kingdom Accreditation
Service (UKAS) or US National Institute of Standards and Technology (NIST) standards
laboratory. If these facilities do not exist it is recommended that the transmitter is
returned to the manufacturer, Michell Instruments, or an approved agent. A calibration
certicate bearing a seven-point calibration is issued with each transmitter.
In most applications, annual re-calibration ensures that the stated accuracy of the SF82
transmitter is maintained.
5.2 Transmitter / Battery Replacement
To remove the transmitter, or battery pack:
1. Ensure instrument is switched o and that the battery charger, current
output and any sampling components are also disconnected.
2. Remove and retain the 10 cross-head screws from the top plate.
3. Lift the small battery pack cover in the top right corner of the instrument.
A small at bladed screwdriver may be required to gently pry the edges
of the cover if it has become stuck to the waterproof seal underneath.
Removing this cover rst will make it easier to lift the entire top panel out
of the instrument.
4. Lift the top plate of the instrument out of the case. Disconnect the battery
pack before removing the top plate completely to prevent it from straining
on the attached cable. If only replacing the battery pack, do not follow
the next two steps.
5. Undo the screw from the centre of the transmitter connecting plug and
pull o the connector.
6. Unscrew the transmitter from the block.
Fitting the replacement is simply a reversal of the above procedure. When tting a new
transmitter it may be necessary to change the alignment of the GDSN connector (see
next page).

MDM50 LP Version User’s Manual
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MAINTENANCE
If the front panel prevents the connector from tting onto the transmitter, follow the
instructions below:
• Completely unscrew the cable gland on the GDSN connector to release
the cable tension.
• Remove the locking screw from the back of the connector (retaining the
metal O-ring).
• Remove the connector block using a small screwdriver.
• Rotate the connector block and push it back into the connector housing.
Take care not to trap any wires against the hole for the locking screw.


MDM50 LP Version User’s Manual
Michell Instruments 13
MAINTENANCE
• Replace the locking screw and cable gland.
5.3 Filter Cartridge Replacement
Unscrew the gas inlet tting to reveal the lter, which can then simply be removed for
checking or replacement.
5.4 Checking the MDM50 Electronics Calibration


Supply
4-20 mA
PIN 3
PIN 1
(GND) Screen/Shield
Optional
Figure 5 2-Wire Connection - View Showing Rear of Connector Terminal Block
To verify if the display electronics are within calibration a 4-20mA current source can be
connected in place of the transmitter.
At 4 mA the display should show -60.0°Cdp ±0.5°C (-76.0°Fdp ±0.9°F)
At 20 mA the display should show +60.0°Cdp ±0.5°C (+140.0°Fdp ±0.9°F)
If the displayed values are outside of this range, please contact Michell Instruments'
Service Department.
5.5 List of Spares
P/N Description
SSF-PF-10PK Pack of 10 particulate lter cartridges
MDM50-BAT Replacement battery pack
MDM50-CHR Replacement battery charger

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MAINTENANCE
5.6 Troubleshooting
Symptom Cause Actions
Display shows “ErrI”
Transmitter disconnected
or sensor element /
sensor thermistor fault.
NOTE: This error is
normally displayed
during the rst few
seconds while the
instrument is starting
up.
Check transmitter cable is securely
connected inside instrument.
Check instrument electronics with
4-20mA source.
Exchange transmitter.
Display shows “ErrL”
Transmitter reading
under-range.
Transmitter current
signal
< 4mA.
Transmitter element
open circuit.
Check instrument electronics with
4-20mA source.
Exchange transmitter.
Display shows “ErrH”
Transmitter reading
over-range.
Transmitter current
signal > 20mA.
Sensor element short
circuit.
Sensor may have been exposed
to saturation conditions or liquid
contamination. Check that lter and
sensor guard are clean and dry.
If liquid water has contacted sensor,
or sensor block, then disassemble
and dry thoroughly. After drying, it
is recommend to purge assembled
instrument with very dry (-75°Cdp
(-103°Fdp) / 1ppmV moisture) air for
12 to 24 hours.
Exposure to other contaminants can
cause lasting damage and may require
transmitter to be exchanged.
Check instrument electronics with
4-20mA source.
Display ickers on
and o Battery voltage low. Connect charger.