How to Test a PT100 RTD With a Multimeter To test a PT100 with a multimeter, disconnect it, null the meter leads and measure across the element: a healthy PT100 reads 100.00 Ω in ice water and about 107.79 Ω at 20 °C, and a PT1000 ten times as much (IEC 60751). OL means an open element, near 0 Ω a short, and any reading from a signal lead to a metal sheath means failed insulation. Updated October 29, 2026. Reviewed by the Focusens Engineering Team. Probe temperature PT100 PT1000 0 °C (ice water) 100.00 Ω 1000.00 Ω 20 °C 107.79 Ω 1077.94 Ω 22 °C 108.57 Ω 1085.70 Ω 25 °C 109.73 Ω 1097.35 Ω 30 °C 111.67 Ω 1116.73 Ω 100 °C 138.51 Ω 1385.06 Ω Values from the IEC 60751 equation. Full tables: PT100 resistance table and PT1000 resistance table. What a good PT100 or PT1000 reads on the bench A good PT100 reads the table value for the temperature it is actually at, within its class tolerance; the "100" in its name applies only at 0 °C. At room temperature that means somewhere between about 107.8 and 111.7 Ω, and the exact figure depends on the room. A reading of 109 Ω means 23.1 °C, 110 Ω means 25.7 °C and 112 Ω means 30.8 °C (our calculation), so a bench check needs a thermometer beside the probe or an ice bath. The thermometer's own error counts too: ±0.5 °C is ±0.19 Ω on a PT100 (our calculation). Let the probe lie untouched beside it until both readings stop moving. The class tolerance in ohms is small. IEC 60751 sets Class A at ±(0.15 + 0.002|t|) °C and Class B at ±(0.3 + 0.005|t|) °C, the same in °C for any nominal resistance: Class At 0 °C PT100 window at 0 °C At 20 °C PT100 window at 20 °C A ±0.15 °C 99.94 to 100.06 Ω ±0.19 °C 107.72 to 107.87 Ω B ±0.30 °C 99.88 to 100.12 Ω ±0.40 °C 107.64 to 107.95 Ω Our calculation from the IEC 60751 Table 2 formulas and the slope at each temperature. A PT1000 window is ten times wider in ohms and the same in °C. A PT100 changes about 0.39 Ω per °C, so 1 Ω of error is about 2.6 °C. That one number explains most of what follows: a tenth of an ohm in the test leads, a corroded terminal or a cheap meter all show up as degrees. How to test an RTD with a multimeter through 2, 3 or 4 leads Testing an RTD with a two-probe multimeter means measuring the element plus some lead resistance and then taking the leads out by arithmetic. The order below works for PT100 and PT1000 probes. Make the loop safe, then disconnect the probe from the transmitter, PLC card or controller. When a transmitter detects "an input sensor failure" it "drives its output to low or high depending on the position of the failure mode switch" (Rosemount 644 manual), so put the control loop in manual and have any temperature trip bypassed under the site's procedure before you lift a wire. In a hazardous area the same manual says: "Do not remove the connection head cover in explosive atmospheres when the circuit is live." An input circuit in parallel changes the reading. Measure first at the panel end with the field cable still on the probe, then at the...

