metwurst is looking to replace its horrid little radial vacuum gauge with something with greater precision.
On offer is an electronic analogue gauge with a range from 5 torr to 2 millitorr, at a reasonable price. Actually the unit does measure to atmospheric pressure, but there's only one graduation on the scale between 5mmHg and ATM.

In the eyes of an experienced bee, is this going to be a useful unit for the kinds of vacuum distillations most frequently carried on here (allylbenzenes, related ketones, and phenethylamines), or is a larger range needed?

metwurst's  mechanical vacuum pump attains in the neighborhood of 2x10-3 torr on paper, so achieving a vacuum in that range isn't a problem, but is it a useful range for this kind of work?

That gauge would really be on my wish list for useful gadgets, especially with a pump like yours. How much is it?

It's about $350 US, which is relatively cheap compared to the >$1000US quotes that have been furnished by local suppliers.
Searching the Hive and net show that digital gauges from 20,000-0 millitorr around ~$145US, but there are no local suppliers of such units that metwurst has been able to find.
metwurst isn't overly concerned about the price if the unit is suitable, but it's a little concerned that it may have to perform distillations at pressures greater than 5mmHg.

When you perform distillations at pressures >5mmHg, you do not have any use for any that precise measurements, and almost any gauge will suffice. The bp difference of a compound between 0.5-1.0 mmHg is larger than between 5-10mmHg.

Guys,

Have you looked at this one (VG64):

http://www.supco.com/images/05%20vacuum%20gauges.pdf

It's cheap and measures down from Atm to 1 micron.  Comes highly recommended!

Thanks!

metwurst has only used the pump twice, for distillation of MDP-2-P, and at both times the distillation temp was 108°C. The nomograph indicates that the system was in the neighborhood of 2 torr if we accept that MDP-2-P boils at 280°C@760mmHg.

I think I will buy the unit, and then seek a full scale (760-0) torr lower precision unit for other purposes at a later date.

Thankyou for your assistance Rhodium.

Ps: Is it just me, or is the Java nomograph at

http://www.chemsoc.org/exemplarchem/entries/pkirby/exemchem/Nomograph/Nomograph.html

inaccurate?
It suggests water will boil at -17°C at 30mmHg!

Yes, there are several members around here saying that applet is on PCP or something... I have removed the link to it from my page today.

The VG64 turns up a lot on TFSE and everybody seems happy with it.
Since metwurst's last post, it's done some more searching, and is now vacillating between the VG64 and the unit being offered to me, since they seem equally well suited to what it's doing.

One final question might clinch the deal:
The VG64 has a thermistor type sensor, which is susceptible to damage by water vapour and everything else.
The unit being offered uses a Hastings DV-23 thermopile sensor, about which I've not been able to find anything regarding damage by vapour.
(datasheet is at

http://www.chell.co.uk/hastings/pdfs/gtubes.pdf

)

Is anyone aware if the thermocouple type gauge is more robust than thermistor types?


PCP indeed. :-)

All I know is that thermistor resistors are generally encapsulated inside a glass bead... so unless you are working in conditions that corrode glass, you should be fine.

As long as you clean the sensor after every operation, it should last for some time.

A little google will reveal a  < 169.00 vacuum gauge that reads 12 scales and operates in the 15 torr down to .001 torr range, is digital and portable... won't work on aspirators( not enough vacuum).. great for your pumps though...

My aspirator pulls about 9-10 torr, FYI.  Or is that uncommon?  But yeah, probably not worth buying the gauge for a vacuum source that will only show up on the weak end of the scale...

so why do some vacuum pumps advertise an absolute torr rating along the lines of 2.2x10^-3 ?  according to the chart on rhodium's site, that vacuum doesn't really exist.

It does exist in a clean, dry argon atmosphere (think physics lab), but as soon as you introduce chem lab conditions with traces of moisture, volatile solvents, rubber tubing and pump oil used more than once, reality interfers and your vacuum drops to 0.1 mmHg or worse. Noone working in a clandestine lab has gotten a better vacuum than at most 0.01 mmHg (and that only the first couple of times with a new pump).