PST MICHELL Instruments MDM25 User manual

MDM25
Hygrometer
User Manual
RH
Ta
Abs H
53.1%
21.0°C
9.7 g:M
3
MDM25
Hygrometer
97234 Issue 4.1
February 2022

Please ll out the form(s) below for each instrument that has been purchased.
Use this information when contacting Michell Instruments for service purposes.
Product Name
Order Code
Serial Number
Invoice Date
Installation Location
Tag Number
Product Name
Order Code
Serial Number
Invoice Date
Installation Location
Tag Number
Product Name
Order Code
Serial Number
Invoice Date
Installation Location
Tag Number

© 2022 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.
MDM25
For Michell Instruments' contact information please go to
www.michell.com

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Contents
Safety ................................................................................................................................vi
Electrical Safety ...........................................................................................................vi
Pressure Safety ............................................................................................................vi
Toxic Materials .............................................................................................................vi
Repair and Maintenance ...............................................................................................vi
Calibration ...................................................................................................................vi
Safety Conformity ........................................................................................................vi
Abbreviations .....................................................................................................................vii
Warnings ...........................................................................................................................vii
1 INTRODUCTION ................................................................................................1
1.1 Description ........................................................................................................ 2
1.1.1 Controls and Indicators ................................................................................. 2
1.1.2 Instrument Display ........................................................................................ 3
1.2 Display Units ...................................................................................................... 4
1.3 Display Settings .................................................................................................. 4
1.4 Device Settings .................................................................................................. 4
1.5 About ................................................................................................................ 4
2 INSTALLATION ..................................................................................................5
2.1 Unpacking the Instrument ................................................................................... 5
2.2 MDM25 Accessories ............................................................................................ 5
2.3 Battery Requirements ......................................................................................... 6
2.3.1 Fitting the Batteries ....................................................................................... 6
2.4 Mounting and Connecting the Probe .................................................................... 7
3 OPERATION ......................................................................................................8
3.1 Preparation for Operation .................................................................................... 8
3.2 Overall Menu Structure ....................................................................................... 8
3.2.1 Display Settings ............................................................................................ 9
3.2.2 Device Settings ........................................................................................... 10
3.2.3 About ......................................................................................................... 11
4 MAINTENANCE ...............................................................................................12
4.1 Probe Calibration .............................................................................................. 12
4.2 Cleaning .......................................................................................................... 12
4.3 Probe Replacement ........................................................................................... 12
4.4 Batteries .......................................................................................................... 12

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Michell Instruments v
Appendices
Appendix A Technical Specications ..............................................................................14
Appendix B Quality, Recycling & Warranty Information ...................................................16
Appendix C Analyzer Return Document & Decontamination Declaration ..........................18
Figures
Figure 1 Controls and Indicators ...............................................................................2
Figure 2 Display ......................................................................................................3
Figure 3 Battery Compartment Location ....................................................................6
Figure 4 Probe Connections ......................................................................................7
Figure 5 Main Menu Structure ..................................................................................8
Figure 6 Display Settings Menu .................................................................................9
Figure 7 Device Settings Menu ...............................................................................10
Figure 8 About Menu .............................................................................................11
Tables
Table 1 Unit Descriptions .........................................................................................4

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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
competent personnel using 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.
Pressure Safety
This product is not suitable for use at pressures greater than atmospheric.
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
An annual calibration is recommended for the RH probe. This should be returned to the manufacturer,
Michell Instruments, or one of their accredited service agents for re-calibration.
Safety Conformity
This product meets the essential protection requirements of the relevant EU and UK directives.

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Michell Instruments vii
Abbreviations
The following abbreviations are used in this manual:
Abs H absolute humidity
ºC degrees Celsius
ºF degrees Fahrenheit
LCD liquid crystal display
g grams
g/m3 grams per cubic meter
lb pound
mm millimeters
oz ounce
RH relative humidity
sec second(s)
T temperature
Ta ambient temperature
Td dew point
Tw wet bulb temperature
V Volts
% percentage
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.

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Michell Instruments 1
INTRODUCTION
1 INTRODUCTION
The MDM25 is a battery operated handmeter.
Available with a number of dierent probe congurations and the ability to display
relative humidity, temperature, absolute humidity (g/m3), dew point and wet bulb
temperature, the MDM25 is suitable for spot-checking humidity in a wide variety of
applications.
The hygrometer is contained within a molded ABS housing with a rubber surround for
drop protection – sealed to IP54 standards. The rubber surround allows for comfortable
hand-held operation and safe transportation of the unit.
It is powered by 3 alkaline ‘AA’ size cells, which are designed to last a minimum of 150
continuous hours between replacement. Continuous battery charge status indication is
provided by a battery indicator icon.
A graphical display presents the RH and temperature readings, in addition to a selectable
third value, in large format characters.