RTD vs Thermistor: PT100 or NTC for Accuracy and Range RTD vs thermistor is mostly a question of temperature span. A Class A PT100 stays within ±0.15 to ±0.45 °C from 0 to 150 °C. A 10 kΩ NTC thermistor with ±1 % on R25 and on its B value is ±0.22 °C at 25 °C but ±1.3 °C at 100 °C (our calculation). The PT100 is a platinum resistor on one IEC 60751 curve; the NTC is a metal-oxide ceramic that changes about 13 times more per degree relative to its resistance and shrugs off lead resistance. Interchangeable NTC grades reach ±0.1 °C only inside windows such as 0 to 70 °C. Updated October 23, 2026. Reviewed by the Focusens Engineering Team. PT100 RTD 10 kΩ NTC thermistor Sensing material Platinum wire or film Metal-oxide ceramic Change per °C at 25 °C +0.39 Ω (+0.35 % of its resistance) −447 Ω (−4.5 % of its resistance) Resistance curve One standard curve (IEC 60751) Set by each maker (B value or R-T table) Tolerance at 25 °C Class A ±0.20 °C, Class B ±0.43 °C ±0.22 °C (±1 % R25, ±1 % B) Tolerance at 100 °C Class A ±0.35 °C, Class B ±0.80 °C ±1.30 °C (±1 % R25, ±1 % B) Typical rated range −70 to +500 °C thin film, to +600 °C wire-wound (Class B) −40 to +125/155 °C epoxy, to +200/300 °C glass Error from 1 Ω of 2-wire lead at 25 °C +2.6 °C −0.002 °C Connection 3-wire or 4-wire for Class A or tighter 2-wire Our calculation for the tolerance and lead rows, from the IEC 60751 class formulas and a Beta model with B25/85 = 3977 K. Ranges are the datasheet ratings cited in the range section below. What is the difference between an RTD and a thermistor? An RTD and a thermistor both turn temperature into resistance, but they are made of different materials and follow different curves. IEC 60751 defines the platinum resistor as a "resistor made from a platinum wire or film", and "a platinum resistor with R0 = 100 Ω is often referred to as a Pt100". A PT100 is therefore an RTD, not a thermistor. TDK describes NTC thermistors as resistors made of "polycrystalline mixed oxide ceramics" (TDK general technical information). The materials behave in opposite directions. Platinum gains resistance as it warms, about 0.385 % per °C on average from 0 to 100 °C (the IEC 60751 α = 3.851 × 10⁻³ /°C). An NTC loses resistance as it warms, and much faster: TDK puts NTC coefficients at "2%/K to 6%/K", "about ten times greater than those of metals". Vishay's 10 kΩ NTCLE100E3 changes by −4.38 % per °C at 25 °C (Vishay NTCLE100E3). Temperature 10 kΩ NTC (B 3977 K), per °C PT100, per °C 0 °C −1,807 Ω (−5.3 %) +0.391 Ω (+0.39 %) 25 °C −447 Ω (−4.5 %) +0.388 Ω (+0.35 %) 85 °C −33 Ω (−3.1 %) +0.381 Ω (+0.29 %) 150 °C −4.3 Ω (−2.2 %) +0.374 Ω (+0.24 %) Our calculation: Beta model for the NTC, IEC 60751 equation for the PT100. Percentages are of each sensor's resistance at that temperature. Comparing ohms per degree makes the gap look larger than it is. At 25 °C a 10 kΩ NTC changes about 1,150 times as many ohms per degree as a PT100, but relative to its own resistance it is about 12.7 ti...

Conductive Level Switch vs Reed Float Switch: How to Choose A conductive level switch only works in a liquid that conducts electricity, while a reed float switch works in conductive and non-conductive liquids as long as the liquid is dense enough to lift its float. Standard conductive models from KOBOLD, Endress+Hauser and VEGA need at least 7.5 to 100 µS/cm, depending on the model. Seawater, inhibited glycol coolant and US river water clear the 7.5 to 20 µS/cm minimums. Oil, fuel and distilled water do not, so they need a float or another sensor that does not rely on conductivity. A float has its own limits: its material has to suit the liquid, and sludge or iron particles can jam it. Choose by the liquid first, then by what is likely to foul the sensor. Updated October 17, 2026. Reviewed by the Focusens Engineering Team. Conductive (electrode) switch Float switch with reed contact What it senses Current through the liquid between probe rods A magnet in the float closing a reed contact in the stem Liquid must Conduct: minimums of 7.5 to 100 µS/cm Float the float: minimum specific gravity 0.45 to 0.90, by float Works in oil, fuel, distilled water No (distilled water only with a special high-sensitivity relay) Yes, with a float material rated for the liquid Moving parts None The float Typical failures Scale on the rods, conductive foam, liquid too pure Stuck float, iron particles, turbulence, welded contact Which liquids each switch type can sense A conductive switch needs the liquid to carry a small current between its rods, so the deciding number is the liquid's conductivity. Published minimums from switch makers: KOBOLD NES conductive switch: "Min. conductivity: 20 μS/cm" (KOBOLD NES data sheet); the compact NEK needs 100 µS/cm. Endress+Hauser Liquipoint T: for "conductive liquids (from 10 µS/cm)" (Endress+Hauser TI00375F). VEGA VEGAKON 61 relay: "Conductance of the medium min. 7.5 µS/cm" (VEGAKON 61 operating instructions). The table sets those minimums against what common liquids actually measure. Liquid Typical conductivity Conductive switch (7.5 to 100 µS/cm minimum) Float switch with reed contact Seawater About 50,000 µS/cm (Boyd, Global Seafood Alliance) Works Works; choose a float and stem that resist salt water Inhibited glycol engine coolant 3,000 to 5,000 µS/cm (Lubes'n'Greases) Works Works Fresh river water, US 50 to 1,500 µS/cm (US EPA) Works with 7.5 to 20 µS/cm switches; the low end is below a 100 µS/cm switch such as the KOBOLD NEK Works Fuel-cell coolant Typically below 5 µS/cm (BASF Glysantin FC flyer) Below every minimum listed Works Distilled water 0.5 to 3 µS/cm (US EPA) Below standard minimums; needs a special high-sensitivity relay Works Ultrapure water 0.055 µS/cm (METTLER TOLEDO conductivity guide) Below every minimum listed Works Hydraulic and lube oil Near zero to about 2,000 pS/m (0.00002 µS/cm), read from HYDAC's chart of example oils (HYDAC white paper) Does not work Works if the float's material and minimum specifi...

