Retrotec DM-2A Instrukcja Użytkownika Pobierz pdf (Strona 48)

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7.4 Wind causes a fluctuating pressure

A wind blowing across the tip of a tube can cause significant error, even if the tube is not moving.

One of the first things to notice when the wind is blowing is that it’s very difficult to establish the test

pressure. For example, when trying to establish 50 Pa, the wind will cause that pressure to go up to 55

Pa and down to 45 Pa, making it very difficult to take a reading.

This type of problem can be rectified by using the @ Pressure key, which will extrapolate the reading to

the desired pressure regardless of the actual pressure being experienced at the moment. This does not

solve the problem completely, but it does help.

A second option is to increase the time averaging to 20 seconds or more. Be aware that if the fan is

increasing or decreasing in speed, it will take 20 seconds at least for the gauge to register its reading.

For example, if during the initial 10 seconds there was no building pressure whatsoever, and during the

second 10 seconds there was 50 Pa, then the gauge will average that to 25 Pa. The rule of thumb here

is to wait for at least double the time average period before a reading is taken.

To learn how Time Averaging can negate the effects of wind

1. Use a fan to create an air stream.

2. Set Time Averaging to one second.

3. Place a tube, connected to the positive port, in the path of the air stream.

4. The gauge will display a pressure that fluctuates. Adjust the "wind" speed until the gauge is

reading between 2 and 5 Pa.

5. Adjust Time Averaging to 20 seconds.

6. Notice that after 20 seconds, the gauge reading fluctuates significantly less.

The Baseline feature can also be used to minimize the effects of wind, especially if the wind is relatively

constant.

To learn how establishing a Baseline can minimize the effects of wind

1. Use a fan to create an air stream that causes between 2 and 5 Pa pressure.

2. Set Time Averaging to one second.

3. Place a tube, connected to the positive port, in the path of the air stream.

4. Establish a Baseline pressure.

5. Notice that once a Baseline pressure is taken, the measured pressure drops closer to zero,

negating the effects of the wind on the actual measured pressure.

6. Stop the fan.

7. Notice now that the measure pressure is negative, even though no pressure is being

received by the gauge. With the Baseline feature still active, the gauge is compensating for

a pressure that does not exist. Clear the baseline reading to eliminate this error.

Another option is to ensure that the outdoor pressure pickup point is sheltered from the wind. In spite

of standards that require tubes be attached to walls, this strategy does not work particularly well for

limiting factor wind. What does seem to work is taking the outdoor pressure pickup tube 25 feet away

from the building, and setting it in a flat area. Cover the end of the tube with a heavy flat sheet of half-

inch plywood, for example, and the wind fluctuations will be reduced.

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Retrotec DM-2A Instrukcja Użytkownika Strona 48