# Trying to understand the effect of heat on solenoid coils

#### Bhante

Joined Dec 15, 2009
42
I am trying to understand the effects of prolonged exposure to high temperatures on the coils of solenoid valves, vibrational pumps and similar devices, and whether the negative effects are reversible.

Most medium sized espresso machines have a variety of solenoid valves for regulating flow of water/coffee, and a vibrational pump usually manufactured by Ulka, usually the EP5. The Ulka - which has a modular construction - has a coil similar to that of a solenoid valve (such as those manufactured by Parker/Lucifer), which moves a steel piston to and fro against a spring to pump water. Between the coil and the plastic tube containing the steel piston there is an iron sleeve (magnetically contiguous with an iron path in a loop around the coil) around 2 to 2.5 mm thick which appears to play a crucial role in the concentration of the induced magnetic field from the coil on the steel piston. The Ulka is particularly interesting because of its modular construction, so that parts can be freely interchanged between different pumps.

After a number of years' use these devices have reduced and/or erratic performance, as a result of prolonged exposure to quite high temperatures. Because of the modular construction, when they fail functioning parts are often interchanged between various old units to make one good unit. I have observed that the maximum pressure that a used Ulka pump can develop varies substantially, and depends essentially on the coil/iron sleeve combination rather than the piston construction, which (if it is visually in good condition) has no effect on the maximum pressure. What is interesting to me is that of the coil/iron sleeve combination, it is the iron sleeve which is crucial rather than the coil itself - I think actually the coil itself has zero effect, although I would not state that categorically. Supposing I have two pumps which in good physical condition and both of which vibrate when current is applied, where Pump A is able to develop 15 bars of pressure, while Pump B (or the same model EP5) develops only 4 bars. Consider the effect of swapping individual components between Pump A and Pump B, and the result on performance of the two pumps. Swapping piston construction (or individual components of the piston construction) has no effect on performance. If I swap the coil/iron sleeve combination, then the performance of the two pumps is reversed: Pump A 4 bars, pump B 15 bars. Interestingly, if I keep the coil itself unchanged and swap only the iron sleeve construction*, the pressure developed is fairly close to or equal to Pump A 4 bars and Pump B 15 bars (from memory, I cannot remember whether all the difference is in the sleeve or only most of it). * EDIT: It is actually not just the sleeve over the piston construction, but also an iron frame that forms a magnetic loop around the coil, and this frame has to be dismantled to remove the iron sleeve; my detailed experiments were some years ago, and I think I also tried swapping the sleeve/frame part-by-part, but cannot remember the results, although I think I am right in saying the inner sleeve is the most critical component which changes in performance.

Here is my question. As far as I can tell, none of the components in the Ulka pump (piston, iron sleeve, iron frame, coil) exhibits any permanent magnetism at all. So (1) what is the nature of the change in the iron as a result of exposure to heat*, resulting in reduced performance? (* There is a little bit of extrapolation involved here, as I don't have definitive evidence that the deterioration in Ulka performance is the result of heat rather than cumulative electromagnetic use; the extrapolation is from my experience of the Parker/Lucifer solenoid valves (such as part no. E121C13 2-way valve and coil part no. 483510, https://parcom.de/kataloge/Parker_Lucifer_Part-IV.pdf page 342) - in this case it seems pretty clear that the deterioration is directly related to heat exposure; with the Parker/Lucifer swapping the coil/magnetic path combination has the same effect as the Ulka, but it is an integrated unit so swapping the magnetic path alone is not possible). And (2) whatever the change in the iron magnetic path is, can that change be reversed? I think this is the main failure mode of the solenoid valve in espresso machines rather than the coil burning out.

(Just thinking about that magnetic path through the iron sleeve and frame for a moment: if my memory of magnetism from some half a century ago is not too far out, there should be a magnetic field induced by the coil of which the most concentrated part would go through the iron magnetic path and presumably also the steel piston; what the relationship is between the magnetic path through the iron and the magnetic path through the steel piston is not completely clear to me, nor the apparent lack of a heat exposure problem on the steel piston - could that have something to do with the speed of build-up/collapse of the magnetic field in iron as opposed to steel?)

#### profbuxton

Joined Feb 21, 2014
418
I don't think that temperature would have much effect on the metal parts but it will degrade coil insulation after a while(depending on type and length of exposure to heat). I am trying to picture the operation, you say its dependent on the piston oscillating against a spring. Maybe the spring is being affected by heat. I assume there are some sort of non-return valves involved, Could they be leaky(heart valves can leak too). Is there a built up of solids from the water clogging things.Veerrry interesting!