Thermistor Wiring: Polarity, Lead Resistance and Connectors Thermistor wiring for a two-lead NTC takes two connections and has no polarity: the thermistor goes to the controller's thermistor input, or in series with a fixed resistor on a bare analog input, with either lead on either terminal. What changes the reading is resistance added in series with it. Extra cable, a loose crimp or a corroded pin makes an NTC read colder, and the error grows as the thermistor gets hotter because its own resistance falls. On a 10 kΩ NTC, 30 m of 24 AWG cable moves the reading by about 0.01 °C at 25 °C and 0.15 °C at 85 °C (our calculation). The same cable on a 2-wire PT100 RTD adds about 13 °C. Updated October 15, 2026. Reviewed by the Focusens Engineering Team. What happens in the wiring Effect on a 10 kΩ NTC reading Size (our calculation, B 3977 K) Leads swapped None 0 °C 30 m of 24 AWG cable added Reads colder 0.01 °C at 25 °C, 0.15 °C at 85 °C 10 Ω of bad contact or crimp Reads colder 0.02 °C at 25 °C, 0.30 °C at 85 °C Open joint or broken lead Reads the lowest temperature or an open-circuit fault Not a number Short between the leads Reads the highest temperature or a short-circuit fault Not a number Measuring current (self-heating) Reads warmer 0.04 °C at 0.1 mW, 0.25 °C at 0.63 mW (5 V, 10 kΩ divider) on a 2.5 mW/°C probe Does a thermistor have polarity? A two-lead NTC or ceramic PTC thermistor has no polarity. It is a resistor whose value changes with temperature, so current can flow through it in either direction and the reading is the same. Silicon KTY sensors, sold as linear PTC thermistors, need a check. KTY83 and KTY84 are single-chip parts whose published curve holds only with the current in the right direction, and genuine parts carry a colored band to mark it; KTY81 and KTY82 use two chips in series opposed and work either way round (see how to test a KTY84-130 with a multimeter). Carrier puts it directly in the installation manual for its immersion thermistors: "These Carrier thermistors and RTD temperature sensors are both non-polarity and non-position sensitive." The same manual gives the lead colors only as part of the sensor type, "10 KΩ nominal (White/Green)", with no plus or minus lead. Murata answers the same question for its PTC chip thermistors in a product FAQ: "There is no polarity." Some thermistor probes do have assigned wires. A probe with a built-in bridge resistor, or two elements sharing a common lead, is no longer a plain two-terminal part. The Apogee SF-110 manual, for example, gives each conductor a function, such as "positive leaf thermistor lead" and "negative leaf thermistor lead", and puts precision bridge resistors in the cable, so its wires are not interchangeable (Apogee SF-110 manual). If a probe has more than two conductors, read its drawing before connecting it. If it has exactly two and the element is an NTC or a ceramic PTC, polarity is not the problem. A Focusens epoxy NTC with an extension cable. The splice unde...

