Reading the MKS 740B baratron vacuum gauge

Cremona, September 2018.
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This page describes how to use the MKS 740B vacuum gauge: we will see the pinout, machine a standard KF-16 fitting and assemble a cute box with arduino to read and display the pressure.
I got gauges and some KF-16 clamps off ebay. Retailers of dismissed laboratory equipment often sell for cheap: 2 gauges for 30 euros. They become very handy when classical needle vacuometers have reached the fullscale. Those probes come in a variety of ranges and can easily reach pressures of 1E-3 Torr.

In the figure you can see the MKS 740B at the left, a Tylon baratron gauge at the right and a KF-16 clamp with sealing o-ring.

Ok, let's get started.

About the gauge

The MKS 740B is a capacitance vacuum gauge. See the manual, HERE. All credit goes to MKS of course.

The code of my gauge is a 740B11TGA2GA, meaning that the model is 740B, has 10 Torr full scale (11T), KF-16 flange fittings (GA), 2% accuracy (2G) and 9-PIN connector (A). Note that accuracy is not with respect to the full scale, but to the measurement.

Electrical connection and pressure range

You power the gauge with two cables and you measure the output from two other cables. Super simple. MKS 740B has some embedded circuitry to measure the capacitance and directly spits out the pressure as a signal from 0 to 10V. 10V is the full scale and 0V is absolute vacuum.
In that case the pressure is simply:

P = FS*Vout/10V.

When the pressure is larger than the full scale you get something more than 10 V (like 11.9V or so). Anyway, refer to the manual. The MKS 740B requires 13 to 30 V.

The pinout is:
Pin 1: Pressure output (0 - 10 V nominal)
Pin 4: Power in (13 - 32 VDC)
Pin 8: Pressure return
Pin 9: Power return
All the other pins are reserved: do not connect them.

The range of the sensor goes from the full scale (10 Torr) to around 0.05%. According to MKS, even lower readings can be performed if the environment temperature is constant and the flow is steady. For a 10 V output, this means the minimum reading at 0.05% * 10V = 0.0005 * 10V = 5 mV. And for a 10 Torr gauge, this is equivalent to 5 mTorr (aka 5 microns). Not bad! Take a look at this table, from the MKS 740B manual: table pressures

Vacuum fittings

The ISO KF (also QF or NW) are standard fittings for medium vacuum up to the lower end of high vacuum. Basically, the pipes have flanges that are hold together by a KF clamp, that squeezes them together acting on the 15deg inclined surface. You should buy the clamps (and the O-rings to put inside), they are not exceedingly expensive, about 10 euros each. Regarding the fittings, you can machine them easily is you wish, as I did. Here are dimensions in [mm] for a simple fitting to connect the vacuum gauge to a hose going into the vacuum jar.

Some pictures of the machining process and assembly..

Pressure measurements

Ok, so here is the box containing the electronics. Yeah, it's just an arduino, shame on me... but you know, it saves so much time!!! The only difference between the MKS and the Tylan gauge is the power supply. The schematic for the MKS 740B electronics box looks like this:

The power supply is written to be 15V. Actually, anything between 13V (minimum supply for the gauge) and 20V (maximum power supply for arduino nano) would do. Note the two resistors making a voltage divider at the output of the baratron: since arduino measures 0 to 5 V we divide the 10V output in the two branches. Actually, to be precise we'd need to avoid the extremes of the acquisition range, but let's neglect this for now. The signal then goes to the analog port A0. You can find the ARDUINO SCRIPT HERE. You will need to download the LiquidCrystal_I2C library. Not sure if it works with windows. With Linux it sure does.

A better schematic employs an operational amplifier to provide a better reading at low voltages. In fact, arduino reads with 10 bit resolution (1024 levels), meaning that the pressure uncertainty due to the digitalization is 10 Torr / 1024 = 10 mTorr. With an OpAmp you can amplify the pressure signal by, say, a factor 100 maybe and you can provide that to another pin. When the signal gets below a certain threshold, you start reading with arduino the amplified signal.
You can see some space left on the perforated board, below, for adding an OpAmp in the near future.

Also thanks to my brother Riccardo for sharing with me the fun of building this small control circuit! :-)

Any comment? Feel free to write me!

Cheers,
Stefano

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