iButton data logger calibration and NIST certification FAQ
This application note answers frequently asked questions (FAQ) about iButton data loggers in terms of calibration, software correction, certification, NIST, and traceability. These questions and answers apply to the DS1922 series and DS1923 iButton data logger.
A data logger is a device that can record data according to pre-configured rules. The data can come from a data source or a sensor that can send data. The DS1922 contains a temperature sensor, and the DS1923 Hygrochron â„¢ contains a temperature sensor and humidity sensor.
Calibration, also known as calibration, is the process in which the data logger is in a reference temperature environment and measures the temperature according to instructions. Calibration is performed at the factory and involves a high temperature reference temperature Tr1 and a low temperature reference temperature Tr2. The third reference temperature Tr3 is used for subsequent processing of temperature readings (see below). Table 1 lists the reference temperature involved in each recorder. Application note 2810: "DS2422 Trim Procedure and Software CorrecTIon" describes the calculation of a single correction parameter and the process of writing the parameter to the internal register of the recorder.
Table 1. Recorder specific reference temperature (nominal value)
| Logger | Tr1 | Tr2 | Tr3 |
| DS1922L DS1923 | + 60 ° C | -10 ° C | + 25 ° C |
| DS1922T | + 90 ° C | + 25 ° C | + 60 ° C |
| DS1922E | + 140 ° C | + 110 ° C | — |
The humidity sensor used in the DS1923 is calibrated by the manufacturer. Sensor accuracy can be improved by subsequent processing (see below).
What is a software fix?
Accuracy can usually be improved by software correction, this process is also called subsequent processing. A detailed description of the temperature correction algorithm is provided in the DS1922L / T and DS1923 data sheets. In addition to the reference temperature Tr1 (nominal value), the algorithm also uses two other pairs of data: the actual reference temperature values ​​of Tr2 and Tr3 (measured under laboratory standard conditions, measured using a calibrated platinum resistance thermometer PRT) and after Calibrated recorder measurements (ie conversion results) (Table 1). After calibration, the four data are determined separately and written to the calibration memory of each recorder in 16-bit format (pages 18 and 19). Note: The software correction process is not suitable for DS1922E, because its calibration points are very close to each other, and the measurement accuracy cannot be effectively improved by software correction.
The DS1923 data sheet introduces a correction algorithm for humidity measurement. This algorithm uses 3 pairs of data: the actual humidity reference value with reference humidity of 20%, 60%, 90% RH (RH = relative humidity) and the measured value of the recorder (ie conversion result). The six data are obtained by the recorder in a + 25 ° C environment and tested in a NIST traceable humidity chamber, and are written into the calibration memory after the temperature data corrected by the software in a 16-bit format. Then, put the recorder in 30%, 50%, 70% and 80% RH environment. Using the data stored in the calibration memory, the humidity conversion results are corrected by the software to verify compliance with the tolerance index listed in the data.
The issued certification document ("certificate") ensures that the production data logger device complies with the NIST traceable serial number requirements (see below). These certificates also confirm that the data loggers listed in the document have been tested in accordance with Maxim ’s quality assurance process and meet the accuracy specifications provided by the data.
NIST is the abbreviation of National Institute of Standards and Technology of the US Department of Commerce. NIST research solves the current scientific and technological problems, so that the US industry and science can produce better products and provide better services and technologies in the future.
Traceability refers to the establishment of an unbroken chain of relationships relative to a specified reference point. The Chemical Science and Technology Laboratory (CSTL), a branch of NIST, calibrates instruments that measure temperature, flow, pressure, vacuum, humidity, volume, and liquid density. NIST traceability is established by NIST calibration instruments.
The certificate lists the serial numbers of all reference devices, the serial number of the data logger (also called ROM ID or registration code), the date when the data logger was finally tested at the factory, and provides a download of the sample certificate (PDF format).
Reference instruments for temperature and humidity recorders include: temperature sensors (RTD) and humidity chambers, which comply with NIST traceability standards. The instrument is used to verify the accuracy of temperature calibration and provide reference data for software correction.
RTD (Thermistor Temperature Detector) is also called PRT (Platinum Resistance Thermometer). RTD has high accuracy and long-term stability. Maxim uses a MINCO thermometer, model # S9689PA5X12.
Humidity tank is a container whose internal temperature and humidity can be accurately controlled. The NIST traceable humidity chamber (Thunder ScienTIfic, model 2500) used by Maxim tests the DS1923 data logger and provides software calibration reference data for the DS1923 logger.
Users can apply for DS1922L, DS1922T, DS1922E, DS1923 certification. These certificates only apply to temperature measurement functions. Humidity certification is currently planned, but has not yet been completed.
Open the web page: Thermochron NIST Traceable CerTIficates. The user can input one or more ROM IDs of the data loggers of the applied certificate (see Figure 1). You can find the 16-character ID on the mark on the surface of the device and enter it manually; you can also read in the device ID number. Then copy and paste into the input area, but for this operation the user needs a 1-Wire® adapter and installs the OneWireViewer software.
Figure 1. ROM ID input area
After entering one or more ROM IDs (you can enter one per line when entering multiple IDs, or you can separate them with commas or spaces), click the "Request CerTIficate" button. After a few seconds, the certification file in PDF format is available, and the user can save the file and rename it.
The DS1922 and DS1923 data loggers have been in production for many years, and we only started providing certification services in 2009. The database used to provide the certificate only contains data logger products manufactured in 2009 and after 2009, and also contains some devices manufactured in 2008.
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