Short answer. Cold junction compensation corrects a thermocouple reading for the temperature where the thermocouple wires meet the instrument's copper terminals. Without it, a Type K at 100 °C with terminals at 25 °C reads 75.9 °C, not 75 °C. The error is close to the terminal temperature, not equal to it, and a wiring fault can push it high as well as low. Cold junction compensation (CJC) corrects a thermocouple reading for the temperature of the point where the thermocouple wires meet the instrument's copper terminals. A thermocouple produces a voltage set by the temperature difference between its tip and that point, so the instrument measures the terminal temperature, converts it to the equivalent thermocouple voltage, and adds that voltage before converting the total to temperature. Without cold junction compensation, a Type K thermocouple at 100 °C with its terminals at 25 °C reads 75.9 °C, not 75 °C (in Fahrenheit, 212 °F with 77 °F terminals reads about 169 °F). A Type T at 300 °C with the same 25 °C terminals reads 282.8 °C, only 17.2 K low. The error is close to the terminal temperature, but not equal to it, and wiring faults can push it high as well as low. Reviewed by the Focusens Engineering Team, 24 September 2026. How cold junction compensation works on a K-type thermocouple Thermocouple reference tables give the voltage a thermocouple produces with its reference junction at 0 °C. The National Institute of Standards and Technology (NIST) ITS-90 thermocouple database publishes those tables and the reference functions behind them, and IEC 60584-1 standardises the same functions. A field instrument never has a 0 °C reference, so it measures the real one and corrects for it. Worked through for a Type K thermocouple with its tip at 400 °C and the instrument terminals at 30 °C, using the NIST Type K table: The thermocouple delivers the difference between 400 °C and 30 °C: 16.397 mV − 1.203 mV = 15.194 mV. The instrument's terminal sensor reads 30 °C. The table voltage for 30 °C is 1.203 mV. The instrument adds the two: 15.194 + 1.203 = 16.397 mV. It looks up 16.397 mV in the Type K table: 400 °C. Skip step 3 and 15.194 mV looks up as 371.5 °C, which is 28.5 K low rather than 30 K low. The correction is made in millivolts, not in degrees. Beckhoff's thermocouple basics (EPP3314-0002 documentation) states that the compensation "is carried out with the voltages and not with the temperature". Adding 30 °C to 371.5 °C would give 401.5 °C, a 1.5 K error created by the arithmetic alone. Without compensation, the error is close to the terminal temperature, not equal to it A thermocouple's sensitivity changes with temperature. Near room temperature Type K produces about 40.5 µV per kelvin; at 400 °C about 42.2 µV/K; at 1000 °C about 39.0 µV/K. Type T rises from about 40.7 µV/K at 25 °C to about 58.1 µV/K at 300 °C. The voltage lost by ignoring the terminals is fixed by the terminal temperature, but the number of degrees it represents depends on th...

Short answer. A 10k NTC thermistor reads 10,000 Ω at 25 °C (77 °F), whichever curve it follows. Away from 25 °C the curves separate. At 0 °C a Type 2 reads 32,651 Ω, a Type 3 29,490 Ω, a 10k 3950 part about 32,755 Ω and a 10k 3435 part about 27,700 Ω. A 10k NTC thermistor reads 10,000 Ω at 25 °C (77 °F), whichever curve it follows. Away from 25 °C the curves separate. At 0 °C a Type 2 thermistor reads 32,651 Ω, a Type 3 reads 29,490 Ω, a 10k 3950 part reads about 32,755 Ω and a 10k 3435 part about 27,700 Ω. The chart below gives all four curves from −40 to 125 °C, followed by the error a controller shows when it expects one curve and gets another. Reviewed by the Focusens Engineering Team, 24 September 2026. 10k thermistor resistance chart, −40 to 125 °C °C °F Type 2 (10K3A1), Ω Type 3 (10K4A1), Ω 10k 3950, Ω 10k 3435, Ω −40 −40 336,098 239,828 343,633 204,700 −30 −22 176,803 135,233 179,267 118,500 −20 −4 97,006 78,930 97,840 71,020 −10 14 55,304 47,549 55,595 43,670 0 32 32,651 29,490 32,755 27,700 10 50 19,903 18,787 19,938 18,070 20 68 12,493 12,268 12,501 12,110 25 77 10,000 10,000 10,000 10,000 30 86 8,056 8,196 8,051 8,301 40 104 5,325 5,594 5,315 5,811 50 122 3,601 3,893 3,588 4,147 60 140 2,487 2,760 2,473 3,011 70 158 1,752 1,990 1,738 2,224 80 176 1,256 1,458 1,243 1,668 90 194 916 1,084 904 1,267 100 212 678 817 668 975 110 230 510 624 500 760 120 248 388 482 380 598 125 257 341 426 333 533 Sources, one per column. Type 2: Measurement Specialties (now TE Connectivity) 10K3A1 datasheet. Type 3: TE Connectivity 10K4A1 datasheet. Both columns agree with Vishay Curve 1 and Curve 9 in the