Imported from GitHub: gbochev/vacuum-gauge · commit 2ebb035 · license GPL-3.0

Description

Simple Arduino-based vacuum gauge for Pirani and Thermocouple sensor types

README

Vacuum gauge

Simple Arduino-based vacuum gauge for Pirani and Thermocouple sensor types

Description

While developing a vacuum system used for homemade PVD sputtering aparatus I felt the need of a simple and cheap vacuum gauge that can measure pressures as low as 1 Pa. I was not able to find online a finished DIY project thus I created one. The main idea is to use a standard thermocouple sensor at first (some of them are expensive) and then try to transition to a simple Pirani type sensor which can even be a small lightbulb (the lightbulbs are commonly available and probably have relatively the same resistance and performance so once calibrated one can have very cheap sensor).

This project is still work in progress. Next steps will be to connect a thermocouple that has a datasheet (specifying the pressure in Pa and the mV output) interpolate the points from the datasheet and then place it into the arduino code. After that experiments with commonly available light bulbs can begin. Since the lightbulbs (and some thermocouple gauges) need to operate under vacuum, two relays are added to stop the supply when the system is not under vacuum.

Updates:

v1.1 released; Added support for soviet vacuum gauge LT-4M (ЛТ-4М). The conversion formula that works about right from 260Pa to 1Pa is Ppa = ([mV] ^-2.55) * 266.6 The tube is sensitive in really small range 260 - 0.1Pa where the usable range for me is 260-1Pa (lower than 1Pa the formula is different and it is not accurate).

Tips

As some chinese Arduino clones do not have connected (or have them wrongly connected) protection diodes please be careful (or best advice - do not supply the board by the external supply and the USB at the same time) when you supply the board externally.

Mount all transistors and regulators on heatsinks as they will heat up during measurement.

Some cheap thermocouple gauges that I found are called: ЛТ-2 (LT-2) and ЛТ-4М (LT-4M). They can sometimes be found on ebay for a good price. Also there are some new chinese ones produced by ZHVAC but I have not tested them yet (ZJ-27, ZJ-54) but they should perform much better.

After you connect the vacuum gauge use a multimeter to calibrate the ADC (basically connect it in parallel to the thermocouple gauge's output then calculate the difference between the multimeter and the ADC and set it in the code (it is defined as constant in the loop). Next if you do not have good measurements swap the + and - of the thermocouple - for some reason it does work better even though differential measurement is taken.

Schematic

You can view it here

PCB

PCB is located in the pcb folder. It is created with KiCAD. You can order the PCB directly by using the production.zip (file that contains the exported gerber files).

Arduino code

The Arduino code is located in the arduino-code folder. The code still lacks the ability to measure pressure and it is a mess (I merged several online projects). It measures the pressure in mV. As next steps conversion tables for different sensor types will be added. In order to use it, please install the libraries (Adafruit and the others) that are referenced on the top of the file.