In case of so-called ZP method (measurements of muscle and fat depth on the carcass split-line) the simple rulers or hand-operated electronic callipers can be used.
The measurements are usually taken on the left half of the carcass. Exceptionally, i.e. in case that the left half of the carcass is damaged, the operator can take the measurements on the right half of the carcass.
The method is used for carcass classification of warm or cold carcasses.
When operators are checked for correctness of measurements (i.e. monitoring or surveillance), the measurements are considered as correct if they are within the tolerance limits (which is Ī3 mm for muscle depth and Ī2 mm for fat thickness) compared to measurements at monitoring.
The problems in monitoring of operators in so called ZP methods are (1) as the method is not invasive, the correctness of the measurement site is not possible (2) when classification on warm carcasses is performed, the monitoring a day after slaughter is not possible
If supplier doesnít agree with a classification he should have a possibility to a complaint, either to the classification body or to the national authorities.
The classification body should ensure that:
- the facilities and equipment are properly identified,
- the access to and the use of the equipment are such that the continued suitability and intended use are assured,
- procedures and/or instructions exist for
(b) maintaining of the equipment
(c) calibration of equipment (first and regular)
(d) traceability to national and international standards
(e) testing of the computer software if in use
(f) protecting data
- measurements taken at the wrong position
By definition of muscle depth - M (distance between the cranial edge of m. gluteus medius and dorsal edge of canalis vertebralis) and fat thickness F (thinnest part over m. gluteus medius ), the operator should search for the shortest distance when taking measurements. In practice, the operator has limited time to take the measurement and therefore he takes the measurements at visually estimated shortest distance, which is not always the actual shortest distance.
Some cases of problematic decisions in taking measurement are presented on the photos:
1) The shortest distance is situated in the area of os sacrum and not in the region of lumbar vertebrae
2) The embossment on the dorsal edge of the canalis vertebralis
3) Fascia glutea veils over the lower edge of the muscle or
4) Fascia glutea interrupts the muscle
5) The muscle has a very thin, hardly visible elongation on the cranial edge
6) The muscle has two shanks
7a) The covering fat is very thin and wavy; thus making difficult to estimate the thinnest part
7b) The covering fat is very thin and wavy; thus making difficult to estimate the thinnest part
8) Compressing fat too much or not enough
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A) Electronic callipers
Usually, the measurements for classification of carcasses are taken shortly after the slaughter when the fat tissue is still warm and soft. When using electronic callipers, compressing fat too much or not enough is among the most important reasons for differences between operators, who have different way of handling (manipulating) the calliper.
The difficulties in measurements of fat are more important in very lean carcasses especially when simple rulers are used. The danger of pressing a ruler too much against the tissue can result in an overestimation of fat thickness.
- bad splitting of the carcass can result in false measurements or no measurements (impossible to measure at the defined positions); the frequency of bad splitting depends on the equipment (saw) in use and/or an operator.
Bad splitting can be such, that the measurement of muscle depth is impossible or due to the cutting angle fat thickness can be over or underestimated
In cases where the measurements canít be taken, carcasses should remain unclassified. Such cases are usually rare (<1%). The record i.e. statistical control of the frequency of bad splitting is recommended.
In Slovenia, the unclassified carcasses (due to bad splitting or incorrect dressing or unusual combination of muscle and fat thickness) are registered; their frequency varies according to the abattoir (in 2002 from 0.10% to 3.0% with average of 1,4%).
- working conditions are essential for good operatorís performance at work.
The abattoirs should be obliged to provide good working conditions for classification. The operator should have enough time and space to perform his tasks. While measuring, the carcass should not be moving or bumping against or by the neighbouring carcasses.
It is advisable, that from time to time the calliper is checked (a quick test) also during the day (during the slaughter).
Statistical monitoring can help to identify problems like
- operatorís effect
- correctness of classification in case of non-automated taking of measurements
In case of callipers without automated recording , i.e. rulers, the attention should be paid to the distribution of data. When using a simple ruler, the operators are inclined to round values (ex. to the nearest 5 mm).
It is difficult to evaluate the operatorís effect when the measurements are not taken on the same items. The collecting of data on long term basis and statistical analysis can provide a tool to alert of the operatorís effect. Example is given for one of Slovenian abattoirs where three operators were compared (>10.000 carcasses/operator).
Every operator should be checked from time to time in order to control over the operatorís effect.
One way to do it would be to perform testing between operators working in pairs.
Another way would be to test each operator in comparison with measurements of (two) reference operator(s).
Every operator should participate in such comparisons at least once every two years.