E+E Elektronik EE891, EE892, EE893, EE871 CO2 Modules Application Note

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APPLICATION NOTE
AN0102
CO2 Modules:
Changing the Measuring Time Interval
Selecting the Output Value and Response Time
Triggering Measurements
Rev. 1.3 03/2014
Relevant for:
This application note applies to EE891, EE892, EE893 and EE871
Introduction:
This application note provides support in setting individually the measuring time interval, selecting the
response time and triggering a measurement. It also discusses the impact of the measurement
interval on the response time.
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1 Setting the measuring time interval
Following steps are necessary for setting the measurement time interval for E+E CO2 modules.
1.1 Check if “Global Measurement Interval” function is supported
 Set the read pointer with control byte 0x50 Set internal custom pointer to address 0x07
 Read the value of byte 0x07 with control byte 0x51 Read from internal custom address
User setup of the measuring time interval is possible if bit 4 of the read byte is ‘1’.
1.2 Check current measuring time interval




Set the read pointer with control byte 0x50 to address 0xC6
Read the Global Measurement Interval low-byte with control byte 0x51
Read the Global Measurement Interval high-byte with control byte 0x51 (pointer increments
automatically after reading a byte)
Calculate the measuring time interval using the following formula
∗ 256
10
Example:
150
0 ∗ 256
10
150
10
15
1.3 Set the measuring time interval
For setting the measuring time interval one must always write both the low and the high byte of the
Global Measuring Interval (Bytes 0xC6 and 0xC7 in Custom Area). Values smaller than 150 (=15s)
and higher than 36000 (=3600s) will be ignored by the firmware.
Calculation of byte values:
∗ 10 256
∗ 10 /256
MOD…modulo operation
For division only integer values are relevant.
Example:
60 ∗ 10 256
∗
600
256
88
2
Writing values to custom area:
 Write the low byte with control byte 0x10 Direct write to custom area and address 0xC6.
 Write the high byte with control byte 0x10 Direct write to custom area and address 0xC7.
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2 Selecting the Output Value
The E2 Interface provides up to 4 output values (measurement value 1 to measurement value 4).
Measurement value 4 (Value Index: 30 acc. E2 bus uniform measurement value table)is the standard
CO2 output value and it is appropriate for most applications. This CO2 output value is optimized for
low noise output signal and it is calculated out of the last 11 measurements
Measurement value 3 (Value Index: 31 acc. E2 bus uniform measurement value table) delivers fast
response CO2 value and is meaningful only for applications with fast changing CO2 concentration.
Measurement value 3 is the last measured CO2 value without any averaging. It implies a noise of
typ. ±30ppm which should be taken into account for the overall accuracy.
NOTE: When using measurement value 3 any disturbance affects immediately to the output
Example: The CO2 level increase caused by the exhaled air of an individual passing by the sensor
will be visible in the measurement value 3.
The choice of using one, the other or both measurement values depends on the application. Please
contact E+E Elektronik or a local distributor for any support you might need.
3 Response time
The response time for measurement value 3 and 15s measurement interval is τ90 ≈ 60s.
For measurement value 4 with 15s measurement interval the response time is τ90 ≈ 105s. Longer
response time is of advantage in climate control applications because it filters out short time peaks
and consequently avoids changes in ventilation based on short time events.
Longer measuring interval increases the response time of measurement value 4.
4 Triggering a measurement
For measuring interval > 15s, the master device can trigger a measurement by reading the status byte
(control byte = 0x71) if the last measurement value is older than 10s. The new measurement value
will be available typically 5s to 10s after the trigger.
After each triggered measurement the interval time counter is set to ‘0’.
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Contact information
E+E Elektronik Ges.m.b.H.
Langwiesen 7
A-4209 Engerwitzdorf
Austria
Tel.:
+43 7235 605 0
Fax.:
+43 7235 605 8
E-Mail:
[email protected]
Homepage: www.epluse.com
For your local contact please visit the homepage.
The information in this document is believed to be accurate in all respects at the time of publication but is
subject to change without notice. E+E Elektronik assumes no responsibility for errors and omissions, and shall
not accept responsibility for any consequences resulting from the use of information included herein.
Additionally, E+E Elektronik assumes no responsibility for the functioning of features or parameters not
described. E+E Elektronik reserves the right to make changes without further notice. E+E Elektronik makes no
warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor
does E+E Elektronik assume any liability arising out of the application or use of any product or circuit, and
specifically disclaims any and all liability, including consequential or incidental damages without limitation. E+E
Elektronik products are not designed, intended, or authorised for use in applications intended to support or
sustain life, or for any other application in which the failure of the E+E Elektronik product could create a
situation where personal injury or death may occur. Should the buyer purchase or use E+E Elektronik products
for any such unintended or unauthorised application, the buyer shall indemnify and hold E+E Elektronik
harmless against all claims and damages.
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