Food spoilage measuring instrument circuit

The circuit design is shown in the resistance measurement circuit in Figure 5-41a . This is a high internal resistance transistor resistance meter, LB is a DC ammeter, and the transistor
plays the role of DC amplification. When the probe does not capture the food being measured, no current flows through the LB. When the probe is inserted into the food to be measured, the internal
resistance of the food is connected to the input end of the meter, causing the base current to increase. After amplification, the collector current also increases. A potential difference is generated between the two ends of LB, the meter needle swings, and
current flows. Since the resistance values ​​of RP1 and Ri remain unchanged when measuring Bin, the base current of the transistor is inversely proportional to the internal resistance of the food. As long
as you read the indication value of the ammeter, you can know the resistance of the food.
    Use the above method of measuring internal resistance of food to determine the degree of spoilage of food leopards. The internal resistance of a certain food can be continuously measured every few hours or a day under certain storage points and ambient temperature conditions,
and the internal resistance curve of the food changing with time can be drawn from freshness to complete spoilage
. Summarize the rules of food spoilage changes. Use and store food scientifically to reduce losses caused by spoilage. Figure 5-41b is
a curve showing the change of the internal resistance of leathery fish in the natural environment with storage time using a food resistance circuit.
． (1) Moisture content measurement circuit Figure s-41a is the moisture measurement circuit. The circuit is composed of NEs55 time base circuit. The oscillation frequency of the vibrator composed of R6, R, and G
is determined by =l/T=l. 43/(R6+2R,)C4. The moisture content is related to the contact resistance R, the resistance value. If the moisture
content is high, the R resistance will be small and the oscillation frequency will be high; on the contrary, the oscillation frequency will be low. When R = about 2Mn, after the power is turned on, the speaker will make
a sound of "rr吂|诂", and the indicator LED will flash red light. Use standard resistors, R02, R. The resistance values ​​participate in the oscillation respectively, and
the oscillation frequency of RI is repeatedly compared to determine the moisture content.
    (2) Determine the standard resistance. Use standard cooking oil, rice, and pork to replace Ror, maggot, and Rr13 in the circuit. Choose one such as Rcu
for measurement. Connect an oscilloscope to the output terminal ③ to measure the oscillation period. Use Formula
    , 1/T=1. 43/ (Ra+ 2R.)C4
calculates the value of Rx which is R at this time. - value. It can be measured multiple times to find the average value and finally determine Rm. Use the same method to calibrate R02 and Rtj3.
Connect them to the circuit. The fHl line of its internal resistance changing with time is shown in Figure 5-41b.
    (3) DC power supply and probe production The DC power supply is shown in Figure 5-41a. The power supply circuit is composed of voltage transformation, rectification, filtering, voltage regulation and other parts
. The output DC voltage can be continuously adjusted within the range of 0-30V, and can be connected to the resistance measurement circuit or the moisture content measurement circuit
via a transfer switch for power supply. The two probes C and D can be made of phosphor bronze wire or brass wire with a diameter of 1 to Zmm. The front end of the probe should be sharpened to facilitate the insertion of food. The insulating
bracket is made of materials such as organic glass or epoxy laminate. There are requirements for the size, as long as it is beautiful and easy to use.
    (4) Testing methods and test results Before using the tester, prepare a variety of common foods, fresh and spoiled. Use fresh food
data as a standard for comparison to determine the degree of food spoilage. During the test, the probe is generally inserted into the food about 10mm (
foods less than 10mm thick, such as vegetable leaves, can pass directly through). It is best to measure several points for the same food and then take the average. The width and length of the application needle can
be appropriately relaxed, the length is 100-200mm, and the spacing is between 100-150mmt. Then turn on the power supply respectively and conduct the above-mentioned resistance and water content
measurements.