This guide provides an overview of the correct measurement procedure for the MDM300 at dew points above -70°C.
For any further information regarding these procedures, measurement below -70°C, or any other aspect of operation,
please refer to the User’s Manual.
Quick Start
1. Follow procedure A and note the result.
2. Turn the instrument off.
3. If you wish to measure at sample pressure then use the Michell Humidity Calculator to convert this result back to
the equivalent at your sample pressure.
4. Using this converted result, and the table below, nd the appropriate procedure to measure your sample.
Dew Point Measure at System Pressure >2.5 barg (36 psig) Measure at Atmospheric Pressure
> -40°C Procedure B Procedure A
< -40°C Procedure C Procedure A
Procedure A: Standard Purge (Atmospheric Pressure)
1. Connect the MDM300 to the sample gas stream.
2. Adjust the ow rate, using the inlet ow control valve, to between 0.2 & 1.2 Nl/min (0.5 & 2.5 scfh) {MDM300
Version} OR 0.2 & 0.5 Nl/min (0.5 & 1 scfh) {MDM300 I.S. Version} and ensure that the outlet ow control
valve is fully open.
3. Turn the instrument on - the initialization period begins automatically. The screen will show a countdown of the
remaining initialization time. The status display will read ‘Initializing internal sensor’.
4. After the Initialization period is complete, the instrument will begin measurement. The ow rate should now be
adjusted, using the inlet ow control valve, to 1.0 Nl/min (2.1 scfh).
5. During the rst few minutes of measurement, the displayed moisture value will drop rapidly below the actual
moisture value, before returning to a stable reading. The instrument display will show an initial measured value,
and the status display will show ‘Measurement in progress’.
6. Once ‘Measurement in progress’ has disappeared, the instrument is close to the nal dew point. Depending on the
conditions, it may continue to respond for a number of minutes before tracking the measured dew point.
Procedure B: Standard Purge (System Pressure)
1. Connect the MDM300 to the sample gas stream.
2. Adjust the ow rate, using the outlet ow control valve, to between 0.2 & 1.2 Nl/min (0.5 & 2.5 scfh) {MDM300
Version} OR 0.2 & 0.5 Nl/min (0.5 & 1 scfh) {MDM300 I.S. Version} and ensure that the inlet ow control
valve is fully open.
3. Turn the instrument on - the initialization period begins automatically. The screen will show a countdown of the
remaining initialization time. The status display will read ‘Initialising internal sensor’.
4. After the Initialization period is complete, the instrument will begin measurement. The ow rate should now be
adjusted, using the outlet ow control valve, to 1.0 Nl/min (2.1 scfh).
5. During the rst few minutes of measurement, the displayed moisture value will drop rapidly below the actual
moisture value, before returning to a stable reading. The instrument display will show an initial measured value,
and the status display will show ‘Measurement in progress’.
6. Once ‘Measurement in progress’ has disappeared, the instrument is close to the nal dew point. Depending on the
conditions, it may continue to respond for a number of minutes before tracking the measured dew point.
Procedure C: Enhanced Purge
1. Connect the MDM300 to the sample gas stream.
2. Adjust the ow rate, using the inlet ow control valve, to between 0.2 & 1.2 Nl/min (0.5 & 2.5 scfh) {MDM300
Version} OR 0.2 & 0.5 Nl/min (0.5 & 1 scfh) {MDM300 I.S. Version} and ensure that the outlet ow control
valve is fully open.
3. Turn the instrument on - the initialization period begins automatically. The screen will show a countdown of the
remaining initialization time. The status display will read ‘Initialising internal sensor’.
4. After 2-3 minutes of Initialization have elapsed, close the outlet ow control valve and fully open the inlet ow
control valve. The ow rate should now be adjusted, using the outlet ow control valve, to 1.0 Nl/min (2.1
scfh).
5. After the Initialization period is complete, the instrument will now begin measurement. During the rst few minutes
of measurement, the displayed moisture value will drop rapidly below the actual moisture value, before returning
to a stable reading. The instrument display will show an initial measured value, and the status display will show
‘Measurement in progress’.
6. Once ‘Measurement in progress’ has disappeared, the instrument is close to the nal dew point. Depending on the
conditions, it may continue to respond for a number of minutes before tracking the measured dew point.
