I expect the OP was using ‘thermistor’ as a generic term for a sensor.
Have a look at the Model Boating live steam practice (Model boat Mayhem forum) where they have been regulating the gas burner with a servo following on from the work of Malcolm and Roy from the St Albans MES controlling water level  must be 40 years ago.

The simple answer is NO, there is no correlation between steam temperature and its pressure, especially as the twmperature would differ throughout the system, but the pressure would be the same everywhere that was sharing the same connections.

Ducan has summed it up well. There is another big issue with this approach. If for any reason the temperature sensor is not fully immersed in liquid it will underread. This could be due to low water, sloshing or movement of the sensor. If it is a mobile boiler then sloshing could give a averaged temperature due to the thermal response of the sensor that would be lower than the actual temperature/pressure but not obviously wrong.
There have been numerous industrial and some aircraft accidents due to incorect temperature readings because the sensor was not immersed in the liquid properly.

Apart from that I can see no advantage in this approach. It’s an indirect measurement that still needs a boiler penetration for electrical connections or a drywell and it needs electrical power to work.

Robert.

Attaching the sensor to the boiler shell measures the local shell temperature not the water temperature. There can be significant, in relation to determining steam pressure, differences beween the two. It can also change with the flame size / position.

Robert.

Hello Duncan,

A cold junction compensation device is straightforward. I have an old Cambridge Instruments thermocouple unit which is simply a millivoltmeter plus the compensation device. Really old fashioned 1950’s style instrument. Large beautifully built and accurate.

Having said that I don’t think that the OP’s idea is going to fly. Especially as he asks for a circuit diagram. Doubt there is such a thing in the amateur world. He would need to do it himself.

Andrew.

In principle, I’m sure it could be made to work.  The relationship between temperature and pressure is well-known, (Boyles Law) and steam temperature/pressure relationships were studied by James Watt, later in much greater detail by the Victorian big name physicists, notably Rankine, also Kelvin and Clausius,

Look up ‘Steam Table’ in Thermodynamics text books and the web.  Clipped off example from Simetric:

From the full table, 150psi is about 1000kpA, at which pressure the steam/water will be at about 185°C.

I don’t know what voltage range comes from a thermocouple operated between 100° and 200°C, or if it’s linear.  I believe resistance varies linearly with temperature.  More research needed.

A short list of other problems to solve:

• The sensor operates in a harsh environment: high temperature and pressure, wet and potentially corrosive.  Robust construction needed!
• The sensor must be carefully placed to read the correct temperature, that of the boiling water or steam, not the boiler shell or pipework.  Probably easiest to put it in the underwater part of the boiler, insulated from the shell.
• Cheap electronics grade thermistors have max operating temperatures too low for this application. Suitable high temperature thermistors are available, but they’re big, encased in stainless steel, and prices start at £350.  Many are ‘price on application’ – if you have to ask you can’t afford it!  Proper Industrial.
• Thermocouples may be a better option.  Not researched properly, by me! Cost about £20 by the look of it, accuracy ±1°C ish.  (Not as good as a thermistor – is 1 degree good enough!)
• Better accuracy from Platinum Resistance Sensors, but I couldn’t find anyone selling them.
• Home-made?  Calibration is a significant problem, but
  • A thermocouple is just two different metal wires twisted together. (Producing a voltage that depends on temperature.)
  • Maybe a coil of resistance wire: resistance varies with temperature.
• All these sensors will need electronics.  An Op Amp arranged to multiply the voltage or resistance reading by a fixed gain, followed by a something to drive a line of LEDs, moving coil meter, or digital display.  (Arduino can do all that and the sums necessary to convert sensor temperature values into steam pressure.)

Not difficult provided the engineer has the right combination of skills.   I’m weak on Op Amps and the mechanical details of a suitable sensor – steam tight, corrosion resistance, practicalities like access for maintenance etc.   I can think of a few members who could probably do everything needed on their own, but a team of Model Engineers with complementary skills would be better.   Sadly it is easier to herd cats than Model Engineers!

