3.3 Connecting Hose

Sensirion recommends a hose with an inner diameter of 3/16 inch. Due to the dynamic measurement principle, a small air flow is required (Figure 6) which leads to a dependence on the length of the hose (Figure 7). Tubes up to 1 m show less than 1 % error of the measured value (Figure 7).

Length of the connecting hose Deviation of Measured Value 0.5 m (20 inch) - 0.4 % 1.0 m (40 inch) - 0.8 %

gas flow [sccm]

2.0 m (80 inch) - 1.6 %

020406080100120 020406080100differential pressure [% full scale]

4.0 m (160 inch) - 3.2 % Figure 6: Typical air flow through the SDP1000 / SDP2000. Please note: 1 sccm = 1 cm

3

/min at 0°C and 1013 mbar pressure (1 sccm = 0.001 norm liter). Figure 7: Influence of the length of the connecting hose on the accuracy (using 3/16 inch inner diameter). Example: a 50 Pa difference pressure is shown as 49.8 Pa when using 0.5 m tube with 3/16 inch diameter.

4 Application Example: A High End Differential Pressure transducer manufacturer

V
supply 5V IC
1 IC
2 C 1 C 1 R 2 C

Example: Output V

out

: 0…10 Vdc Range: 0…500 Pa, linear characteristics Power supply V

SDP1000 / SDP2000
supply

: 14.5… 27 Vdc

1 2 3 1 2 3 1 R

Component Value

1 R IC
1 LM78L12 IC
2 LP2980 SDP SDP1000 5V 12V 12V12V12V OP
1 , OP
2 , OP
3 LMC6044
2 OP C
1 200 nF
2 R 1 OP 3 1 R

+ - + -

4 R V
out C
2 2.2 uF

+ -

R
1 10 k (5%, 10 ppm)
3 R 5 R R
2 18 k (5%, 10 ppm)
1 R R
3 1 k (5%, 10 ppm)
6 OP R
4 100

V = (V – 5V (R /(R +R )) (R +R )/R

R
5 6 k (3x18 k 5%, 10 ppm in parallel)
out SDP * 3 2 3 * 5 6 5 R
6 10 k (5%, 10 ppm)

Figure 8: Schematic of a high-end differential pressure transmitter using the SDP1000 / SDP2000. With only a few external components and no additional calibration the transmitter features an accurate and temperature compensated 0…10 Vdc output signal. www.sensirion.com v 2.7 June 2005 7/8