Technical Note Vol.4 January, 2012
Tips for Eﬀective Use of Metal Detectors
- Countermeasures Against False Detection Vol.2 -
Last time we introduced factors in external environment that aﬀect metal detector performance. This time we feature another factor, product effect.
 False detection caused by product eﬀect
Metal detectors detect contaminants by catching the changes of magnetic ﬁelds in detection heads. The magnetic ﬁelds change by the eﬀect of not only metals but also products themselves. This eﬀect of products on magnetic ﬁeld is called product eﬀect. Its strength varies depending on the product characteristics such as temperature and salinity, regardless of physical appearance.
When the characteristics of a product greatly varies, false detection can occur because a metal detector sets the limit value based on the product eﬀect of non-defective products. The product eﬀect can be visualized on the product eﬀect screen of the metal detectors (Fig. 1).
Let's take a look at an experiment with water in container to see how much product temperature and salinity aﬀect detection sensitivity.
Fig. 1: Product eﬀect screen
 Eﬀect of product temperature
First, look at the product eﬀect of temperature. Fig. 2.1. and 2.2 show that when water temperature rises 10 degrees C, product eﬀect becomes double and detection sensitivity becomes lower. For instance, when you leave products to be inspected out of refrigerator or freezer for long hours, the product temperature will have changed at the time of inspection.
If your facility's temperature is aﬀected largely by seasonal climate changes, you should be careful in handling your products.
Fig. 2.1: The relationship between water temperature and product eﬀect
Fig. 2.2: The relationship between water temperature and detection sensitivity
 Eﬀect of salt content
Next, we take a look at salinity eﬀect. A pinch of salt, generally used in cooking, is around 6 grams. If you put 6 grams of salt in 100 ml of water, the salinity level increases by 0.6%. You can see how this works with product eﬀect and detection sensitivity in Fig. 3.1 and 3.2. When salinity level becomes 0.6% higher, product eﬀect signiﬁcantly increases and detection sensitivity decreases, which can lead to false detection.
This experiment was conducted under the extreme conditions for explanation purposes; however, please be aware that changing or adjusting seasonings can signiﬁcantly aﬀect the detection performance of metal detectors.
Fig. 3.1: The relationship between salinity level and product eﬀect
Fig. 3.2: The relationship between salinity level and detection sensitivity
 Solutions for product condition change and variability
As mentioned above, temperature and salinity of products greatly contribute to detection sensitivity. In order to prevent false detection, you can take following measures.
(A) To perform auto-settings with the same product condition as when fed into inspection
(B) To perform auto-settings with a product having larger product eﬀect (e.g. deliberately raise product temperature or salinity level)
When you change the ingredients or amount of seasonings, perform (A) each time so that your metal detector remains stable. When temperature and salinity level can vary and diﬃcult to control, (B) is eﬀective. However, it's important to check detection sensitivity after changing the settings because setting product eﬀect larger than actual one can reduce detection sensitivity.
 duw-h series overcome product characteristic variation
Let us introduce Anritsu duw-h series metal detector featuring "auto-tracking function". The left graph in Fig .4 shows that the peak of the detection signal has shifted to the right as product temperature has risen. At the timing of the vertical dashed line, where the metal detector measures the eﬀect of a product that may include metal contaminants, the evaluation value falls below the limit value. This causes the overlooking of defective products.
The right graph in Fig. 4 shows the auto-tracking function senses the amount of shift and adjusts the timing to measure the product eﬀect. As a result, the evaluation value goes beyond the limit, which means the metal detector successfully detects contaminants.
Fig. 4: Auto-tracking function
With this automatic compensation capabilities, the duw-h series ensure stable detection performance at all times even in facilities where product temperature tend to change during production.
As we have learned in the last and this issues, metal detector is simple in mechanism but susceptible to external factors and product eﬀects. Why don't you take this opportunities to review your metal detector operation?
Vol.4 PDF >
Technical Note (For general use) >
Products & Solutions > Metal Detection >
Metal detector >
New Metal Detector "M5 series" >
Glossary: Metal Detectors >