#### WBahn

Joined Mar 31, 2012
24,974
Why do you believe that the devices develop erratic behavior primarily because of exposure to high temperatures? Boiling water is not a particularly high temperature for most mechanical devices, particularly if they are designed to operate at those temperatures. Is there some reason why simple wear or residue build up isn't a possible explanation?

#### Bhante

Joined Dec 15, 2009
42
Thanks both of you for your replies. In response to your questions I'll give some of my detailed evidence - it requires a lengthy description so please bear with me! (Hope you don't fall asleep!)

All of the moving parts (both the pump and the solenoid valve) are contained within what I call the piston assembly, and the whole of this assembly is unaffected by heat exposure. I say that on the basis of experience of maintaining/renovating my coffee machines over the last 30 years, including the revovation of numerous old Ulkas acquired for spare parts over the years. Of course they need to be maintained, that goes without saying, which means complete dismantling, soaking in citric acid to remove deposits, scrubbing as necessary, rinsing, and careful inspection. Actually both the Ulkas and the Lucifer valves (at least the old ones, I have no experience of the new ones!) last many years, and are made from very high quality materials.

Any degradation in performance of the mechanical parts - on the basis of my experience - either will be completely alleviated by thorough refurbishing, or (in rare cases, probably from a machine sitting unused and unrenovated for long periods) there will be visible damage to one or more parts. If all the mechanical parts are in visibly good condition they will always work well (provided the coil/magnetic path combination is good). That in itself is a pretty clear indication that heat does not really affect the mechanical parts. The parts inside the Ulka are standard, and if any one part is not in good condition it can be exchanged with a part from a pump that does not work, provided that part is in good condition, and the result will always work well.

The same is not true of the coil/magnetic path construction - the coil which is fully encapsulated, the inner iron sleeve (which goes through the hole in the middle of the coil, and the plastic-encased piston assembly goes inside the iron sleeve), and the rest of the magnetic path which consists of iron which is screwed together around the coil, in contact with the ends of the iron sleeve. There are no moving parts, no contact with water, and no visible changes in condition occur. Obviously it would be possible for a coil to burn out resulting from wrong voltage, electrical malfunction, blockage, overheating due to lack of priming, etc. But I have never seen a coil that does not work. I have also never seen any variation in the resistance of the coil. Despite that, the pressure that a mechanically perfect pump is able to develop (measured with a manometer) varies, and deteriorates over a period of time (minimum several years). I have even experimented with a brand new pump, swapping different parts in turn with an old pump that was mechanically in excellent condition but developed very low pressure. The mechanical parts have no effect, the coil itself (apart from magnetic path) has no effect, all the performance effect is in the iron magnetic path, especially the inner sleeve. If I swap the magnetic path alone onto a brand new pump, the performance dives. The new magnetic path put on the old pump (in excellent mechanical condition ofter thorough renovation) results in excellent performance. Swap the magnetic path back, performance is again reversed.

Therefore there is absolutely no question that the changes we are discussing concern the iron magnetic path alone. The coil is encapsulated and so I cannot comment on what happens inside, but performance-wise there is no change.

The situation with the solenoid valve is identical, except for two factors: (1) the magnetic path is encapsulated with the coil and therefore cannot be experimented with separately, and (2) the adverse effect of heat exposure is beyond question. My current coffee machine dates from 1996, and has two solenoid valves manufactured by Lucifer. One is a three-way valve which has both hot water (120 to 130 degrees celsius, under pressure) and hot coffee flowing through it. It is bolted on the side of a very hot component, and some parts of it would normally be held at well over 100 degrees for several hours a day, over a period of quite a few years. The temperature exposure of the coil would be less extreme, but still even the coolest parts of the coil would be quite hot for several hours in the day; the inner surface of the coil in contact with the coil would be very hot.

The other Lucifer valve is a two-way valve located behind the boiler. It is exposed to significant heat, but substantially less than the three-way valve. There are several indications (such as an original sticker) that the coil on this valve is most probably the original component, i.e. 1996. After 23 years it is still working perfectly. The coil is marked with the correct part number for the two-way valve, i.e. E121C13 (in addition to the coil part number). For at least two reasons it is visibly evident that it is of older manufacture than the three-way valve (markings on the coil and type plate, and valve construction).

The three-way valve by contrast is evidently of newer construction (change of company name), and it has the type plate from the two-way valve, E121C13 - meaning that sometime between 1996 and 2013 when I acquired it the original coil of the three-way valve failed and was replaced with the coil from a most probably used two-way valve from another machine. (If it was a new coil it would have no type plate so the original type plate would have been used; if the entire new solenoid valve was replaced then the new type plate would be correct for the three-way valve). It is interesting that the replaced coil is from a two-way valve - most probably someone had a disfunctional second machine - possibly purchased as scrap for spare parts - and used the coil from the two-way valve not from the three-way valve, one supposes because the coil of the other three-way valve would be likely to fail sooner (and I am supposing had already failed).

