|信息来源： 液位变送器 | 2020-06-11 点击量： 529|
After the marketization of the means of production, fierce competition has intensified, and it is difficult to distinguish between the true and the false. Because the transmitter is a marginal discipline, many engineering designers are unfamiliar with it. Some manufacturers confuse the industrial level and commercial level indicators of their products (the price of industrial level is 2-3 times that of commercial level)
Taking the current and voltage transmitter with 0.5 level accuracy as an example, the following methods are used to distinguish the true and the false.
(1) The benchmark should be stable. 4mA is the corresponding input zero benchmark. The benchmark is not stable. How about the accuracy linearity? The zero drift change of 4mA within 3 minutes after cold start is not more than 4.000ma 0.5%; (i.e. 3.98-4.02ma). The pressure drop on the load 250 Ω is 0.995-1.005v. Foreign IC chips mostly use expensive energy gap benchmark, and the temperature drift coefficient changes 10ppm per degree;
(2) The total current consumption of the internal circuit is less than 4mA, which is equal to 4.000ma after setting. Moreover, the current consumption of the active rectifier filter amplification constant current circuit does not change due to the change of the original side input. The foreign IC core adopts constant current power supply;
(3) When the working voltage is 24.000v and the full range is 20.000ma, the reading of the full range is 20.000ma, which will not change due to the change of load 0-700 Ω; the change is not more than 20.000ma 0.5%;
(4) When the full scale is 20.000ma and the load is 250 Ω, the reading of the full scale is 20.000ma, which will not change due to the change of the working voltage 15.000v-30.000v; the change is not more than 20.000ma 0.5%;
(5) When the primary side is overloaded, the output current shall not exceed 25.000ma + 10%. Otherwise, the 24V working power supply and a / D input clamping circuit for the transmitter in PLC / DCS will be damaged due to excessive power consumption. In addition, the output of the transmitter will also be damaged due to excessive power consumption, and there will be no further damage to the A / D input clamping circuit;
(6) When the working voltage is connected reversely at 24V, the transmitter shall not be damaged, and polarity protection must be provided;
(7) When the induced lightning and induced surge voltage between two lines exceed 24V, clamp them and do not damage the transmitter; generally, 1-2 TVs transient protection diodes 1.5ke in parallel between two lines can suppress the impact of positive and negative pulse with 20 millisecond pulse width every 20 seconds, and the transient withstand impact power is 1.5kw-3kw;
(8) Whether the linearity of 0.5% of the product mark is absolute error or relative error can be identified at a glance by the following methods: the true linearity of 0.5% conforms to the following indicators
When the primary input is zero, the output is 4mA plus or minus 0.5% (3.98-4.02ma), and the pressure drop on the load 250 Ω is 0.995-1.005v
Output 5.6ma plus or minus 0.5% (5.572-5.628ma) with 10% primary input, the pressure drop on the load 250 OM is 1.393-1.407v
When the primary input is 25%, the output 8Ma is plus or minus 0.5% (7.96-8.04ma). The pressure drop on the load 250 Ω is 1.990-2.010v
When the primary input is 50%, the output is 12mA plus or minus 0.5% (11.94-12.06ma). The pressure drop on the load 250 Ω is 2.985-3.015v
When the primary input is 75%, the output 16mA is plus or minus 0.5% (15.92-16.08ma). The pressure drop on the load 250 Ω is 3.980-4.020v
When the primary side input is 100%, the output 20mA is plus or minus 0.5% (19.90-20.10ma), and the pressure drop on the load 250 Ω is 4.975-5.025v
(9) The current must be limited when the primary input overload is greater than 125%. When the primary input overload is greater than 125%, the output overcurrent is limited to 25mA + 10% (25.00-27.50ma). The pressure drop on the load 250 Ω is 6.250-6.875v;
(10) Identify whether there is clamping when the induced surge voltage is more than 24V: connect two lines of AC 50V output port with an AC 50V pointer meter, and touch the two lines of output port with two lines of AC 50V in an instant to see if there is clamping, and the clamping voltage can be seen at a glance;
(11) Identification of whether there is polarity protection or not: measure the two-wire output port by multiplying 10K gear with the pointer multimeter Ω, and once the Ω resistance is infinite, there is polarity protection;
(12) Long time short circuit protection with or without pole output current: when the primary side input is 100% or the overload is greater than 125% - 200%, short circuit the load 250 Ω, and measure whether the short circuit protection limit is 25mA + 10%;
(13) Identification of industrial grade and commercial grade: the working temperature range of industrial grade is - 25 ° C to + 70 ° C, and the temperature drift coefficient is 100ppm per degree change, i.e. the temperature changes by 1 ° C, and the accuracy changes by 1 / 10000; the working temperature range of commercial grade is 0 ° C (or - 10 ° C) to + 70 ° C (or + 50 ° C), and the temperature drift coefficient is 250PPM per degree change, i.e. the temperature changes by 1 ° C, The accuracy changes to 2.5% of 10000; the temperature drift coefficient of current and voltage transmitter can be verified by constant temperature box or high and low temperature box test.
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