Measuring Fish with Torrymeter

Principle Of Operation

The original research leading to the development of the Fish Freshness Meter was carried out at Torry Research Station, in Aberdeen, Scotland.

It was found that certain dielectric properties of the fish skin and muscle alter in a systematic way during spoilage, as tissue components degrade. These alterations, occurring at microscopic level, are strongly associated with the gross changes in appearance, odour, texture, and flavour which take place during spoilage and which are normally used to judge freshness. Hence, determination of the appropriate dielectric properties gives a measurement of the freshness of the fish.

The base of the instrument (sensing head) has two pairs of concentrically arranged electrodes. This sensing head is applied directly on to the skin of the fish. An alternating current is passed through the fish, between the outer pairs of electrodes and the resulting voltage sensed by the inner pair. The phase angle between the current and voltage is measured and converted electronically to allow digital display on a convenient scale in the range 0 – 18. The phase angle and hence the meter reading decreases with spoilage. The current passed through the fish is approximately 1 milliamp and so cannot harm the operator or affect the fish.

Between the measuring electrodes are two auxiliary electrodes. These electrodes, in conjunction with one of the carbon electrodes, sense whether there is proper contact with the fish. The fish sensing device prevents readings being taken under inappropriate conditions e.g. in air, or in ice. Since the phase angle depends on temperature as well as freshness, the reading displayed by the instrument must be corrected to the value it would show at a reference temperature of 0°C.

The meter is powered by rechargeable batteries within the instrument. They have sufficient capacity to allow a full working day’s operation.