ROTRONIC MBW 373 - Manual (Page 31)

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Connecting Hoses
Your success with dew or frost point measurement may likely depend on proper selection and
connection of the hoses you use in configuration of your gas sampling system.
Types of Hoses
There are various types of hoses available for use in your gas sampling system. You should
consider the suitability for your application.
Teflon
Teflon tubing is flexible, easy to use, and is well suited for frost/dew point measurements in the
range of approximately –60 to +20 °C. However, for frost point measurements below
approximately –60°C, the hygroscopic effects of Teflon tubing may begin to induce
measurement errors due to permeation of water vapor into the gas stream. For dew point
measurements above 20°C, heated hoses will be required.
Stainless Steel
Stainless steel tubing, while less convenient to use, is a good choice for all measurement
ranges. For measurement of frost/dew point temperatures below approximately –60°C, electro-
polished stainless steel tubing should be used to reduce dry down times. For extremely low frost
point measurements, also keep bends, fittings, and couplings to only the necessary minimums.
Heated Hoses
When measuring dew points that are higher than room temperature, heated hoses will be
required. If your 373 is intended to be used for high dew point measurements, it will be
equipped with heated internal components and controls for external heated hoses. Once the
hoses are connected pneumatically and electrically, you can set the temperature and enable
control. See Heater Controls on page 17.
Pneumatic Connections
For high dew point measurements, connect one end of the heated hose to the gas input fitting
on the back panel. Connect the other end of the heated hose to the sample gas. When
sampling a chamber, insert the end of the hose fully into the chamber if possible.
Wrap a piece of tubular insulation around the exposed fitting between the back panel and the
tube’s insulating sheath. Insulate all exposed tubes, fittings, and other connections. If any
section of tubing cools to a temperature below the dew point of the gas being measured,
condensation will occur within the tube, altering the measurement.
If desired, also connect a heated hose to the gas outlet connection of the 373. However, if
simply exhausting the gas to ambient, a heated hose on the gas outlet is generally not required.
Also note that it makes no sense to put a heated hose on the gas outlet if there is not a heated
hose on the gas inlet.
Electrical Connections
Attach the electrical cable from the heated hose to the electrical plug immediately adjacent to the
gas fitting. The 373, if properly equipped, will control the temperature of the hose, when
enabled, to a specified value.