This three-way valve failed in June 2019 (almost certainly for at least the second time since 1996, because of the type plate). When the machine was cold, the valve worked perfectly, but when the machine was hot (with effect from quite close to normal operating temperature) it suddenly ceased to operate. I thoroughly cleaned the mechanical parts - no effect (worked perfectly cold, worked while warming up, suddenly failed when hot). Several times I observed that when newly hot (almost but not quite operating temperature), pulling the first shot would work, but less than one minute later the valve would no longer operate - the water from the boiler is hotter than the valve, therefore heats it up further.

I had no spare part, therefore I swapped the coil with the two-way valve - i.e. the very hot three-way valve now has the good coil from the cooler two-way valve, while the two-way valve now has the defective coil from the three-way valve. Resistance of the two coils is identical. The result: now the three-way valve functions perfectly, whether hot or cold. The two-way valve functions perfectly when fully cold, but completely fails when slightly warm. This proves beyond question that the coil construction is defective, while the mechanical parts function correctly. This coil is fully encapsulated, and the manufacturer's catalogue states that the magnetic path is included within the encapsulation, therefore unlike the Ulka I cannot test the magnetic path separately. But taking together the results for both the Ulkas and the Lucifers, it is reasonable to conclude that (a) the heat exposure is responsible for the degradation, and (b) it is the iron magnetic path alone that is affected, not the electrical coil itself.

So what is changing in the iron, and can the changes be reversed?

Attached is a pdf about renovation of the Ulka, in German but with extensive pictures of the inner workings, taken from https://www.kaffee-welt.net/index.php/Thread/513-Schwingankerpumpe-ULKA-Zerlegen-Zusammenbau/
The iron sleeve can be seen in picture 2 (top in two parts upright) and picture 18 (yes - I'd forgotton the sleeve is in two parts - probably forces the magnetic field to go through the piston in the middle).

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#### Alec_t

Joined Sep 17, 2013
10,598
coil/magnetic path construction - the coil which is fully encapsulated, the inner iron sleeve (which goes through the hole in the middle of the coil, and the plastic-encased piston assembly goes inside the iron sleeve), and the rest of the magnetic path which consists of iron which is screwed together around the coil, in contact with the ends of the iron sleeve. There are no moving parts
I must be missing something. If there are no moving parts, what does the piston do and how does this assembly act as a pump? Can you post a sketch of the assembly?

Edit: Sorry, no sketch required. Pic 4 gives enough info. I reckon wear or distortion of the plastic cylinder would increase the clearance around the piston and account for the pressure drop.

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#### Bhante

Joined Dec 15, 2009
42
I must be missing something. If there are no moving parts, what does the piston do and how does this assembly act as a pump? ... I reckon wear or distortion of the plastic cylinder would increase the clearance around the piston and account for the pressure drop.

#### Alec_t

Joined Sep 17, 2013
10,598
Perhaps the effect you observe is down to irreversible magnetic aging (though I haven't found any relevant info on the magnitude of aging effects) ?

#### Wolframore

Joined Jan 21, 2019
1,479
Keep in mind that the resistance of copper wire will rise with temperature. Coil temperature increased by 1C is approx 0.393% higher per degree above 20C. so your Rt-hot = Rt20C(1+ 0.03939 (t-hot-20)) The increased resistance means that you are losing current to and therefore the torque needed to activate your solenoid. It is also possible that the iron core has degraded from corrosion which reduces the effect of the remaining magnetic flux. Also possible that your coil is damaged. Perhaps you are no longer getting cooling to your solenoid for whatever reason... it could be bad coils leading to too much current which would make things hotter... there are still many variables.

A diagram would help those of us with limited time to read your detailed but very long descriptions... picture = 1000 words.