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INTRODUCTION
1.1 Description
1.1.1 Controls and Indicators
The controls and indicators associated with the MDM25 hygrometer are located on the
front panel.
The probe connection to the MDM25 hygrometer is made to the top panel, and the
battery charger cover is located on the rear of the instrument.
Figure 1 shows the layout of the controls.






1
2
3
4
5
6
1Display
2Left and Exit Key
3Enter Key
4Down Key
5Up Key
6Power Key
Figure 1
Controls and Indicators

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INTRODUCTION
1.1.2 Instrument Display
RH
Ta
Abs H
53.1%
21.0°C
9.7 g:M
3




1
2
3
4
1
Battery Charge Indicator Icon – displays 3 dots
when full. When battery icon is empty, replace
the batteries.
2RH Display
3Temperature Display
43rd Output Display – presents the selected 3rd
output, either in ºC / ºF Td, g/m3, or ºC / ºF TW
Battery Indicator
Full battery power
2/3 battery power
1/3 battery power
Replace batteries
Figure 2 Display

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INTRODUCTION
1.2 Display Units
The MDM25 measures relative humidity and temperature. The third output, when
displayed, is calculated from these two parameters.
The table below gives a description of all available units.
Signal Symbol Units Description
Relative Humidity RH %
The ratio of water vapor pressure to the
saturation water vapor pressure (the
maximum amount of water vapor that can
exist at a given temperature) over a liquid
water surface, expressed as a percentage.
Temperature Ta ºC or ºF
The temperature of a gas determines the
maximum quantity of moisture that can
be present as vapor in that gas.
Dew Point Td ºC or ºF
The temperature at which dew, or
condensation, forms, on cooling a gas.
This is, therefore, the temperature at
which air becomes saturated in equilibrium
with water.
Absolute Humidity Abs H g/m3
The mass of water vapor present in unit
volume of moist air of a given temperature
and pressure. SI (metric) units are grams
of water per cubic meter of air (g/m3).
Wet Bulb
temperature Tw ºC or ºF
Temperature indicated by a thermometer
sheathed in wet wicking, and inuenced by
the rate of evaporation from the wicking.
Table 1 Unit Descriptions
1.3 Display Settings
The Display Settings Menu contains options to change the contrast ratio, backlight
intensity and orientation of the display.
1.4 Device Settings
The Device Settings Menu contains options to change the period of time before power-
o, temperature units and 3rd value units.
1.5 About
The About Menu gives information about the rmware version and serial number of the
hygrometer.

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INSTALLATION
2 INSTALLATION
2.1 Unpacking the Instrument
Open the box carefully and unpack the contents. Check that the following items are
included. Report any shortages immediately.
• MDM25 Hygrometer
• 3 o Alkaline ‘AA’ cells
• User Manual
• RH & Temp Probe (options available)
• Calibration certicate (optional)
2.2 MDM25 Accessories
Filters and Protection Caps
For 12mm probe For 19mm probe
Slotted protection cap, black Slotted protection cap, black
Mesh lter Mesh lter
Flat SS sintered dust lter Flat SS sintered dust lter
Arrow 20μm SS sintered lter Arrow 10μm SS sintered lter
Arrow 20μm SS sintered lter
PVDF lter
Probes
• Fixed probe
• Standard probe
• Sword type probe
• Remote high temperature probe, 300mm (18”)
• Remote high temperature probe, 500mm (19.6”)
Other
• Carrying case
• Batteries (3 ‘AA’ Alkaline cells)
• Control Kit of calibration salts

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INSTALLATION
2.3 Battery Requirements
The MDM25 is supplied with 3 Alkaline ‘AA’ size cells. These non-rechargeable Alkaline
cells have a high energy density and long shelf-life.
Do not mix dierent type of batteries.
All three batteries must be of the same type.
2.3.1 Fitting the Batteries
Unscrew the battery compartment and insert the three ‘AA’ cells, according to the image
on the inside of the battery compartment.
Ret the compartment cover ensuring the seal is fully compressed.
Figure 3 Battery Compartment Location

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Michell Instruments 7
INSTALLATION
2.4 Mounting and Connecting the Probe
All probes insert directly into the connector on the top of the MDM25.
Align the locating peg with the locating notch and then rotate the knurled thumb wheel
until the probe is securely held in place.
Locating Peg

Locating Notch

Figure 4
Probe Connections
WARNING: Do not over-tighten as this could cause damage
to the probe body.

