Temperature Transmitter Introduction

About Precision
At present, the accuracy of smart temperature transmitters is generally mentioned in the market as 0.1, 0.2, 0.5 or 1%, multiplied by the corresponding range is the actual digital accuracy. In fact, this is not standard and will cause great confusion to users. Most users are also in the "muddleheaded" use. Only smart transmitters (with processors) are discussed here. Analog transmitters are not included in the scope of discussion due to limited indicators.
The accuracy of a smart transmitter actually depends on two components: analog-to-digital conversion (ADC) and digital-to-analog conversion (DAC). The transmitter first converts the analog quantity of the sensor into an operable digital quantity for processing through ADC, and then outputs an analog quantity of 4 ~ 20mA or 0~10V through DAC.
These two parts have their own error and temperature drift index.
For example, when PT100 is input, the ADC accuracy is 0.1 ℃, the DAC output is 4 ~ 20mA, and the DAC accuracy is 0.05. So,
When the range is set to 200 ℃, the total accuracy is 0.1/200*100%+0.05%= 0.1%
When the range is set to 100 ℃, the total accuracy is 0.1/100*100%+0.05%= 0.15%
It can be seen that the total percentage accuracy of a transmitter changes with the change of range, and the smaller the range, the accuracy will decrease accordingly, which is why some manufacturers will give the minimum range allowed by the transmitter, the purpose is to ensure compliance with the given accuracy index.
At present, most users still do not have a thorough understanding of accuracy indicators, so in view of this situation, this article is hereby added to describe "about accuracy". I hope this simple explanation can help the majority of users and enable the majority of friends to obtain accurate data indicators in actual demand and use.
The following is the calculation method of the accuracy of the intelligent temperature transmitter:
Analog output type, such as 4 ~ 20mA or 0~10V:
Total percent accuracy = analog-to-digital conversion accuracy/range * 100% + digital-to-analog conversion accuracy
Total Digital Accuracy = Total Percent Accuracy * Span = Analog-to-Digital Conversion Accuracy Digital-to-Analog Conversion Accuracy * Span
Bus output type, such as RS485, RS232, HART, FF, etc:
Total percent accuracy = analog-to-digital conversion accuracy/range * 100%
Total digital accuracy = analog-to-digital conversion accuracy
It can be seen that the analog-to-digital conversion accuracy can be directly obtained through the bus output type, without superimposing the digital-to-analog conversion accuracy, so if high-precision measurement is to be achieved, it is best to use the bus type transmitter.
Accuracy example
1. Input PT100, range 0~200 ℃, transmitter analog-to-digital conversion accuracy 0.05 ℃, digital-to-analog conversion accuracy 0.02%. then,
Total Percent Accuracy = 0.05 / (200 - 0) * 100% +0.02=0.045%
Total digital accuracy = 0.045 * (200- 0) = 0.09 ℃ or total digital accuracy = 0.05+0.02% * 200=0.09 ℃
2. Input PT100, range 0~200 ℃, transmitter A/D conversion accuracy 0.05 ℃,RS485 output. then,
Total Percent Accuracy = 0.05 / (200 - 0) * 100% = 0.025%
Total Digital Accuracy = 0.05°C
Accuracy of integrated intelligent temperature transmitter
The above discussion is the accuracy of the transmitter itself, but in actual use, the transmitter should be connected to the sensor as a whole, and the sensor itself has manufacturing errors, so the total accuracy = the accuracy of the transmitter itself. For example, the error of Class A PT100 is 0.15 ℃ at 0 ℃ and 0.4 ℃ at 100 ℃. If the total digital accuracy of the transmitter itself is 0.05 ℃, the total digital accuracy of the integrated transmitter is between 0.2 and 0.45 ℃. This is for general smart transmitters. AndAdvanced Smart TransmitterYes canCorrect the error of the sensor, that is, the sensor matching function. The overall accuracy can be improved by more than 60% or even infinitely close to the digital accuracy of the transmitter itself. This makes the transmitter "not picky" sensor, that is, no matter how low the sensor accuracy level, can achieve high-precision measurement, can save a lot of cost. When using this function, the requirements for the operator and the operating equipment used are very high.

"Zero temperature drift" intelligent integrated temperature transmitter

"Zero temperature drift" high precision intelligent integrated temperature transmitter (invention patent number: CN 201310655974.9) implementation standard "JWTB01-2014" is higher than "JJG (oil) 31-94" and "JJF1183-2007" agreed all indicators.
The difference between the "zero temperature drift" intelligent integrated temperature transmitter and other manufacturers' products is that it can correct the temperature drift error, match with the sensor and give the maximum error of the full range and all elements. The total factor refers to the overall final maximum error including the sensor error of the thermocouple or platinum resistance, the temperature compensation element error, the temperature drift error of the transmitter electronic circuit and the error of the transmitter itself, that is, the maximum error finally obtained when the user uses it.
The "zero temperature drift" intelligent integrated temperature transmitter measures its own temperature in real time and corrects the temperature drift that occurs. The measured value can be compared with the internal extremely stable reference element to calibrate the measured value; It can be directly matched with the thermal resistance element to realize ultra-high precision measurement. The patented non-standard thermocouple calculation model is embedded to overcome the problem of inaccurate compensation of non-standard thermocouple and realize precise matching to achieve the purpose of high precision and wide range measurement.
Example 1: regular integrated temperature transmitter, 0~600 ℃ range, working environment temperature -30~80 ℃, Class I K thermocouple sensor. Maximum error of thermocouple: 0.4 × 600=2.4 ℃; Transmitter accuracy is 0.1, error is 0.1 × 600=0.6 ℃; The cold end compensation error is generally 1 ℃ given by the manufacturer. The temperature drift error is 0.0025 FS/℃ (produced by a better manufacturer). Compare the measurement data with an ambient temperature of 40 ℃ with that at 0 ℃, A temperature drift error of 0.0025 × 40 × 600=0.6 ℃ can be obtained. Of course, if the temperature range is widened or a transmitter with a larger temperature coefficient is used, the temperature drift error will be larger. So the conclusion is:
Total error = (2.4+0.6+1+0.6) = 4.6 ℃, converted to percentage 4.6 / 600 × 100% = 0.77
Example 2: regular integrated temperature transmitter, 0~200 ℃ range, working environment -30~80 ℃, Class A PT100 sensor. The maximum error of PT100 is 0.55 ℃; Transmitter accuracy is 0.2, error is 0.2 × 200=0.4 ℃; Temperature drift error is 0.0025 FS/℃ (produced by a better manufacturer). Comparing the measurement data with ambient temperature of 40 ℃ and 0 ℃, a temperature drift error of 0.0025 × 40 × 600=0.6 ℃ can be obtained, of course, widening the temperature range or using a transmitter with a larger temperature coefficient will result in a larger temperature drift error. So the conclusion is:
Total error = ±(0.55+0.4+0.2) = ± 1.15 ℃, converted into percentage of ± 1.15 / 200 × 100% = 0.575;
Through the above two examples, it can be seen that after the integrated temperature transmitter is connected to the sensor, the accuracy obtained by the actual end user is far from the accuracy of the original design, because the error of the sensor and the influence of temperature drift are not considered. To reduce this error, it is necessary to require the transmitter to be able to fully suppress the influence of temperature drift, and to match with the sensor, reduce or even eliminate the manufacturing error of the sensor.