16
votes
Accepted
Why does a Pt100 RTD fail after immersion in thermally conductive and electrically isolating paste?
These are "naked" RTD and are exceptionally fragile. Look at them in the wrong way and they break. Essentially, there's the platinum coated chip and the wires are simply bonded to it. No ...
- 8,754
12
votes
Is it normal to have this much fluctuation in an RTD measurment in boiling liquid?
That's a pretty mediocre chip to be expecting 0.1°F stability from. At 1mA 0.1°F is about 8uV. The chip has typical noise of 150uV over the bandwidth, and thermal EMFs can cause errors of some uV here ...
7
votes
Accepted
How do Resistance temperature detectors (RTD) work?
There are two categories of sensors which can be used in an electric circuit:
Those which impact an existing current, the measure then consists in interpreting the current variation into something ...
- 386
7
votes
Accepted
Why does a resistor have this value in a circuit with an operational amplifier?
In theory it’s to balance out offset due to bias current so the resistance is equal to the parallel combination of R5 and R6. 49.9K is the closest value to 50K in the E96 series that many 1% resistors ...
5
votes
Why does an RTD sensor circuit use a reference resistor that is 4x the RTD value?
It may be some ratio that they think works okay with their chip. To my judgement it represents far too high a measurement current. For example, with a PT100-DIN the measurement current is 4mA at 0°C ...
4
votes
Accepted
self-heating effect of dual PT100 temperature probe in a moving mass of water
You would be better off to get actual self-heating numbers from the probe manufacturer (or make measurements yourself). The coefficient can vary over a range of 10:1 depending on RTD element and probe ...
4
votes
How does 3-wire RTD or resistance measurement works in bridge of wheatstone?
Start from the voltage divider equation (since it is Vo you are balancing), and see where you get to...
If bridge is balanced (without \$ R_L \$): \$ \frac {R_2}{R_1+R_2} = \frac {R_3}{R_g+R_3} \$
...
- 5,493
4
votes
Accepted
Accurate yet relatively simple way to use RTD probes
The MAX31865 seems like a very good choice. The datasheet does give more detailed accuracy specifications—the first page of a datasheet is generally just marketing material.
On page 3, we have some ...
- 2,641
4
votes
Accepted
Why does an RTD sensor circuit use a reference resistor that is 4x the RTD value?
For example, a PT1000 platinum RTD which has resistance = 1000Ω at 0°C has R=3900Ω at 850°C, and 850°C is the maximum temperature for platinum RTDs. 4X sufficiently covers that range for all types of ...
- 556
4
votes
Why does a Pt100 RTD fail after immersion in thermally conductive and electrically isolating paste?
Seems pretty benign. Zinc oxide (the usual component in thermal paste) in a silicone grease carrier. The element itself in those bare sensors is passivated except where the connections are, and ...
4
votes
PT1000 drifts more than 100°C
The whole system heats up to about 135°C ...
... and ...
Figure 1. From datasheet.
- 180k
4
votes
PT1000 drifts more than 100°C
You need some numbers. Either instrument the temperature inside the box or find a way to self-monitor. That will confirm or refute your expectations of system temperature.
Run wires out from the ...
3
votes
Alternate way to read Resistance temperature detectors (RTDs) with Microcontroller
What you are talking about is called charge time measurement. There are even microcontrollers with built-in hardware to assist with this. Usually it is intended for measuring changes in small ...
- 315k
3
votes
Accepted
Can long cables induce that much noise?
It's difficult to fully understand your RTD cabling, since you've mentioned "Two of them have a multi-core and the other three have a single core", yet you would need at least two wires to connect to ...
- 18.3k
3
votes
Accepted
Purpose of capacitors connected to both inputs of Op-Amp
Typically filters look like this:
Source: https://www.planetanalog.com/improving-the-cmrr-of-instrumentation-amplifiers/#
Which you could count the resistors of the bridge to make your RC filter.
