Short answer. On a 2-wire PT100, 1 Ω of loop resistance reads as 2.56 °C. On a PT1000 the same ohm is 0.26 °C, because the element is ten times more sensitive. Twenty-five metres of AWG 24 copper is about 4.2 Ω of loop, which is 10.8 °C on a PT100 and 1.1 °C on a PT1000. The conversion is one division. What is usually missing is not the ratio but the rest of the arithmetic: which gauge, over what length, and which part of the error a third wire or a commissioning trim actually removes. This page gives the number, the table, and the two cases where the usual rule of thumb is wrong. The one division you need A platinum RTD changes resistance at a rate set by IEC 60751. At 0 °C that rate is 0.3908 Ω per kelvin for a PT100 and 3.9083 Ω per kelvin for a PT1000. Divide the loop resistance by that slope and you have the error in degrees. Element dR/dT at 0 °C (Ω/K) at 25 °C at 100 °C 1 Ω of loop reads as PT100 0.3908 0.3879 0.3793 2.56 °C PT1000 3.9083 3.8794 3.7928 0.256 °C Slopes calculated from the Callendar-Van Dusen coefficients in IEC 60751. The slope falls slightly as the sensor heats, so the error a given cable causes grows a little with process temperature. Full values are in the PT100 and PT1000 resistance tables. The slope is why the factor of ten exists and why it never goes away. It is not a tolerance, a grade, or a manufacturing difference. A PT1000 simply moves ten times further for the same degree. What your cable actually adds On a 2-wire connection both conductors are in series with the element, so the loop is twice the one-way length. Copper resistance at 20 °C from the standard AWG table gives the numbers below. Every solid line is a PT100. The whole PT1000 range fits inside the shaded band at the bottom. Cable, one way Loop Ω, AWG 18 / 22 / 24 PT100 error °C PT1000 error °C 5 m 0.21 / 0.53 / 0.84 0.5 / 1.4 / 2.2 0.05 / 0.14 / 0.22 10 m 0.42 / 1.06 / 1.68 1.1 / 2.7 / 4.3 0.11 / 0.27 / 0.43 25 m 1.05 / 2.65 / 4.21 2.7 / 6.8 / 10.8 0.27 / 0.68 / 1.08 50 m 2.10 / 5.30 / 8.42 5.4 / 13.6 / 21.5 0.54 / 1.36 / 2.15 100 m 4.19 / 10.59 / 16.84 10.7 / 27.1 / 43.1 1.07 / 2.71 / 4.31 Two-wire connection, copper at 20 °C, both conductors counted. Read the three numbers in each cell as AWG 18, AWG 22 and AWG 24. In metric terms those are roughly 0.82, 0.33 and 0.20 mm². A 10 °C error on a Class A PT100 is roughly seventy times the element's own tolerance at 0 °C. At that point the sensor grade you paid for has stopped mattering. Why 3-wire cancels less than people assume A 3-wire connection does not remove lead resistance. It measures one conductor and subtracts it from the other two, which works only while all three conductors are identical. They usually are, to within a fraction of a percent, when the cable is a single homogeneous run. The mismatch shows up somewhere else: a crimp that is not quite the same as its neighbour, a junction box where one conductor was extended, a terminal that has oxidised on one leg only. What survives on a 3-wire r...

