JOFRA The world leader and innovator in temperature calibration manufactures and supplies high accuracy temperature sensors and meters as part of their calibration range. Below is a description of the process undertaken in the manufacture of high accuracy temperature sensor.
It is not easy to make a good quality reference sensor. The main requirement of a reference probe is stability. This means minimal drift as a function of operating time at the actual temperature. The less the probe drifts, the lower the measurement uncertainty.
Small diameter - fast response
The STS-050 A/B and STS-100 A/B series has a relatively small diameter. This leaves optimum space for sensors-under-test in the dry-block and ensures a fast response time. A fast reacting sensor will optimize the measurement information. In addition to straight probes in 4 mm and 1/4 in, AMETEK also offers a 90° angled version specifically developed for use with dry-block temperature calibrators. This probe allows the user to have both the sensor-under-test and the reference probe in the thermowell at the same time: even if the sensors have a connection or a transmitter head.
Reduced hysteresis and drift
The main reason for drift within a sensor assembly is im-purities within the element, especially at temperatures above 350°C. All internal parts must be cleaned thoroughly. The assembly of the components is performed in a clean room. These precautions ensure minimum contamination of the element during use and provide the user with the best possible performance.
Once the probes are assembled, they are subjected to a long approval process. This includes mechanical stress reduction of the entire assembly as well as ageing the sensor element itself. The purpose of ageing the sensor is to remove the initial drift. The procedure involves heating the sensor up to maximum and holding it for 1 hour before cooling down. This process is repeated over a period of several days. The resistance is then measured at 0°C (32°F) and recorded. The sensor is again heated up to maximum, and this time the temperature is held constant for 100 hours. Finally, the output from the sensor is again measured at 0°C and recorded. The difference between the first and the second measurement is recorded. The difference between these two measurements is our verification of the stability qualities of the sensor. To be accepted for final calibration and certification, the probe must meet our minimum tolerance.
Electrical isolation resistance (parasite-resistance-error), when measured at the highest operating temperature, should be as high as possible. A low isolation resistance would cause the output signal to be incorrect in relation to the temperature. JOFRA STS-050 A/B and STS-100 A/B series probes meet the IEC-751 requirements of isolation resistance by several hundred percent.
The final quality-certificate-check
Upon completion of every certificate, after final calibration of the probe, examination and approval cycles are performed according to our established procedures. The critical verification is to ensure that the difference between the initial and the final 0°C measurement on the certificate meets our minimum tolerance. These requirements are based on a vast amount of data, which has been evaluated statistically. This value indicates if the probe has a sufficient long-term stability. AMETEK also checks that the linearization coefficients have values that correlate to an acceptable curve sequence in accordance with our requirements.
The final documentation on the probe is the calibration certificate. The JOFRA STS probes can be delivered with either an accredited certificate or a traceable certificate. In some cases, the customer may prefer to calibrate the probe. It is then possible to purchase the probe without any certification.