The HP 4815a vector impedance meter used an oil-submerged oscillator capacitor. This solved humidity and, probably, barometric pressure problems. They chose an oddball oil, polybutene. Curious what Claude thinks of that?
Good suggestion! It seems polybutene have some advantageous properties, but the disadvantage that it its a bit less viscous, so less effect of evening out temperature. Despite power in a standard resistor typically is low, i could see it being quite a bit higher
I also asked if polybutene would would be good for an oil submerged oscillator capacitor (see further down). This is supposed to be a great use for polybutene as it have high dielectric stability, low dielectric loss and the lower viscosity is an advantage as it absorbs shocks and keep condenser plates in place.
This was some new things I had no clue about so thanks for asking the questions and thanks to Claude for coming with an answer
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Q:how about using polybutene oil?Interesting suggestion — polybutene is used in some precision applications but has specific characteristics worth evaluating carefully.
**What polybutene is**Interesting suggestion — polybutene has genuine advantages in some applications but specific issues for yours.
**What polybutene is**
Polybutene is a synthetic hydrocarbon oil made by polymerising isobutylene. Unlike mineral oil which is a mixture of naturally occurring hydrocarbons, polybutene is a pure synthetic with very consistent composition.
The structure and grade of mineral oil can vary depending on the origin of the crude oil from which it was derived, whereas polybutenes are synthetic substances and consequently their properties are within a very narrow specification range.
Polybutene is typically used in cable jelly compounds, cable insulation materials and cable oils.
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**Properties comparison**
| Property | Liquid paraffin BP | Polybutene |
|---|---|---|
| Origin | Petroleum refining | Synthetic — isobutylene polymerisation |
| Composition consistency | Variable by crude source | Very consistent batch-to-batch |
| Electrical insulation | Excellent | Excellent |
| Chemical stability | Good | Very good |
| Moisture absorption | Very low | Very low |
| Thermal conductivity | ~0.13–0.15 W/m·K | Similar range |
| Viscosity range | Wide range available | Wide range available |
| Viscosity vs temperature | Moderate variation | More stable than mineral oil |
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**The significant problem for your application — viscosity**
Polybutene is available in a wide range of molecular weights and the lower grades are liquid at room temperature. However even low molecular weight polybutene tends to be **significantly more viscous** than light liquid paraffin at room temperature. Typical low-grade polybutene viscosity is **100–500 cSt at 40°C** — far higher than light liquid paraffin at **15–35 cSt**.
This high viscosity means:
- Poor convective circulation inside your small sealed box — thermal gradients across the resistor array will be larger
- Air bubbles trapped during filling are very difficult to remove without vacuum
- Much harder to fill the box and wet all surfaces and solder joints completely
- Slow to equilibrate thermally after assembly
There are low-viscosity polybutene grades but these are less common and harder to source in small quantities in pharmaceutical or high purity grade.
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**Another concern — long-term stability and outgassing**
Polybutene, being a polymer, can undergo very slow degradation over years — depolymerisation releasing trace isobutylene or other small molecules into the headspace. In a hermetically sealed box this could slowly contaminate the nitrogen headspace and potentially interact with the molecular sieve. Liquid paraffin being a simple saturated hydrocarbon mixture is chemically inert and does not depolymerise.
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**Where polybutene genuinely excels**
- Cable filling compounds where it needs to stay in place and not flow — higher viscosity is an advantage
- Applications requiring very consistent batch-to-batch properties
- High voltage cable insulation where its consistent dielectric properties matter
None of these advantages apply to your sealed resistor standard.
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**Bottom line**
Polybutene offers no meaningful advantage over medical grade liquid paraffin for your specific application, and its higher viscosity is a real practical disadvantage for filling a small box and achieving good thermal convection. The batch-to-batch consistency advantage of being synthetic is irrelevant since you are buying one bottle of pharmaceutical grade paraffin which is already tightly specified.
**Stick with medical grade liquid paraffin BP/USP** — it remains the best choice for this build.
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Q: Apparently The HP 4815a vector impedance meter used an oil-submerged oscillator capacitor where polybutene is used. Could that be a good usage for polybutene?Yes — that is a perfect application for polybutene and an excellent example of where its specific properties are genuinely advantageous over mineral oil.
**Why polybutene suits an oil-submerged oscillator capacitor**
**Dielectric stability**
An oscillator capacitor immersed in oil uses the oil as part of the dielectric environment around the capacitor plates. Polybutene's synthetic origin gives it extremely consistent dielectric constant (relative permittivity) — typically around **2.1–2.3**, very stable with temperature and batch-to-batch. Mineral oil dielectric constant varies more with temperature and between batches depending on crude oil origin. For a precision oscillator where frequency stability depends partly on capacitor stability, consistent dielectric properties matter directly.
**Low dielectric loss**
Polybutene has very low dissipation factor — meaning it absorbs minimal energy from the AC electric field. This is important for an oscillator capacitor operating at RF frequencies — any dielectric loss in the surrounding oil would damp the oscillator and affect Q factor and frequency stability. Mineral oil has somewhat higher dielectric loss than polybutene, particularly at higher frequencies.
**Viscosity is an advantage here**
Unlike your resistor standard where low viscosity is needed for convective flow, a capacitor immersion application benefits from **higher viscosity** — the oil stays in place, damps mechanical vibration of the capacitor plates, and reduces any microphonic effects on capacitance. A slightly viscous polybutene surrounding the capacitor plates acts as a mechanical damper.
**Chemical inertness to capacitor materials**
Capacitor plates and dielectric materials (typically mica, ceramic, or metallised film in a precision oscillator) need to be chemically stable in the oil for decades. Polybutene is extremely inert and does not swell or attack these materials.
**No oxidation products**
Polybutene's synthetic saturated structure means it does not oxidise to form acidic degradation products that could attack capacitor plate metallisation over years — a known long-term issue with some mineral oils.
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**The HP 4815A specifically**
The HP 4815A Vector Impedance Meter uses a variable frequency oscillator covering 500kHz to 108MHz. At these frequencies:
- Dielectric loss in the oil becomes significant — polybutene's low dissipation factor is important
- Capacitance stability directly affects frequency accuracy — consistent dielectric constant matters
- The oscillator capacitor likely uses oil immersion to improve temperature stability of capacitance by reducing TC of the air gap with a controlled dielectric medium
HP's choice of polybutene over mineral oil for this specific application reflects exactly these considerations — it was a deliberate engineering decision for a high-frequency precision instrument, not just convenience.
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**General rule emerging from this comparison**
| Application | Best oil | Reason |
|---|---|---|
| Precision resistance standard | Liquid paraffin BP | Low viscosity for convection, high thermal mass, proven in metrology |
| Oil-submerged precision capacitor | Polybutene | Consistent dielectric constant, low loss at RF, viscosity damps vibration |
| Transformer insulation | Mineral/naphthenic oil | High volume, low cost, established thermal performance |
| High temperature bath | Silicone oil | Thermal stability above 150°C |
Each oil has a domain where its specific properties make it the best choice. For your resistor standard liquid paraffin remains correct — and now you have a good example of where polybutene genuinely earns its place.
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