Short answer. A healthy KTY84/130 reads 577-629 Ω at 25 °C and 970-1030 Ω at 100 °C. Anything near 1000 Ω at room temperature is a different part, not a faulty one. The reading that actually separates a good sensor from a counterfeit is taken above 200 °C, where it must still be climbing toward 1641-1803 Ω. The most-read question about this sensor on NXP's own community is some version of "how do I tell whether it still works". The published answer points at Table 7 of the data sheet and warns not to exceed 10 mA. That is correct, and it is not enough. A table of nominal values does not tell you which readings mean a dead part, which mean the wrong part, and which mean the wiring. This page fills in that gap. What a healthy reading looks like NXP publishes an acceptance window, not just a nominal curve. Table 7 of the data sheet gives minimum, typical and maximum resistance at every step from -40 to 300 °C. It lives in a PDF, which is why most people quote the typical column and stop there. Here is the window itself, for the KTY84/130. Temperature (°C) Min (Ω) Typ (Ω) Max (Ω) TC (%/K) -40 340 359 379 0.84 0 474 498 522 0.79 25 577 603 629 0.74 100 970 1000 1030 0.61 150 1282 1334 1385 0.54 200 1641 1722 1803 0.48 250 2046 2166 2286 0.44 300 2456 2624 2791 0.29 KTY84/130 at a 2 mA sense current, from Table 7 of the NXP KTY84 series data sheet. Bold rows are the two measurements that decide most questions. The shaded band is what a good part is allowed to read. Note how much it widens above 150 °C. The last column is the one to notice. The temperature coefficient falls steadily with temperature, and then drops sharply between 250 and 300 °C, from 0.44 to 0.29 %/K. The curve flattens at the very top of the range. That has a practical consequence. Near 25 °C the part moves about 4.6 Ω per degree, so a 10 Ω reading error is roughly 2 °C. At 300 °C the acceptance window itself is 335 Ω wide, which is about 44 °C. A KTY84 used at its ceiling is not a precision sensor, whatever its grade. Why your meter will not read the data-sheet number exactly Every figure above is specified at a 2 mA sense current. A handheld meter on its ohms range does not supply 2 mA. It supplies whatever its own design chooses, typically well under 1 mA, and it does not tell you what that is. This is not small print for a KTY sensor. The data sheet carries a curve for it. Figure 4, "deviation of sensor resistance as a function of operating current in still liquid", has separate traces at 2 mA, 1 mA, 0.5 mA and minus 1 mA. The dependence is real and published. What is not published is a correction table. What this means on the bench. Treat a handheld reading as a comparison, not as a calibration. The useful test is a known-good part and the suspect part measured on the same meter, minutes apart, at the same temperature. A 3 % offset from the table is the meter. A 40 % offset is the part. Self-heating pushes the same way. At 2 mA through 603 Ω the sensor dissipates about 2.4 mW, an...

Short answer. A KTY84 cross-reference table tells you which part has a similar curve. It does not tell you what will break. Four things do: the KTY84 family is specified at R100 = 1000 Ω and not at 25 °C, its ceiling is 300 °C and no silicon successor reaches it, the KTY83 and KTY84 parts are polarised while KTY81 and KTY82 are not, and any change of base resistance means the controller has to be relinearised. The KTY silicon sensors are being withdrawn, and the designs that used them are now being re-sourced under a deadline. Siemens states in its own product note that the KTY sensor is discontinued and no longer available in the market, and TI publishes an end-of-life replacement note for the KTY81 and KTY82. Most of what is published about this is a table of "equivalent" part numbers. That table is useful, and we publish one ourselves in the KTY83-110 cross-reference guide. It is also the part of the job least likely to cause a field failure. The four problems below are the ones that come back after the new parts are already fitted. Break 1: KTY84 is specified at 100 °C, not at 25 °C Nearly every other thermistor on your bench is quoted at 25 °C. KTY81 and KTY83 follow that habit: R25 = 1000 Ω or 2000 Ω. KTY84 does not. Its 1000 Ω figure is the value at 100 °C. At 25 °C a KTY84/130 sits at about 603 Ω. If an engineer reads "1 kΩ" on a KTY84 line and pairs it with a 1 kΩ part specified at 25 °C, the two curves are roughly 40 % apart at room temperature. The bridge still reads something. It reads the wrong thing. These are the published values for a KTY84/130 at a 2 mA sense current, with the tolerance NXP states in kelvin at each point. Note how the tolerance widens at the top of the range. Temperature (°C) Min (Ω) Typ (Ω) Max (Ω) Tolerance (K) -40 340 359 379 ±6.48 0 474 498 522 ±6.07 25 577 603 629 ±5.84 50 694 722 750 ±5.59 100 970 1000 1030 ±4.90 150 1282 1334 1385 ±7.10 200 1641 1722 1803 ±9.71 250 2046 2166 2286 ±12.73 300 2456 2624 2791 ±22.12 KTY84/130 at a 2 mA sense current, from Table 7 of the NXP KTY84 series data sheet. The 1000 Ω nominal appears at 100 °C, not 25 °C. The practical check takes one minute: put the candidate part and the original on the same bench at room temperature and compare readings before anything is soldered. A 40 % gap is visible immediately. Worth reading the last column honestly. At 300 °C the tolerance is ±22 K and the acceptance window is 335 Ω wide. A KTY84 reaches that temperature, but it is not a precision sensor when it gets there, and a design that needs accuracy at the ceiling was never well served by this part. Break 2: the 300 °C ceiling that silicon does not follow KTY84 runs to 300 °C. That is why it ended up in motor windings, spindles and brake-adjacent sensing in the first place. It is also the specification that no drop-in successor matches. Four replacement routes and what each actually covers. The shaded band is the range a KTY84 design may be using and no silicon PTC can reach. TI names t...

