Key features of TELNA-designed thermometers

The present page presents the main characteristics common to all the thermometers designed by TELNA Instruments.

q Stability

All the TELNA thermometers offer very high stability performances. Very low long-term drift makes it possible to perform measurements reliable and repeatable over a long period of time. This allows to reduce the need for re-calibration by a National Laboratory (thus non-negligible saving in cost) provided that the instrument temperature coefficient is very small and does not introduce errors in measurements.

q Temperature coefficient

The temperature coefficient of our thermometers are so small that the measurement are accurate over the full ambient temperature range (at 10°C, 25°C, 35°C, …). It is pointed out that high temperature coefficients can alter the measurements. NB: it is suspect and even worse when the temperature coefficient of a thermometer is not specified and warranted.

q Jitter

The least significant digit of our thermometers is always highly stable. Example: if the thermometer resolution is 0.001°C, the thousandth of a degree digit fluctuates less than 0.0002°C( which makes this last digit very stable). In order to verify it, the platinum probe just needs to be replaced by the very high stability simulation resistance (Vishay or equivalent) of 100W or 138.5W (or other). You will then notice that the display is rigorously STABLE even with the TN750S (0.0001°C resolution). This is due to the system noise that is often called "short-term stability" or "jitter" which represents only a small fraction of the resolution (whether this latter is 0.01°C, 0.001°C, or 0.0001°C).

q Connection between probe and thermometer

The connection between the platinum prove and the thermometer is a 4-wire shielded cable. The common-mode rejection ratio is very high. Thanks to these 2 characteristics, the connection cable can be 30cm or 1km long without any readable shift in the measured value.

q Sensor current

The Pt100 current input is nominally 1 mA. However it can optionally be divided by 4 and reduced to 0.25 mA which divides the self-heating by 16. Therefore, the measurements in air will be shifted 16 times less than with 1mA. This is of paramount importance for the actual accuracy of such measurements.

q Linearization

The TELNA thermometers implement a probe linearization function based on ITS-90 but not only. This function is SELF-ADAPTATIVE. The parameters are self-corrected (during calibration) based on the probe characteristics without requiring to introduce those parameters in the thermometer software. The first benefit is that it is not necessary to buy a calibrated probe (much more expensive…). During calibration by a National Laboratory, the probe specifications are automatically accounted for by the TELNA thermometer and the linearization function is slightly modified for this purpose.

q Adjustments

It is illusory to think that it is possible to get the required accuracy by assembling a calibrated probe (more expensive) and a thermometer without adjusting the whole system. This is a major source of error at such an accuracy level. In particular, leakages in the metallic body of the probe may seriously influence the measuring results. Only the adjustment of the whole system allows to take this into account. It is thus necessary to assemble the probe (not calibrated) and the thermometer and to take them to a Laboratory that will adjust the whole and deliver a Calibration Certificate. All our thermometers can be calibrated through front panel keys without special software.

q Probe

The probe must be "strain free" for taking benefit of the exceptional stability of the TELNA thermometers. Molded industrial probes drift would drift a lot especially if the measurements are taken at high temperatures (higher than 100°C). If a cheap probe is used it is useless to associate a TELNA thermometer to it: the drift of this probe is such that the stability of the TELNA thermometer becomes useless.

The probe needs also to be long enough so that it can be immersed to 20/25 cm within the measurement liquid. For instance, a 20cm long probe cannot allow to ensure an uncertainty of 0.01°C during adjustments (still because of leakages).

q Computer interface

The TELNA thermometers communicate with a PC without requiring any specific software. It is only needed in Windows to click on Start followed by Programs, and Accessories followed by HyperTerminal. The communication is in R-S232 serial mode (with ASCII characters). The command circuit is ISOLATED from the rest of the instrument by means of optocouplers and isolated supply. This allows to eliminate the parasitic ground currents and their influence.

However, if the RS-232 standard is not convenient to you, and if you wish a more elaborated and reliable communication mode, we can propose in option the JBUS communication mode (with a CRC checksum).

q Reliability

All the TELNA thermometers consume very small currents. The power consumed is of the order of 1 Watt. Therefore the thermometer does not heat at all and the voltage regulators are hardly tepid…Consequently the reliability of our instruments is very high and failures are extremely rare.

q Case

In order to comply with the European Standard on Electromagnetic Compatibility (EMC), the aluminium cases of our instruments act as a faraday cage: there is a full electrical continuity between all parts of the case. And a very efficient line filter interfaces with the power supply connector.

Victor JOURNO, the designer of all TELNA thermometers.