If you’ve ever priced a Resistance Temperature Detector (RTD) from a reliable supplier, you already know they’re not just a wire wrapped around a ceramic core. There’s a whole lot more going on behind the scenes—especially when it comes to how you connect that little sensor to your readout device. As someone who’s been supplying RTDs for over a decade, I’ve lost count of how many customers have come to us saying, “My RTD reads 10 degrees off—why?” More often than not, it’s not the RTD itself that’s the problem. It’s the measuring circuit holding it back. Resistance Temperature Detector
Let’s break this down, no fancy jargon, I promise. First, remember that RTDs work on a super simple principle: their resistance changes predictably with temperature. A standard PT100 (the most common type we supply) has 100 ohms at 0°C, and about 38.5 ohms per degree Celsius above that. But to measure that tiny change in resistance, you need a circuit that sends a small current through the RTD and measures the voltage drop across it. If that circuit isn’t designed right, you get errors—sometimes small, sometimes big enough to throw your whole process off.
The big one most folks don’t think about is lead wire resistance. If you’ve got an RTD that’s 10 feet away from your control panel, those two (or three, or four) wires carrying current to and from the sensor have their own resistance. Say each lead is 2 ohms. That adds 4 ohms total to your measurement, which at 0°C makes your RTD look like it’s 104 ohms instead of 100. Translated to temperature? That’s a ~10°C error right there, no matter how good your RTD is.
Now, here’s where the type of measuring circuit makes all the difference. Two-wire RTD circuits are the cheapest and most common, but they merge the RTD resistance and the lead resistance into one measurement. They work great for short distances—like under 3 feet—where lead resistance is negligible. But if you’re running longer wires, you’re eating into accuracy fast. That’s why we usually steer customers toward three-wire circuits when they need sensors further out. The third wire lets the circuit measure the resistance of one lead separately and subtract it from the total, canceling out most of the lead error. Four-wire circuits take it a step further, using two leads for current and two for voltage measurement. Since the voltage leads carry almost no current, their resistance doesn’t show up at all—making them the most accurate, even for really long runs. But they’re also pricier, so it’s all about balancing your needs.
Another error source is self-heating. The measuring circuit has to send current through the RTD to measure resistance, and that current makes the RTD warm up a little. If the current is too high—like more than 1mA—you can add a degree or two of error, especially in low-temperature or low-fluid environments where heat can’t dissipate. A lot of cheap measuring circuits crank up the current to get a stronger signal, which sounds like a win but actually ruins the RTD’s accuracy. We test every RTD we send with a small, optimized current (usually 0.5mA) so self-heating is under 0.1°C, but if your circuit doesn’t match that, you’re losing that precision.
Noise is another sneaky culprit. If your RTD is in a factory with motors, variable frequency drives, or other electrical equipment, the measuring circuit picks up that electrical noise and adds it to the RTD’s signal. A circuit with good filtering can cut that noise way down, but a cheap one might let a few millivolts of noise slide through—enough to throw off a temperature reading, especially if you’re measuring small changes. For example, a millivolt of noise on a PT100 translates to almost 3°C of error. I’ve had customers come to us with RTDs that were working fine until they added a new conveyor belt near their sensor—turns out the old circuit didn’t have enough noise rejection, and swapping the measuring circuit fixed the problem way cheaper than replacing the RTD.
Wait, let’s talk about common mistakes we see all the time. A customer last year bought a batch of our high-precision PT1000s (which have 1000 ohms at 0°C, so smaller resistance changes are easier to measure) and complained they were 5°C off when installed. Turns out they used a two-wire circuit for a 15-foot run. The lead resistance was 3 ohms per wire, adding 6 ohms total, which on a PT1000 was a 0.6% error—translated to 5°C. Swapping to a three-wire circuit fixed it immediately. That’s not the RTD’s fault; that’s the circuit.
Another thing: reference resistance. Some measuring circuits use a fixed resistor to calibrate the RTD’s resistance. If that reference resistor is cheap or drifts over time, your accuracy goes out the window. We always recommend pairing our RTDs with circuits that have stable, calibrated reference resistors—especially if you’re running a process that needs tight tolerances, like food manufacturing or pharmaceutical testing. A $10 reference resistor can save you thousands in rework if your measurements are off.
Now, let’s get into what this means for you, as someone using an RTD. It’s not enough to just buy a “high-accuracy RTD” from a supplier. You have to match the measuring circuit to your setup. If you’re not sure which circuit is right, here’s a quick rule of thumb: under 3 feet, two-wire works. 3 to 20 feet, go three-wire. Over 20 feet, or if you need ±0.1°C accuracy, four-wire is non-negotiable. And always check that your circuit’s current is low enough to avoid self-heating—we specify on every RTD datasheet the maximum safe current, so make sure your circuit stays under that.
As a supplier, we don’t just ship sensors. We help customers troubleshoot when their readings are off, because we know the circuit is usually the issue. We’ve seen so many folks waste money on new RTDs when all they needed was a better measuring circuit. That’s why we include free guidance on circuit selection with every order, and we can even recommend compatible circuits if you’re building a new system.
If you’re dealing with temperature readings that feel inconsistent, or you’re worried about accuracy for your next project, don’t guess. Reach out to us—we can walk through your setup, suggest the right RTD and matching circuit, and make sure you get the precision you need without overpaying. We’ve worked with everything from small breweries needing consistent fermentation temps to large manufacturing plants monitoring industrial ovens, so we’ve got the experience to help you.
Smart Thermometer References
- RTD Fundamentals for Temperature Measurement. Omega Engineering.
- Resistance Temperature Detector Circuit Design Considerations. Measurement Science and Technology.
- Lead Wire Error Compensation in RTD Circuits. Journal of Electronic Testing: Theory and Applications.
Jiangsu Zhaolong Electric Co., Ltd.
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