Vaisala DMP248 Wartungshandbuch PDF herunterladen (Seite 11)

CHAPTER 2_______________________________________________________ PRODUCT DESCRIPTION

VAISALA __________________________________________________________________________ 5

The function Pws(T) is well known from literature. Thus, it is

possible to determine the offset (RH

) if measurements are made at

two or (preferably) more temperatures assuming a constant Pw

during the process.

30°C

20°C

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

1/Pws(T)

RHout(%)

FIGURE 2-1 Device output during hypothetical offset calibration

cycle.

For example, for a transmitter with a -0.2%RH offset error is made 11

measurements at temperatures 20...30 °C. The frostpoint is assumed to

stay constant at -40 °C and that the 'Gain' is nominal 100%.

As the temperature increases the RH drops. Since the response is

linear, a straight line is fitted through the data points. The line

intercepts the y-axis at RH

as shown in FIGURE 2-1. Now the RH

known and it can be subtracted from the indicated value 0.35% RH to

get the correct value of 0.55% RH.

FIGURE 2-2 shows the DRYCAP



sensor as mounted on a DMP248

probehead. Through a combination of the polymer sensor and Pt 100

sensor, the DRYCAP



will accurately measure the water vapor

pressure used in determining low dewpoints.

During auto-calibration

the Pt 100 element is used to first heat and then measure the

temperature of the sensor while cooling back to ambient temperature.

A complete cycle of auto-calibration takes 60...70 seconds. When the

auto-calibration cycle is active, the transmitter locks the output values

to those measured prior to auto-calibration.

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