Possible operational defects for nearly all electrical thermometers
Defects | Possible reason | Remedy |
| Faulty measuring signals (measuring signal is floating, drifting, too high or too low) | electric/ magnetic interference connections | check cable installation min. 0.5 m distance between measuring and supply cables electrostatic screening; grounding at one point use cables with twisted cores cross disturbing supply cables right-angled |
ground circuits by multiple contact by insulation defects in grounded measuring circuits | preferably floating measuring circuit only one grounding point in measuring circuit | |
| insulation fault by humidity | dry gauge slide apply new sealing | |
| thermal stress | choose appropriate sensor | |
| Wrong temperature indications in contrast to comparable measuring points | protecting tube in „flow shadow” | choose installation place with undisturbed flow. The medium must be able to transfer the temperature undisturbed to the thermometer |
| immersion depth too small | choose longer protecting tube or more favourable installation place | |
| influence of an additional heat source | choose other installation place | |
| Extremely inert time behaviour | sediments on protecting tube | clean protecting tube during inspection |
| protecting tube „too thick” | choose smallest possible protecting tube with respect to process choose different protecting tube construction, for example tapered versions | |
| too small immersion depth | change immersion depth a) for measurements in liquids EL + 5 x protecting tube diameter b) for measurements in gasesEL + 10 x protecting tube diameter | |
| gauge slide without sufficient contact to protecting tube | gauge slide must be in contact with the protecting tube ground and if possible shall touch the protecting tube wall | |
| too high heat dissipation | use contact medium: liquids, heat conducting paste, metal foils, - sleeves | |
| Interruption of measuring circuit | vibrations caused by machine parts or flowing | possibly choose different mounting place dampen equipment use more rigid protecting tube use gauge slides with strengthened springs special constructions of gauge slide and protecting tube |
| Protecting tube strongly corroded or damaged by abrasion | medium does not correspond to original specification | check operational conditions check control medium |
wrong material chosen composition of medium | choose more appropriate protecting tube material choose appropriate construction provide appropriate surface protection | |
| Protecting tube broken | too high flow velocity solid particles in the medium swirls of the medium | reduce immersion length choose different protecting tube construction choose different installation place |
Specific defects of thermocouples
Defects | Possible reason | Remedy |
| Temperature indication too low in case of thinner thermo wires or sectional reduction due to consumption | resistance of the measuring circuit too high incoming or inner resistance of instrument too low | choose instrument with high incoming resistance (≥ 1MΩ) |
| Faulty reading gets higher with rising temperature (reading too low) | decreasing insulation resistance at with rising temperature (has the effect of a shunt and reduces the electromagnetic force) | dry gauge slides and seal tightly against humidity |
| impurities or corrosion and humidity at cable connections and clamps | clean contact points and protect against humidity | |
| Temperature reading deviates strongly from table values | parasitic tensions wrong material combination bad electrical contacts | check thermoelectric voltage examine compensating or extension cable and exchange if necessary |
right compensating cable connected with wrong polarity wrong compensating cable connected too high ambient temperatures | control type and polarity of compensating cable, if necessary exchange cable relay connection points | |
| Temperature reading deviates in time | thermal ageing causes structural transformation | oxygen, sulphur, silicium, hydrogen, etc.changes the chemical and metallurgic structure of the thermoelectric material. Best known „green decay” (selective oxidation of |
| impact of harmful substances | „rinse” the protecting tube by air or operate under overpressure in order to avoid the diffusion of harmful substances | |
| defect in short range order | use preliminary annealed thermo wires use thermocouple type N | |
| Unsteady temperature reading within a correct measuring circuit | unsteady temperature at cold junction | temperature at cold junction must be kept steady |
Specific defects of resistance thermometers
Defects | Possible reason | Remedy |
| Process temperature is too low with correct indication of the electrical thermometer | high conductor resistance impact of circuit, especially in case of 2-wire circuit | choose cable with bigger section shorten feeding line choose 3-or 4-wire circuit, possibly only from connection socket of gauge slide |
| self-heating by a too high measuring current | control measuring current and reduce if necessary | |
| Variable temperature reading | feeding in the field of unsteady temperatures with 2-wire circuit | choose 3-wire circuit in order to avoid environmental temperature impacts |
| unsteady current supply (full influence on measurement) | use appropriate supply unit | |
| Measuring fault gets bigger with rising temperature (reading too low) | decreasing insulation resistance has the effect of a shunt to the measuring resistance (doubles the tolerance at temperatures of more than 600 °C) | control gauge slides/sensor units, respectively dry and apply new sealing against humidity |
| Questionable measuring values | impurities or corrosion and humidity at cable connections and clamps | clean contact points and protect against penetrating humidity eliminate causes of impurities |
| thermoelectric voltages at the connection points by temperature gradients | take care of a steady temperature | |
| The indicated temperature decreases in time (process temperature rises) | ageing of the sensor by thermal or chemical influences | ensure a reliable measurement at important measuring points by short maintenance intervals or by exchange of the gauge slides ensure that the thermometer operates within the allowed temperature range |