My question is what is usually in the industry. I could measure it with a ultrassonics, but I don't know if that's a standard. There are some 4-20mA sensors to dive in the tank?
Problems with ultrasonic devices are fumes, and though not sure on the concentration of H2SO4 you are using, this can cause a problem and you might even have ashes as well.
How much can you spend and how accurate does it need to be? Is it only H2SO4 - or a mix of other ionic compounds?
1. Use an autotitrator and sampling system - probably accurate to 1%, I'd guess you'd spend about USD$15k on the bits (excl installation)
2. OR - A combination of temperature and conductivity should give you a rough concentration. I think I've used this before I just can't remember where I got the lookup chart from.
In-Line Process Refractometers are also used to measure H2SO4. Through the use of intrinsic safety barriers (if needed), the sensor can be operated in any hazardous area. Should anyone want more information please visit our chemical document, http://www.lsc-e-net.com/pdf/chem.pdf.
I would be inclined to go with the ultrasonic (or radar), but forst check with a few local reps. Make them earn their wage and get them do give you some advice.
Ultrasonic sound velocity is the recommended method for concentrations above 70%. The sound velocity is just about linear with the concentration of H2SO4 in this range without any strange effects as with density, conductivity or refractive index. Rhosonics sells analyzers including Hastelloy probes that can be fit in a tank or in a pipe line. Check our their website http://www.rhosonics.com.