#### Janis59

Joined Aug 21, 2017
895
Have no any knowledge about cofefee (as spelled Trump the mighty) but I have it about gas relays of LPG vehicle feed systems. The poor constructors overwhelmingly lacks the good materials therefore they instead of strong magnet with good coil are using the dribbling hair-sized wire and pencil size magnet core producing good if the few grams what are intended to pen probably 20 kg/cm2 pneumatic circuit. So after few years it may start to fail. Reasons are few:
1) the smear over core surface is degraded and filled with dust & teardowns so becoming thick and black. Core just strucks in. Clean it and it will become "refurbished".
2) As I told the magnet is very weak, thus there is pneumatic amplifier used, consisting of tiny hole into core and needleshaped cone in saddle. So, magnet must open not a 8x8 hole but only 0,5x0,5 mm hole instead. And when it is opened then pressure difference is doing the main job. Logically the hole and saddle are gradually wearing off thus the permanent leak is the result. Defect is hardly refurbisheable - just changing the details.
3) About coil insulation - the lacker serves well if dark sort 120C, if orange-light then 150C and if colourlessly transparent sort then 180C. Most probable You have a orange-yellow one, then cofee T of 100 plus magnet compulsive-obsessive own heat means hardly over 150C in bottom layers. Result - ONE shorted turn= giant no-load current plus manyfold decreased magnetic force. That is factually, transformer with shorted turn and wonder - why nothing is getting out. Refurbish by rewinding.

#### Bhante

Joined Dec 15, 2009
42
Thanks for your colourful comments Janis, but the coil (electrical part) is fine. Self-heating is fairly minimal - without the heater on it is well under body temperature. These Lucifer valves are very high quality, with ruby valve seating according to the catalogue.

#### Wolframore

Joined Jan 21, 2019
1,479
But your issue is when the system gets hot! So self heating might not be a problem but your coils are affected by heat.

#### Bhante

Joined Dec 15, 2009
42
According to a paper I found on the internet the magnetic flux INCREASES with heat. Somewhat counterintuitive, but that is what the paper claims. It is highly unlikely the tiny increase in resistance of the copper will have a significant effect. The problem evidently has something to do with changes in magnetic permeability of the iron as a result of prolonged heat exposure, or something along those lines.

#### Wolframore

Joined Jan 21, 2019
1,479
Hmmm would love to see that...

From what I recall magnetic force is not affected by heat:

F = (n x i)2 x magnetic constant x a / (2 x g2)

Where: F = Force, i = Current, g = Length of the gap between the solenoid and a piece of metal, a = Area n = Number of turns, Magnetic constant = 4 x PI x 10-7.

there is no heat in this formula however current is affected by resistance which is affected by heat.

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#### shortbus

Joined Sep 30, 2009
7,237
From what I recall magnetic force is not affected by heat
But steel is, only not at the coffee temperatures. It was once used to judge the critical temperature when heat treating.

"It is magnetic at normal temperatures, but critical temperature (about 1420° F for simple carbon steel) also makes steel non-magnetic. But it doesn’t become magnetic again until it drops to about 500° F." From, https://koboldpress.com/real-steel-the-mystery-of-steel-magic-and-magnetism/

#### shortbus

Joined Sep 30, 2009
7,237
But your issue is when the system gets hot! So self heating might not be a problem but your coils are affected by heat.
That would be my take too. In the beginning of the thread he had 2 valves, and when switching the coils the valves stopped working when hot with one of the coils, tells me it's that coil going open at temperature, a pretty common thing.

#### Bhante

Joined Dec 15, 2009
42
when switching the coils the valves stopped working when hot with one of the coils
Do please read the original post. If you post claims that contradict what I said it becomes nothing more than idle gossip!

F = (n x i)2 x magnetic constant x a / (2 x g2)

Where: F = Force, i = Current, g = Length of the gap between the solenoid and a piece of metal, a = Area n = Number of turns, Magnetic constant = 4 x PI x 10-7.
Where is permeability in this equation? Where is any representation of the qualities of the magnetic path, which affects the result by many orders of magnitude. Even different types of ferrous metal make a difference of the order of well over 1000 fold. Therefore there clearly must be something drastically wrong with your equation.

I think the problem is probably in your "magnetic constant" which is not a constant but permeability, which varies with temperature. In which case your equation is valid only in a vacuum.

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#### Janis59

Joined Aug 21, 2017
895
Magnetic materials indeed have some individual temperature at which the magnetism are instantly lost until cooled. This is kind of Curie temperature like in dielectrics. But for steel based cores it stand about 600 C to 800 C - no any least suspiction it may happen to Yours. However another happens if we have a deal with ferrites, there the 150-250 C may be enough.

#### Alec_t

Joined Sep 17, 2013
10,598
Did you find any useful info on the irreversible magnetic aging effect, beyond what is in the post #7 link?

#### Janis59

Joined Aug 21, 2017
895
Counted which is the 7th (somehow I cannot see anywhere its number) and looked into link. Need to say, in all my radiofhysicist carrier of about 50 years long I never heard or seen anything like that. If it is true, then effect is simply negligible small. Oldest transformer I kept in the hands was from 1897, and still it is operable.