General description
Carbon Dioxide measurement
Oxygen measurement
Handling
Electrical connections
Gas sample
Sample pressure and flow
Calibration
Calibration gases
Entering and displaying set points
Operation of external selects


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1. General description

The 481 and 482 Analysers are designed to measure and display either Oxygen or Carbon Dioxide and are intended for mounting in a standard DIN cutout.
As well as providing an analogue signal output corresponding to the gas concentration the Analysers have the facility for operator entry of up to
six high and low set points to enable relay control under the direction of external select inputs.
Measured gas values and set point examination/change are indicated on the eight character backlit LCD.
Using appropriate calibration gas, zero and span may be adjusted by the front panel pots and in the case of the 482 analyser, course adjustment of the span may be made through the rear panel.
Entry to, and examination/change of set points is made using the 4 buttons below the display.

2. Carbon Dioxide measurement
.
The Carbon Dioxide is measured in the 481 for the range 0 to 10% using the principle of infra-red absorption. This physical method of measurement offers high selectivity, good accuracy and a fast response.

3. Oxygen measurement.

The Oxygen is measured in the 482 for the range 0 to 21 % using a galvanic cell with a typical life of 5 years. The operation of the galvanic cell is similar to a regular primary cell battery in that a voltage is
produced from an electrochemical reaction. In this case, however, the reaction is regulated by the presence of Oxygen.

4. Handling

The 481 and 482 analysers contain sensitive measuring equipment and must be handled with care.
The analysers are of robust construction but are not designed to be waterproof or shock resistant.

5. Electrical connections

The connections are all concentrated on the rear panel and are marked accordingly.
Mains supply (L, N, E) is 220/240 V AC (115 V AC available to order). Ext Sell to 6 and Ext Ov are the external +24 Volt set point selection inputs.
Analog Out is the signal out corresponding to gas concentration.
Relay 1 and Relay 2 are volt free contacts for control applications

6. Gas Sample

The sample to be measured must be reasonably clean and free from water and be available at a pressure of between 0.03 to 0.3 bar.
There is an internal filter to protect the analyser cells but if the sample is noticeably dusty or dirty an additional filter should be fitted before the analyser.
Free water in the sample can cause extensive damage to the ana1ysers and should be carefully avoided.
Water from condensation is most likely to be formed when the gas sample is taken from a high humidity environment that is at a higher than ambient temperature.
F or continuous sampling from a fruit storage environment an external filter and water catchpot should be fitted.

7. Sample Pressure and Flow

The analysers are supplied with a restrictor fitted to limit the flow.
The sample pressure can be between 0.03 and 0.3 bar and the flow will be in the approximate range of 0.1 to 1 litre/min.

All gas analysers require regular calibration since they are measuring the physical properties of gas and these vary with temperature, pressure and time.
The span or full scale of an analyser is the measurement of the partial pressure of the gas in question, and therefore, as the barometric pressure varies the value of the gas measurement in percentage terms will vary directly in proportion to this pressure.
The frequency of calibration depends upon the usage and accuracy required. The general recommendation is as follows:
Air calibration daily
Full calibration every 7 days or
full calibration if temperature changes by 10°C or more from the temperature at calibration.
Full calibration if there is doubt in readings, or, if maximum possible accuracy is required.

9. Calibration Gases

Two calibration gases are needed for each analyser range. The analysers need to be set to ZERO with none of the measured gas present and SP AN to set the scale of the analyser.
The ZERO gas for Carbon Dioxide is fresh air.
Fresh air has a very fixed and stable oxygen concentration of 20.95% and this is used to set the span of the 482 to 21 %.
A mixture of Nitrogen and Carbon Dioxide can be used as the second calibration gas. This will have a known quantity of Carbon Dioxide, usually 10%, and can be used for either analyser.

10. Entering and Displaying Set points

Set point entry is made by pushing and holding the forward arrow (FA)
button until the display indicates the set point number. Use the up arrow (VA) and the (DA) buttons to select the set point number.
When the set point number has been selected, press the FA button to display the high set point.
Use the UA and DA buttons to examine /change the high set point. When the high set point has been examined/changed, press FA button to
examine/display the low set point. Use the UA and DA buttons to change the low set point.
When the low set point has been examined/changed press FA button and Run Y/N) will be displayed. Press UA to return to run mode or press DA to continue with set point examination/change.
The back arrow (BA) may be pressed during examination/change to move to the previous entry operation.
All set points are retained in non-volatile memory.

11. Operation of external selects.

Six external select signals are available on the rear panel and are intended
for the application of +24 volts from the control system to select which set points will determine the operation of control relay 2.
The external selects numbered I to 6 are optically coupled to the 481/482 logic and the +24 volt select must be applied between one of the selects and the select 0 volts.
When the 481/482 is running normally the application of an external control will bl indicated on the left side of the display in the form #x where x is the select number.
Depending on the measured gas value and the high and low set points for the particular external select the operation would be as follows:
481 Carbon Dioxide Analyser.
If the measured gas value is equal or greater than the high set point, control relay 2 contacts will be closed.
If the measured gas value is equal to or less than the low set point, control relay 2 contacts will be open.
482 OxygenAnalyser
If the measured gas value is equal to or greater than the high set point, control relay 2 contacts will be open.
If the measured gas value is equal or less than the low set point, control relay 2 contacts will be closed.

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