Does anyone have any information on electrical conductivity of 50% propylene glycol with inhibitors? I have a system that is using heated glycol to heat make-up air. The glycol is heated in a shell and tube heat exchanger, and I need to detect any glycol leaks into the condensate by measuring conductivity.
Alternatively, is there something I can add (without deleterious effect) to increase the conductivity?

Measuring glycol concentrations by electrical conductivity is not recommended, especially in process systems, heat exchangers, etc/Forged Steel Valves.  The quality of the water will almost completely determine the conductivity, rather than the glycol. 

The equipment referenced above is OK for small stuff, but large chilled water systems are often measured by specific gravity - the same method typically used to check the antifreeze in your car.  However, this is not a reliable indicator of leaks, and it requires periodic sampling - not something that can be used in a control loop.

A second method in favor these days - one much more accurate -  is measurement with a refractometer.  Handheld refractometers are economical at only $200-$300 each.  For process applications, an online digital model is preferred.  Those can cost a thousand dollars or more.  However, they can continuously monitor mixtures under a wide variety of conditions.

Well, I suppose you can try testing for it yourself - put in a conductivity meter output to a DDC system.  Check for your typical water conductivity (including chemical treatment), then mix in a little glycol.  If you can measure it with the conductivity meter output, then make that threshold your alarm setpoint.

My concern is that you will not be able to reliably measure increasing concentrations to detect a leak, or reliably enough to prevent damage to your boiler - but maybe so.

The best that I could find several days ago was the graph below.  This one is for Ethylene Glycol, not propylene.  I found it at:
http://www.meglobal.biz/literature/product_guides/MEGlobal_MEG.pdf and
http://www.meglobal.biz/literature/product_guides/MEGlobal_DEG.pdf

The second one is for "Diethylene Glycol", but I don't even know what that is.  In any event, these are not with inhibitors.  What you can deduce from the graph is similar to everything I've read about glycol.  You may get a small spike in conductivity as glycol concentration is initiated, then it rapidly declines to very little.  That would seem to be a contradictory mechanism to test for a leak.

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