Steam pressure using thermistor

[michael howarth 1Participant @michaelhowarth1]

Is there a workable circuit incorporating a thermistor which would effectively replace a pressure gauge with led indicators on a model steam boiler? I am looking at 4 Bar operating pressure which would give a water/steam temperature of about 145 deg.C.

Mick

[Andrew TinsleyParticipant @andrewtinsley63637]

If you are confident that steam or water temperature can be correlated against pressure. Then why use a thermistor? A simple thermocouple and op amp should give you a voltage output that can be calibrated against temperature. The voltage can be used to trigger further op amps which will switch LEDs at preselected voltages (pressures).

Having said that, I would not rely on such a system, too many things could go wrong! I can’t see why you do not want to use a traditional pressure gauge. After all they have been used for at least 150 years and have proved their reliability.

Andrew.

[BazyleParticipant @bazyle]

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.

[Andrew TinsleyParticipant @andrewtinsley63637]

Controlling water level is one thing, but relying on temperature measurement to record steam pressure is quite another. I seem to remember a couple of articles on measuring water level in a boiler using electronic means. By all means give it a try, I suspect you may find you are pioneering such a method.

With regard to thermistor versus thermocouple, the voltage output of a thermocouple is essentially linear and well documented. I may well be wrong, but I think that the resistivity of a thermistor is non linear with temperature and varies depending on the particular thermistor chosen. So really the thermocouple is a no brainer. I will be intrigued to see how you get on with such a project. Good luck.

Andrew.

[old martParticipant @oldmart]

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.

[duncan webster 1Participant @duncanwebster1]

Oh yes there is, provided the steam isn’t superheated, but it’s not linear, so either use an approximation or a processor. I think it’s getting a bit warm for a normal thermistor.  Those used on 3D printers are I’ve been told suitable for higher temperature.

To use a thermocouple you’d need a cold junction compensation thingy. If you just use ambient that can change by 30 or more deg C, which would compromise the pressure calculation

[Robert Atkinson 2Participant @robertatkinson2]

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.

[duncan webster 1Participant @duncanwebster1]

I think one could get over the immersion problem by spot welding the thermocouple wires to the boiler shell near the bottom. This doesn’t overcome the other issues. You can buy pressure sensors fairly cheaply, they need protecting from the heat, but all that needs is a length of pipe.

[Robert Atkinson 2Participant @robertatkinson2]

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.

[duncan webster 1Participant @duncanwebster1]

I’m perhaps wrongly assuming an internally fired boiler with the sensor nowhere near the flame. The heat transfer between water and shell will be massively higher than between shell and air, and the temperature drop through the shell will be negligible, so I’d be surprised if the shell temperature was significantly different to the water temp. After all, that’s how a hot well works.

I’m not advocating it, better use a pressure sensor

[Andrew TinsleyParticipant @andrewtinsley63637]

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.

[MacolmParticipant @macolm]

Assuming the boiler contains only water (liquid and vapour), it forms a powerful isothermal volume. As soon as the temperature at one point exceeds elsewhere, the pressure will also be higher there, and the resultant flows thus cause the temperature to equalise throughout the volume.

[SillyOldDufferModerator @sillyoldduffer]

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

 

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