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OPERATION
3 OPERATION
3.1 Preparation for Operation
Press the power-on button and observe the readings on the LCD. Some 3rd values may
take a few seconds to calculate, depending upon the %RH and ambient temperature
(Ta).
If using the MDM25 alongside the S503 RH calibrator, adjust the display orientation to
allow for the product to be read correctly when upside down.
If there is there is no reading on the screen check the battery orientation. Replace with
new batteries if required.
3.2 Overall Menu Structure
DISPLAY DEVICE ABOUT
CONTRAST AUTO OFF TEMP UNITS 3RD VALUE FIRMWARE VERSION
BACKLIGHT DISPLAY
ORIENTATION
MAIN MENU
Figure 5 Main Menu Structure

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Michell Instruments 9
OPERATION
3.2.1 Display Settings
DISPLAY
CONTRAST
Set the contrast for
the display settings
by pressing the Up
and Down Keys
0 = no contrast
5 = full contrast
BACKLIGHT
Set the brightness
of the backlight by
pressing the Up
and Down Keys
0 = dark
5 = bright
DISPLAY ORIENTATION
Press
Press Press Press
Special feature:
The display
turns automatically
when hand meter
is positioned
upside-down
To store these settings in the memory, press the Menu
button which will open the Main page. When the hand
meter is turned on again this setting is saved.
To store the settings for ONLY the current session, press
the Left Key
The orientation
of the display
can be rotated
by 180ºby
pressing the Up
and Down keys
Figure 6 Display Settings Menu

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OPERATION
3.2.2 Device Settings
DEVICE
AUTO OFF
To save battery life the
Auto Off automatically
switches off the device if
it is not in use.
By pressing the Up and
Down Keys the period of
inactive time before
power-off can be set to
between 1 and 20
minutes or disabled
entirely.
Steps of 1 min up to 10
Steps of 5 mins from 10
to 20
∞ - AUTO OFF disabled
TEMP UNITS
By pressing the Up
and Down Keys
temperature units
can be selected as
either Celcius or
Fahrenheit
3RD VALUE
By pressing the Up
and Down Keys the
3rd value option can
be set to read:
Dew point
Abs Hum
Wet Bulb or
None
Press
Press Press Press
To store these settings in the memory, press the Menu
button to return to the Main page. When the hand meter
is turned on again this setting is saved.
To store the settings for ONLY the current session, press
the Left Key
Figure 7 Device Settings Menu

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Michell Instruments 11
OPERATION
3.2.3 About
ABOUT
FIRMWARE VERSION
View the firmware
version and serial
number of the hand
meter.
This number relates to
the hand held unit ONLY,
not the attached probe
Press
Press
Press the Menu button to return to the Main page.
Figure 8 About Menu

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MAINTENANCE
4 MAINTENANCE
The MDM25 is designed to operate either in static ambient conditions, or in a owing
gas stream.
The sensors are designed to have excellent performance in ambient conditions.
Therefore, a ow is not necessary to ensure that the probe is reacting rapidly to changes
in measured conditions.
If the sensor is installed in a owing sample, then it is not crucial to regulate the
ow rate in order to ensure consistent performance, although it is preferable to avoid
excessive ow rates past the sensor during operation.
The use of appropriate ltering for the sample conditions is always recommended – so,
where the sample or ambient conditions may contain dust, steam or liquid water, a
suitable mesh or sintered lter should be tted. Protection caps are also available to
defend against the possibility of handling damage.
Care should be taken to avoid mechanical shock (impact) or thermal shock (sudden
temperature changes).
4.1 Probe Calibration
Salt solutions are commonly used for the calibration of RH sensors. Michell Instruments’
calibration Control Kit is available to provide a traceable reference to verify the accuracy
of the probe.
The Control Kit can be purchased from Michell Instruments (for contact information go
to www.michell.com).
4.2 Cleaning
The enclosure can be cleaned using a mild household detergent and a damp soft cloth
or absorbent paper. Remove the connector before cleaning and dry o any moisture
before reassembly. NOTE: Do not use any abrasives – this may cause the LCD
display to become obscured.
4.3 Probe Replacement
The probe should be returned to Michell Instruments annually for recalibration. This
will ensure continued traceability, and will verify that it is still performing within its
specied accuracy.
4.4 Batteries
When the battery charge indicator icon on the display is empty, the batteries should be
replaced (see Section 2.3). Always install 3 new batteries of the same type.

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Michell Instruments 13
APPENDIX A
Appendix A
Technical Specications