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# Data recording methods
Three different methods of data recording are generally possible. All the methods have advantages and disadvantages, and are detailed in the following.
## Cyclic recording
In this method the process values recorded by the provider are recorded at fixed intervals. The recording interval can be selected freely between one second and one hour. If you select a recording interval of 60 seconds, for example, a value is recorded in the ACRON database every minute. The value is recorded in this interval even if it does not change.
#### Advantages
This is a safe and simple method.
#### Disadvantages
To record data changes as accurately as possible, a short recording interval must be selected. This creates large data volumes on the hard disk, resulting in degraded performance.
## Delta Event recording
In contrast to cyclic recording, values are only recorded when the absolute value change is greater than or equal to the defined value.
Up to three recording ranges can be specified: Delta Event 1, Delta Event 2 and Delta Event 3.
Note
Delta Event 2 and Delta Event 3 are only available in the 'Object properties' of the configuration.
When using multiple recordings, the value range of a higher-level Delta Event recording must not contain the range of a subordinate recording, as the subordinate recording would then not be applied. In such cases ACRON delivers a relevant error message.
In the following example the value "1.0" was configured as the value change amount (dead band) and the following values were registered by the Provider:
| Pos. | Time | Value | Recording | Reason |
| ---- | -------- | ----- | --------- | ------------------------------------------------------------------------------ |
| 1 | 12:15:05 | 1.7 | X | Start value |
| 2 | 12:15:06 | 3 | X | Change by 1.3 relative to pos 1 |
| 3 | 12:15:07 | 3.3 | | Change by 0.3 relative to Pos 2 results in no recording, because less than 1.0 |
| 4 | 12:15:11 | 3.6 | | Change by 0.6 relative to Pos 2 results in no recording, because less than 1.0 |
| 5 | 12:15:35 | 4.1 | X | Change by 1.1 relative to pos 2 |
| 6 | 12:15:59 | 3.5 | | Change by 0.6 relative to pos 5 results in no recording, because less than 1.0 |
| 7 | 12:16:30 | 2.7 | X | Change by 1.4 relative to pos 5 |
The smaller the selected dead band, the more accurately will value changes be recorded, and the greater will be the hard disk space requirement.
**Advantages**
With the ACRON Delta Event recording method you record the values which are outside the dead band and which have a certain rise, without reducing the recording interval correspondingly.
This method enables you to avoid large data volumes and the resulting increased memory requirement.
**Disadvantages**
If the value does not change for a long time, it can no longer be retrospectively ascertained whether data recording failed generally.
## Advanced Delta Event recording based on cyclic and Delta Event recording
This process is a combination of cyclic and Delta Event recording. In this way, ACRON provides a method optimally combining the benefits of the two methods.
First, as in Delta Event recording, values are only recorded if they change within a configured band. A control value is additionally recorded cyclically. The interval for cyclic control value recording can be set quite long (e.g. 1 hour).
#### Advantages
Based on the additional cyclic recording, the relevant values from the Delta Event recording are supplemented by a control value providing information on the recording process without unnecessarily increasing the data volume.
In many chemical processes the temperature characteristic is key. Some temperature ranges are of no importance, while changes in other temperature ranges are key.
Within a value curve determined in the course of each recording interval the rise changes suddenly by several increments for a specific period of time, whether in the positive or the negative range. It is precisely those value changes which you want to record.
By means of the following configuration you can set up Delta Event recording so that in the unimportant temperature ranges values are only recorded in the event of major changes (here at least 1 degree) and in the key temperature ranges recording takes place in response to small changes in value (here at least 0.1 degrees):
The configuration shown in the example registers the following values:
Cyclic recording records a value every 300 seconds.
An additional recording is made in the event of a value change >= 1 in the ranges from 10 to 35 degrees and from 38 – 60 degrees, as well as in the event of value changes >= 0.1 in the range 35 – 38 degrees.
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