Sampling Lines; Ensuring Heat Transfer - GE OptiSonde User Manual

October 2007

Sampling Lines

Ensuring Heat Transfer

2-12
Keep the length of sample tubing between the source and the sensor
short, for quick response and highest accuracy. All sampling line
compression fittings provided with the sensor are for ¼-inch diameter
tubing, unless otherwise specified at the time of order.The material
used for the inlet lines can have an important effect on the validity of
the readings. Do not use rubber hose or plastic tubing such as PVC or
Tygon, because of their hygroscopic nature.
When measuring frost points below –30°C, sample gas leaving the
sensor outlet should be vented through a coil approximately two feet
long, since backflow of ambient moisture into the sensor can take
place even under positive pressure. Use stainless steel tubing and
fittings, and ensure that all plumbing is completely free from leaks.
Stainless steel is the material of choice.The sampling system should
allow for periodic cleaning. It may be helpful to install a tee and
closing valve on the inlet side, to permit the sensor to be shut off
while the sampling lines are flushed. At very low humidities, even a
trace amount of contamination can alter measured frost point, so
cleanliness is particularly important.
Be sure the sensor has an adequate heat sink when operating in hot
environments. The sensor must never be allowed to reach a
temperature above its rated limit. It is not sufficient merely to ensure
that the sensor is in an environment whose temperature is below the
rated limit; a means must be supplied to remove heat from the sensor.
When the Model 1111H, 1211H or D-2 sensors are used at ambient
temperatures of 20° to 24°C, full rated depression can be achieved by
mounting the sensor on a smooth, thermally conductive surface (such
as metal), which tends to remain at the ambient temperature.
If possible, do not operate the sensor continuously at or near full
depression. Doing so may decrease the anticipated life of the
thermoelectric heat pump.
Installation