A PTC thermistor is a resistor whose resistance rises as it gets hotter. PTC stands for positive temperature coefficient. That single definition covers two devices that behave nothing alike and are bought for opposite reasons. A silicon PTC changes by about 0.76% per kelvin across its whole range, smoothly, and you use it to read a temperature. A ceramic switching PTC sits under 100 Ω all the way up to its rated point, then crosses 1330 Ω within five degrees and 4 kΩ within fifteen, and you use it to trip something. Specifying one when you needed the other is the most common mistake in this category. Published 10 September 2026. Reviewed by the Focusens technical department. The two families side by side Both are sold as "PTC thermistors". These are real figures from two Focusens series, one from each family. Silicon PTC (FTY series) Ceramic switching PTC (MZ6 series) What it is for measuring temperature detecting that a limit was crossed Curve virtually linear flat, then a step at the rated point Coefficient 0.76 %/K at 25 °C not meaningful; specified as thresholds instead Range −40 to +150 °C rated points from 60 to 190 °C Resistance at 25 °C 1000 Ω nominal for the 1 kΩ types ≤100 Ω single, ≤300 Ω triple Resistance above the rated point rises smoothly, R100/R25 = 1.67 typical ≥1330 Ω at +5 K, ≥4 kΩ at +15 K Rated current at 25 °C 5 mA, 10 mA maximum continuous driven by the relay, ≤30 V DC working Response 60 s thermal time constant ≤5 s Output you get a resistance you convert to degrees a state, below or above the limit Specifications from the Focusens product catalogue. The 60 s figure for the silicon series is quoted against different media on the series page and the element table, so confirm the medium against a part code before designing a control loop around it. A response time without its medium is not a specification. Read the resistance rows together. A silicon PTC moves from 1000 Ω to about 1670 Ω across 75 degrees. A switching PTC moves from under 100 Ω to over 4000 Ω across fifteen. The first is a measurement. The second is a switch that happens to be made of ceramic. Built from the Focusens catalogue threshold figures. The switching curve carries no usable temperature information below its rated point. Which one you actually need Answer three questions in order. Do you need a number or a decision? If a controller has to display, log, or regulate against the temperature, you need a sensor, and that means a silicon PTC, an NTC or an RTD. If something has to happen when a limit is crossed and nothing has to happen before that, a switching PTC does it with no conversion and no calibration. Where does the device sit? A sensor sits on a signal input. A switching PTC for protection usually sits inside the thing being protected, embedded in a motor winding or clamped to a transformer core, wired to a relay that only reads two states. Does it have to reset itself? A switching PTC returns to its low resistance once it cools, which is why it ...

Three PTC thermistors go into a three-phase motor, one per phase, embedded in the winding heads and wired in series to a protection relay. Below the rated temperature the string sits at a few hundred ohms. When any one of the three passes its rated point, the string crosses about 4 kΩ and the relay drops the contactor. The numbers in that paragraph are the whole design. This page gives them, explains why a series string triples every threshold, and covers how to choose the rated temperature against the motor's thermal class. Published 10 September 2026. Reviewed by the Focusens technical department. Why a PTC and not a current-based relay A thermal overload relay infers winding temperature from line current. That inference holds while the motor is cooled normally and fails in the cases that actually burn windings: blocked ventilation, high ambient, a clogged filter, rapid start-stop duty, or a motor running at rated current in a hot room. Current looks normal. The winding does not. An embedded PTC measures the winding itself, so none of those cases hide from it. It does not replace a current-based device, because it cannot see a locked rotor fast enough and it does not protect against short circuits. Standard practice is to run both. A PT100 in the winding gives a continuous temperature reading instead of a threshold, which is what you want on a large machine with a monitoring system. The PTC is the cheaper last line: no scaling, no calibration, two states. What the relay actually reads A switching PTC is specified by resistance at fixed offsets from its rated temperature, written TK. The relay is built around those offsets rather than around a temperature. MZ6 series values for rated points between 90 and 160 °C, TK tolerance ±5 °C: Measured at Single element Three in series 25 °C ≤100 Ω ≤300 Ω TK − 5 K ≤550 Ω ≤1650 Ω TK + 5 K ≥1330 Ω ≥3990 Ω TK + 15 K ≥4 kΩ ≥12 kΩ From the Focusens MZ6 series datasheet. Rated points of 60 to 85 °C and 165 to 190 °C use a different threshold set with a wider tolerance band, so read the row for the TK you are ordering. Every value in the right column is exactly three times the left. That is the property the whole scheme depends on: a relay whose trip point was set for a single element still reads a three-element string correctly, because the string scales linearly and only one element has to go high to carry the sum past the threshold. Threshold values from the Focusens MZ6 datasheet. Only two wires leave the motor regardless of how many elements are in the string. Why cold resistance tells you almost nothing The 25 °C row is the row people test against, and it is the least informative one on the page. Cold resistance is specified only as a maximum, ≤100 Ω for a single element and ≤300 Ω for a string. A healthy element and a marginal one both sit well under it. Nothing about the value at 25 °C predicts whether the step will land where it should, because the step is a property of the ceramic's transition and the c...