Whether or not an electronic gauge is a good idea depends on the application.   Unless there’s a mass-produced commercial product available, not a cheap alternative to a Bourdon Gauge where a model loco driver only intermittently needs a rough idea of pressure.    However, much better than a Bourdon Gauge if the goal is to study performance by logging data to a computer or to automate a boiler to maximise efficiency.

An interesting project, and might be valuable.   An electronic gauge would eliminate the need for clunky pipework feeding a bourdon gauge.  The pipe penetrates the boiler weakening it and needs a Boiler Inspector satisfying seal.

Does anyone know how much heat is wasted by a Bourdon Gauge installation?  Ideally heat should be cooled by the piston doing work, not leaked up the chimney, out of the boiler shell, into the structure, or radiated by pipework!   An electronic gauge would eliminate Bourdon Gauge heat losses.

Dave

From an electronics point of view, thermocouples are the worst to try and use accurately unless they are being used with a proper thermocouple display or converter & correct wire types throughout.

Each and every junction between two dissimilar metals is another thermocouple and the relative temperatures affects the final voltage reading, which is in the millivolts range. (Approx. 6mV @ 145’C for a Type K).

PT100 sensors are the next best, due to the small change in resistance with temperature; the resistance change from 140 to 150 degrees C is less that 4 Ohms.

PT1000 could be the best, as the higher resistance means lower bias currents and less self-heating. It’s the same percentage change as PT100, but a bit easier to work with.

The nominal resistance values near 145’C are:

140’C = 1535.8 Ohms, 145’C = 1554.6 Ohms, 150’C = 1573.3 Ohms

(PT100 values are 1/10th those).

Example – search amazon for these, two for a tenner; 4mm diameter x 30mm long body, rated to 650’C

SUCHUANGUANG m x30mm Pt1000

A typical 3D printer “10K” thermistor has a similar percentage change to the PT1000; 413 to 423 Ohms from 140 – 150’C, but generally lower maximum temperature ratings than PT are available with.

Steam in a boiler is a saturated vapour, not a permanent (or perfect) gas, so it doesn’t obey Boyles law. There are equations which relate T to P, but a bit more complicated

Had a quick Google. This looks like a useful module, does the cold junction compensation and outputs thd temperature in a 12 bit serial to be read by an Arduino

Serial com just took it beyond my pay grade, but the clever people on here will understand. To try it out all that is needed is a sealed vessel with a safety valve and pressure gauge and a gas torch. I’d still use a pressure sensor.

Someone has stated above that thermocouples are the worst thing to measure temperature, from an electronics point of view. I totally disagree with such a statement. I have used multiple thermocouples on individual lamps under life test. An absolute doddle to get accurate measurements.  Added to that there were anything up to 20 lamps on each test, so we are talking around 120 individual thermocouples The results were fed into a data logger. Simple, accurate and easy to implement.

Andrew.

Thermocouples are esay to use when used with instruments, connectors and cables designed for the particular thermocouple in use. Type K would suit this application but would have to be sheathed.
DIY is not easy unless a specialist IC is used. It has to measure the temperature at the cold junction (instrument end). Even then the signals are very small.
A self-contained, unpowered electromechanical indicator may look simple but isn’t It will have internal cold temperature sensing (often a thermistor) and linearisation. In addition it will only be accurate if the total resistance of the thermocouple and connections is a specific resistance.

On aircraft turbine temperature indicator installations there is an adjustment to ensure the resistance is correct. Adjusting this is interesting because the thermocouple voltage affects the resistance reading with any standard ohmmeter. You have to measure it twice with reversed connections and take the average. So it’s two measurements, maths, adjust, two measurements mathss, adjust….. And just to make it more fun some installations actually use it to deliberately offset the reading at the high end. This is done to a value determined when the engine is rig tested. This is done so all engines have the same “redline” temperature regardless of the performance of the specific engine.

Robert.