The ...
- 88.8k
3
votes
Accepted
pt100 RTD transmitters not working - two in a row - am I doing something wrong?
32mA is typical of what a transmitter will output with an open sensor. Connect a 100 ohm resistor directly across the terminals A to B and short B to B' and measure the current (preferably with a ...
3
votes
Why does an RTD sensor circuit use a reference resistor that is 4x the RTD value?
I don't agree with that statement that "A reference resistor equal to four times the RTD's 0 degC resistance is the optimum for a platinum RTD."
In what follows, I assume that the reference ...
- 11.1k
3
votes
Is it normal to have this much fluctuation in an RTD measurment in boiling liquid?
The physics side of the problem:
Boiling as a process arises from local overheating of the liquid and cooling the place by evaporating inside a growing bubble. The temperature needed to create a new ...
- 9,686
3
votes
Why is RTD immune to electro-magnetic fields?
Typically the voltages involved are an order of magnitude higher. A 100 ohm RTD is still relatively low impedance (though nowhere near as low as a typical thermocouple which is a few ohms typically).
...
3
votes
Wheatstone Bridge with Instrumentation Amplifier
In the top circuit, U2A is saturating at the negative rail. If you add a negative 12V supply rather than using ground it should give close to the expected values.
The second circuit also won't work in ...
3
votes
Wheatstone Bridge with Instrumentation Amplifier
Spehro Pefhany's answer is correct.
The voltage above R9 is 1/11 * 3.3 = 0.3 V. The voltage above the PT1000 is 0.352 V. The common mode voltage is thus (0.3 + 0.352)/2 = 0.326 V.
The front end of the ...
- 27.9k
3
votes
Accepted
Using solder to extend RTD cable?
There's no problem with soldering to splice the wires. Just avoid adding mismatched length (a few cm won't matter) and match up the colors so the lead resistances are reasonably well matched.
3
votes
How to connect heaters and RTD sensors from an EMC perspective
Generally you want to run the low level signals separately and perhaps shielded. Aside from EMI, depending on your jurisdiction there are safety regulations that may require more or different ...
3
votes
Accepted
PTC Sensing circuitry
Depending on the current and temperature range, you can try 3 Nickel RTDs 1k ohm @ 0'C in parallel with 1k ohm @ 0'C + 500 ohm @ 0'C platinum RTDs.
That's a total of 5 RTDs at about $3-4 each in small ...
2
votes
ADC variable reference voltage
Page 5 of the data sheet (https://www.analog.com/media/en/technical-documentation/data-sheets/AD7705_7706.pdf) specifies that the Analog in range is \$\frac{\pm V_{REF}}{\textrm{gain}}\$ for bipoolar ...
- 30.6k
2
votes
Accepted
RTD sensor circuit design
DIN standard 100 ohm RTDs are 100 ohms at 0'C and 138.50 ohms at 100'C.
Starting at the beginning, your switched current sink looks quite inadequate. You need it steady to within +/-0.03% for 0.1'C, ...
2
votes
Sallen-Key filter for PT100 RTD measurement
The schematic you found has the lowpass filter schematic drawn wrong:
- 4,446
2
votes
Accepted
Which temperature transducer has these characteristics?
Remember with multi-choice questions the instruction is to choose the answer which you think is best,
PTC thermistor is a generic term that includes metal RTDs and Polyswitches, - anything with a ...
- 36.4k
2
votes
Why is there a negative value on the scada when the analog input wire is disconnected from a field instrument?
It's common in control system design to want to indicate a faulty sensor in a positive (unmistakable) manner. In an analog system the only way to do that is to send a signal that's well out of the ...
2
votes
Working of INA826 instrumentation amplifier for Pt100 RTD
Is this circuit amplifying only the voltage across the RTD or are any
other voltages involved?
It's amplifying the voltage across the RTD. The 10k resistors in conjunction with the 100nF caps have a ...
- 88.8k
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