A thermistor assembly is an NTC element packaged into a housing with leads so it can be fitted to a machine. The element decides the resistance curve. The housing decides almost everything else: how hot the assembly can go, how fast it reacts, whether it survives washdown, and how it mounts. That is the part most selection goes wrong on. The same NTC chip in a silicone tube reaches +200 °C and takes about 60 seconds to settle in air. The same chip in a TPE overmould is rated IP67 and stops at +105 °C. Nothing about the thermistor changed. Published 26 March 2025. Updated 10 September 2026. Reviewed by the Focusens technical department. Scope: custom assemblies, not appliance service parts Two different jobs share this term. If you are replacing a thermistor in an LG refrigerator or a Whirlpool dryer, you need that appliance's service part number from the appliance maker or a parts distributor, and a generic assembly will not fit the connector or the bracket. This page is about the other job: specifying a purpose-built assembly for a product you are designing or manufacturing, where you choose the element, the housing, the cable and the mounting. The six encapsulation types and what each one costs you Every housing is a trade. Sealing costs you temperature range. Temperature range costs you response speed. Speed costs you mechanical protection. Series Encapsulation Sealing Temperature range Response, water 0.4 m/s, T0.63 Dielectric strength MFE-1 TPE or PVC overmould IP67 standard, IP68 on request −30 to +105 °C 30 s 1500 VAC MFE-2 Epoxy encapsulation IP66 −30 to +105 °C 30 s 1500 VAC MFT Metal housing, SUS304 or SUS316, copper or plastic waterproof, rating by build −40 to +105 °C ≤30 s 1500 VAC MFP-1 Silicone rubber or Teflon tube tube seal, no IP class quoted −20 to +200 °C ≤60 s in air 1000 VAC MFP-2 Ring lug, surface mounting with screw contact mount, no IP class quoted −30 to +150 °C ≤30 s 1500 VAC FWZ (RTD) Stainless sheath, PT100 / PT500 / PT1000 element sheathed −50 to +200, +350 or +600 °C by build about 10 s at 0.63τ per build Figures from the Focusens product catalogue. Insulation resistance is ≥100 MΩ at 500 VDC across the NTC series, and long-term stability is quoted as ≤3% drift after 1000 hours of heat or cold storage at 80 °C or −30 °C. Rated ranges from the Focusens catalogue. The element is the same class of NTC across the five NTC series; the housing sets the ceiling. Read the MFP-1 row against the others carefully, because the two numbers are not comparable. Its ≤60 s is measured in still air; every other row is measured in water flowing at 0.4 m/s. Air carries heat away far more slowly than moving water, so the same assembly always looks slower in an air figure. A response time without its medium and flow rate is not a specification, and comparing two suppliers' numbers taken in different media will mislead you every time. Ask which medium and what flow rate before you compare anything. The decision, in the order it actually h...

