Michell Instruments MDM300 I.S, User Manual, Page: 46 / 72

MDM300 I.S. User’s Manual
38 97213 Issue 5, October 2017
GOOD MEASUREMENT PRACTICE
4 GOOD MEASUREMENT PRACTICE
The MDM300 I.S. is designed to operate in a flowing gas stream with flow rates between
0.2 and 0.5 Nl/min (0.5 and 1 scfh). Flow regulation is provided with the MDM300 I.S.
instrument between 2.5 and 10 barg. Outside of this, the sample flow must be regulated
outside the unit.
For all applications, the sample gas is taken into the instrument via the Gas In port
(2) Figure 5 located on the top of the instrument, from where it passes through the
internal sensor and out via the Gas Out port (1)
Figure 5. Gas flow rate (if monitored)
is normally measured externally, on the output side of the instrument.
There are a number of options for sample conditioning including a purpose built sampling
system and a variety of application kits. The flow rate can be regulated on the inlet
or outlet depending on whether the intention is to measure at atmospheric or sample
pressure. Up to 10 barg, this can be achieved by use of the orifice fittings supplied with
the instrument. Section 2.5.1 and Figure 8 show a typical arrangement for measuring
non-hazardous gas at atmospheric or elevated pressure.
In applications where the sample gas has a very high flow rate it is advisable to use
an instrument by-pass arrangement to provide the minimal flow rates which the sensor
requires. Excessive flow restriction can cause back-pressure across the sensor, leading
to a discrepancy between the readings recorded and those expected.
Note: Dew-point is dramatically affected by changes in pressure - in order to
ensure the best performance from the instrument it is essential to adequately
regulate the sample pressure.
Ideally, a good quality pressure regulator should be used in conjunction with a fine
adjustment needle valve. Using a needle valve exclusively is acceptable, but only if the
sample pressure downstream of the nearest regulator is not susceptible to pressure
fluctuations greater than ±250 millibar.
Measurement of Dew Points Drier than -70°Cdp
The MDM300 I.S. Sampling System is optimized for fast response. Therefore, when
measuring very dry dew points, certain steps need to be taken to ensure that the best
accuracy is achieved:
Make sure that all tubing and fittings used are made from stainless steel.
This should reduce the possibility of any moisture being trapped upstream of the sensor
prolonging the dry-down process, or permeating into the sample line through unsuitable
tubing - such as PTFE.
Due to the significantly lower levels of moisture present at dew points of this level, the
response times of the sensor will be significantly increased. The table below offers an
approximate guide to the times taken for the instrument to stabilize at a given dew
point (from a starting point of 10°Cdp ambient):
Target Dew Point (°C) T100 response time from ambient (hours)
-80 12
-90 48
-100 96