Vishay NTC resistance/temperature conversion tables to within 0.3%, under 0.1 K, from −20 to 100 °C. 10k 3950: typical values from the Zhuhai Hongjiacheng HNTC-103F3950FB datasheet, a B25/50 = 3950 K material, rounded to the ohm. 10k 3435: Semitec 103JT datasheet, B25/85 = 3435 K, rounded to the ohm. Fahrenheit values are converted and rounded to the whole degree. The 3950 and 3435 columns each come from one manufacturer's material. Other materials sold under the same beta number follow a slightly different curve, because a beta value fixes two points and the rest of the curve depends on the ceramic. Plotted from the datasheet values in the table above. The lower panel is the same four curves measured against Type 2. Ask for the part's own resistance table; the beta alone does not define it. Which curve is which: Type 2, Type 3, 3950 and 3435 "Type 2" and "Type 3" are building-automation names, not a standard. In many North American HVAC output tables, a table labelled 10K-2 or Type II carries the values of the 10K3A1 curve (Vishay Curve 1), and a table labelled 10K-3 or Type III carries the 10K4A1 curve (Vishay Curve 9). The beta values below put all four materials on the same footing. Common name Element curve Published beta B25/50 B25/85 B0/50 10K Type 2 (10K-2, Type II) 10K3A1, Vishay Curve 1 B25/85 = 3976 K 3936 K 3977 K 3892 K 10K Type 3 (10K-3, Typ...

Short answer. A KTY84-130 and a PT1000 both sit in a motor winding, both are two-wire, and both land in the same one to three kilohm band. They cross near 208 °C and disagree everywhere below it. Fit the wrong one and a winding at a real 150 °C can display as 87 °C, or a cold motor can display as 100 °C and refuse to start. The consequence is not symmetrical, and that asymmetry is the point of this page: A KTY84 element read by a PT1000 input under-reads. At a real winding temperature of 100 °C it reports 0 °C. At a real 150 °C it reports 87 °C. The overtemperature alarm never fires. A PT1000 element read by a KTY84 input over-reads. At a real 0 °C it reports 100 °C. The machine trips on a cold motor and will not start. One failure mode is loud and stops production. The other is silent and removes the thermal protection while every instrument on the panel shows a plausible number. Why this is happening now Siemens has migrated its motors and spindles from the KTY84-130 to a PT1000. The migration document is public, and its wording about the two characteristics is unambiguous: the KTY sensor "is discontinued and no longer available in the market", the PT1000 is the successor to be used in its motors and spindles, and "the temperature characteristic is not compatible!!!". See Change of temperature sensor KTY84-130 on PT1000. The same document sets out the transition: delivery of the new configuration from 2016, phase-out from around 2018, product discontinuation in 2020, product cancellation in 2028, with ten years of spare-part availability. Motors and spindles without a DRIVE-CLiQ interface received new article numbers; those with the interface kept theirs because the sensor type is handled on the far side of the interface. Affected lines named in the document include SIMOTICS S, M, L and T, the 2SP1 spindles, SINAMICS S and G, SINUMERIK and SIMOTION. What the document does not do is say how far wrong the reading goes when the sensor and the input disagree. That is a number an engineer needs when a rebuilt motor comes back with the wrong element in it, or when a spare from the shelf predates the changeover. The rest of this page is that number. The two curves, side by side Temperature KTY84/130 typical (Ω) PT1000 per IEC 60751 (Ω) −40 °C 359 842.7 −20 °C 424 921.6 0 °C 498 1000.0 25 °C 603 1097.3 50 °C 722 1194.0 100 °C 1000 1385.1 150 °C 1334 1573.3 200 °C 1722 1758.6 250 °C 2166 1941.0 300 °C 2624 2120.5 KTY84/130 typical values from Table 7 of the NXP KTY84 series datasheet. PT1000 values computed from the reference function in IEC 60751, with A = 3.9083 × 10⁻³, B = −5.775 × 10⁻⁷ and C = −4.183 × 10⁻¹² below 0 °C; the same function produces our PT1000 resistance table. KTY84 values from the NXP data sheet, PT1000 computed from IEC 60751. Through the range a motor actually operates in, the KTY84 always reads lower. Read down the two columns and the shape of the problem appears. The PT1000 starts high and climbs gently at about 3.85 Ω per kelvin...

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. A 10 kΩ NTC thermistor is about a thousand times less sensitive to the same cable at 25 °C; that case is in our thermistor wiring guide. 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 ...