Beckhoff EPP3504-0023 Short Manual page 56

Product overview
-6
Remarks: ppm ≙ 10
In general, you can calculate this as follows:
• if use at 23°C alone is to be considered:
Total measurement accuracy = basic accuracy & noise according to above formula
• If use at 23°C is to be considered with slow measurement (=averaging/filtering):
Total measurement accuracy = basic accuracy
• If general use within a known temperature range and incl. noise is to be considered:
Total measurement accuracy = basic accuracy & noise & temperature values according to above
formula
Beckhoff usually gives the specification data symmetrically in [±%], i.e. ±0.01% or ±100 ppm. Accordingly,
therefore, the unsigned total range would be double this given value. A peak-to-peak specification is a total
range specification; the symmetrical value is thus half of it. In the quadratic calculation below, the
symmetrical value for "one side" is to be inserted without a sign. Noise is usually specified in peak-to-peak
form, therefore the equation for the noise value already contains the division factor 2.
Example:
• symmetrical specification: ±0.01% (equivalent to ±100 ppm) e.g. for offset specification
• total range: 0.02% (200 ppm)
• to be used in the equation: 0.01% (100 ppm)
The total measurement accuracy calculated in this way is also to be considered as a symmetrical maximum
value and thus to be provided with ± and ≤ for further use.
Example:
• E = 100 ppm
Total
• For further use: "≤ ± 100 ppm"
Expressed in words: "The offset of the individual accuracy specifications under the given conditions
produced a range of 200 ppm that lies symmetrically around the individual measured value. The measured
value specification x thus has an uncertainty of x ±100 ppm; it is thus 95% certain that the true value thus lies
in this range".
The noise component can be omitted
The noise component F
for a set of samples is used instead of a single sample. The averaging can take place in the PLC, or
it can be done by a filter in the analog channel. The output value of a moving average of many
samples has a noise component that is almost entirely eliminated. The achievable accuracy thus
increases when the noise component is decreased.
Error coefficient of ageing
If the specification value for aging from Beckhoff has not (yet) been specified, it must be assumed to be 0
ppm when considering measurement uncertainty, as in the above example, even if in reality it can be
assumed that the measurement uncertainty of the device under consideration changes over the operating
time, or colloquially stated, the measured value "drifts".
Experience has shown that the basic accuracy of the instrument under consideration, provided it is
operated according to specifications, can be taken as the order of magnitude for an annual change (10,000
h). This is an informative statement, does not constitute a specification, and exceptions may occur. In
general, the change in ageing will be very application-specific. A general ageing specification from Beckhoff
is therefore to be regarded as a guideline rather than a guaranteed upper limit, when published.
If the measurement uncertainty consideration in the application shows that aging over the desired operating
time can put the measurement accuracy at risk, Beckhoff recommends a cyclical check (recalibration) of
the measurement channel, with regard to sensors, cabling and the Beckhoff measurement modules. In this
way, potential long-term changes in the measurement chain can be detected early and, in some cases, the
trigger (e.g. overtemperature) can even be eliminated. See also Continuative documentation for I/O
components with analog in and outputs [} 245].
56
-2
% ≙ 10
can be omitted from the above equation (= 0 ppm) if the average value
Noise
Version: 1.2
NOTE
EPP3504-0023