---
title: "Environmental SEM: Keep Water, Control Charge Interactive Lab | Sandbox Physics"
description: "Can chamber gas stop a wet insulator from drying out and charging without blurring the probe? Partly."
canonical_url: "https://sandboxphysics.com/labs/environmental-sem/"
language: "en"
page_type: "lab"
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  zh-CN: "https://sandboxphysics.com/zh/labs/environmental-sem/"
last_modified: "2026-10-27"
---

E14 · Specimen environment

# Environmental SEM: Keep Water, Control Charge

**Can chamber gas stop a wet insulator from drying out and charging without blurring the probe?** Partly. Water vapor near saturation keeps the specimen wet, and the ionized gas amplifies the signal and neutralizes charge. The same gas also scatters some beam electrons into a wide skirt around the probe. A stronger signal does not mean the specimen is better preserved.

Tune water vapour, specimen temperature and gas amplification to keep a wet insulator hydrated and its charge neutralized. Watch what each change does to the signal from the same specimen.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

Model assumptions

The Lab shows expected teaching values, not acquired images or real instrument settings. Water saturation pressure follows Murphy–Koop. Scattering cross sections, gas gain, charge-neutralization feedback and the rate of drying or wetting are assigned. The specimen repeats periodically, and scattered electrons form a Gaussian skirt around the beam. Discharge, calibrated transport, heat flow, real fluid motion and quantitative sample chemistry are not modeled. Hardware and surface relief are each enlarged by their own factor.

Physics tutorial

## Environmental SEM: Keep Water, Control Charge

**Background** In an environmental SEM the chamber holds water vapour or another gas instead of a vacuum. The gas can keep a wet specimen from drying and supply positive ions that cancel the charge on an insulator, but it also scatters part of the electron beam.

**Why it matters** Water vapour keeps the specimen wet, and ions made in the gas neutralize its charge. The same gas also scatters the probe, so each setting trades one effect against another.

Start with the essentials

- **Focus question**: Can chamber gas stop a wet insulator from drying out and charging without blurring the probe?
- **One-sentence intuition**: More signal does not mean a better-preserved specimen.

### Core mathematical model

#### Unscattered fraction along the gas path

$$
\begin{aligned}n_g&=\frac{p}{k_B T_g}\\f_0&=\exp(-n_g\sigma L)\end{aligned}
$$

The ideal-gas law gives the number of molecules per volume. With independent collisions, the fraction of electrons that crosses unscattered falls exponentially with density, cross section and path length. The cross section is assigned, not taken from a fitted gas table.

#### Charge balance on the specimen

$$
\begin{aligned}C\frac{du}{dt}&=I_- - I_{\rm ion}-g u\\I_{\rm ion}&=\min(I_{\max},g_i u)\end{aligned}
$$

The specimen potential is negative, so the equation follows its size, which is never negative. The ion current can never exceed the assigned supply of ions from the gas.

#### Humidity and water retention

$$
\begin{aligned}r&=\frac{f_w p}{p_{\rm sat}(T_s)}\\H&=\exp\!\left[-\frac{t}{\tau_h}\max(0,1-r)\right]\end{aligned}
$$

Humidity is the physical ratio of water partial pressure to saturation pressure. Water loss below saturation uses an assigned ten-second relaxation time; it is not a predicted evaporation rate.

### Common difficulties

#### What the model leaves out

**Typical misconception** If my settings pass the task, they will work on a real instrument.

**Better mental model** The Lab shows expected teaching values, not acquired images or real instrument settings. Water saturation pressure follows Murphy–Koop. Scattering cross sections, gas gain, charge-neutralization feedback and the rate of drying or wetting are assigned. The specimen repeats periodically, and scattered electrons form a Gaussian skirt around the beam. Discharge, calibrated transport, heat flow, real fluid motion and quantitative sample chemistry are not modeled. Hardware and surface relief are each enlarged by their own factor.

### Run the experiment

1. 01

   #### Predict the tradeoff

   Start in vacuum. Look at the negative specimen potential and how the deposited charge splits into neutralized, leaked and stored charge.

   **What to observe:** Without gas the probe has no skirt, yet the specimen is still not stable.

2. 02

   #### Balance the environment

   Try the balanced preset, then warm the specimen or replace water vapour with nitrogen.

   **What to observe:** Gas can neutralize charge while the specimen still dries.

3. 03

   #### Keep the probe

   Lengthen the gas path and inspect the scattered contribution below the apparatus.

   **What to observe:** Detector multiplication cannot restore a scattered probe.

4. 04

   #### Check the full target

   Adjust the settings until humidity, retained water, condensate, unscattered fraction and potential all meet the target.

   **What to observe:** Display gain changes colours only.

Physical conclusion

Read the environment, specimen and collected signal together.

Fields & matter · Starter path

## From a field to an electron microscope

Map an electric field and steer a charged particle. Then find the electron's wavelength and use electron beams to see a crystal and a surface.

[Start here → **Make an Electric Field Visible** Move the probe around a dipole and check that the field always points downhill on the potential landscape. Observe · 8 min](https://sandboxphysics.com/labs/electric-field/)

1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/)
2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/)
3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/)
4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/)
5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/)
6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/)

[View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)