The KTY83-110 is a silicon PTC temperature sensor with a nominal 1000 Ω at 25 °C in a hermetic glass axial package. NXP has discontinued the KTY8x family and states plainly that it has no replacement offering of its own. The Focusens LPTC83-110 matches it electrically at R25 = 1000 Ω over the same −55 to +175 °C range, in a DO-35 glass body rather than the KTY's DO-34. That package difference is the one detail that decides whether your existing board takes the part without a change. It is covered below, with the full family cross-reference. Published 10 September 2026. Reviewed by the Focusens technical department. What NXP actually says about the discontinuation NXP's product page for the silicon temperature sensors carries the notice that this is a product that is no longer manufactured. On that page NXP names KTY82 and KTY81 as end of life and not recommended for new designs, with last order shipment on 31 March 2026, and directs customers to Rochester Electronics to check remaining stock. Two things follow that matter for a buying decision. The last-order window for the parts NXP names has already closed, so what remains in distribution is legacy stock at broker pricing rather than production supply. And NXP states it has no replacement offering, which means any drop-in claim you read comes from a third party rather than from the original manufacturer. The temperature range in circulation is wrong by 25 °C Ask a search engine for a KTY83-110 replacement today and the answer block returns −55 to +150 °C, sourced from a distributor listing. NXP's own KTY83 series datasheet, Rev. 06, dated 4 April 2008, gives the ambient temperature range as −55 to +175 °C. The difference is not academic. An engineer specifying a replacement for a 160 °C winding or a 165 °C process would read the circulating figure, conclude the KTY83 route is closed, and go looking at a different technology. The manufacturer's datasheet says otherwise, and the Focusens LPTC83 series is specified to the same −55 to +175 °C, so two independent documents agree against the widely repeated figure. Where a number matters, take it from the datasheet rather than from a listing. KTY83-110 specification, from the NXP datasheet Parameter Value Sensor resistance R25 990 / 1000 / 1010 Ω (min / typ / max) at Isen = 1 mA Ambient temperature range −55 to +175 °C Package SOD68 (DO-34), hermetically sealed glass, axial leads Temperature coefficient at 25 °C 0.76 %/K R100 / R25 ratio 1.65 / 1.67 / 1.69 (min / typ / max) Recommended sensor current 1 mA continuous All values from NXP KTY83 series datasheet Rev. 06. The datasheet notes that 1 mA is recommended to keep temperature error low above 100 °C. Before committing a substitution, verify the curve rather than the endpoints. The NXP datasheet carries a resistance-versus-temperature table for the KTY83 series at 1 mA; check it against the LPTC curve for the same part at the temperatures your design actually operates at, not just at 25 °C. Two pa...

Set the meter to ohms, disconnect at least one lead of the thermistor, and measure at room temperature. A healthy NTC reads close to its rated R25. Then warm it and watch the reading fall, or chill it and watch the reading rise. An open circuit, a near short, or a reading that does not move with temperature means the part has failed. That answers the question. What follows is how to know what “close to R25” should be for your specific part, why a room-temperature check passes a drifted sensor, and when a measurement taken without unsoldering can be trusted. Published 5 September 2026. Reviewed by the Focusens technical department. Before you probe anything Kill the power and let the equipment sit. Resistance measurement injects the meter's own small current, so a live circuit gives a meaningless reading and can damage the meter. Watch for stored energy. Power supplies, motor drives and anything with a large bulk capacitor hold a charge after the mains is disconnected. Wait for the discharge indication the equipment provides, or check across the bus with the meter on volts before switching to ohms. Note the ambient temperature. Every judgement below is relative to the temperature the thermistor is actually at, not to the number printed on the datasheet. You need a digital multimeter with an ohms range, a way to change the part's temperature by a controlled amount, and the part's R25 and B value. A cup of iced water and a thermometer beats a hot air gun for this, because you know what temperature the part reaches. The procedure Isolate the thermistor. Unplug the connector, or unsolder one leg. Anything in parallel is measured together with the sensor and will drag the reading down. Measure at room temperature. Set the meter to a range above the expected value, touch the probes to the two leads, and let the reading settle. Polarity does not matter. Record the value and the room temperature. Compare against the part's rated R25. A 10 kΩ part at 25 °C reads near 10 kΩ. At 20 °C the same part reads around 12.5 kΩ, so a few degrees of room-temperature difference is not a fault. The next section gives the ratios. Change the temperature and watch the reading move. Put the tip in iced water, or hold it between finger and thumb. Resistance must fall as it warms and rise as it cools, smoothly and within a few seconds for a bare bead. Let it return. Bring the part back to ambient and confirm the reading comes back to where it started. A part that does not return has a thermal or moisture problem even if both individual readings looked reasonable. What “correct” actually looks like “The resistance should change” is not enough to judge a part, because the amount it should change is set by the B value. For a 10 kΩ thermistor with B25/85 = 3988, the Beta equation gives these ratios against its own 25 °C value. Terminology, test methods and the generic requirements for these devices are defined in IEC 60539-1:2022. Check point Temperature Resistance Ratio to R25 I...