"In all the literature I've read to date on the Johanson Hang-Up Indicizer it seems to suggest that is ideal for figuring out outlet orifice size"

If the literature is from the seller then to use the famous quotation - "They would say that wouldn't they" ■

I have the very greatest respect for Jerry Johanson as one of the principle pioneers of bulk technology. His introduction of the indicizer is based on many years of experience in this field and a fundamental understanding of the theory of particulate solids in relation to hopper design. He has seen to heed for a simpler industrial tool to evaluate the flow properties of a bulk material and uses a technique that has a quite separate approach to the Jenike method, and hence does not give a comparable answer.

It must be remembered that the Jenike calculations are based on a developed flow stress field, that is, the conditions pertaining when flow has already taken place and then stopped. From this situation, the strength of the Jenike test procedure is that it can specify the conditions that are required to guarantee flow in respect of orifice size and hopper wall angle based on the yield loci in a specifically prepared state of density that reflects the operating condition of interest.

An inherent weakness of the basic method is that it cannot relate to initial fill conditions and features such as impact surcharges, this is why considerable expertise is needed by specialists to provide the interpretation and reliability in important installations.

The Johanson device essentially measures the shear strength of a prepared compact that, in effect, is similar to the mechanics of a consolidated bulk material over an orifice. The situation there is that self weight will induce collapse if the opening size is large enough, whatever the strength of the product, because the vertical stress above the orifice boundary increases linearly with diameter. Unfortunately for clarity, the Johanson apparatus is linked to a 'black box’ and unsubstantiated formulae for the production of predictions, and is therefore not well accepted by the scientific community. This test is however easy to conduct and produce results, so has become a useful tool for industry. A simpler piece of kit that adopts this form of test and also publishes that basis on which it is founded is the Ajax vertical shear test. See www.ajax.co.uk for a description and the fundamental calculations. As a 'cheap and cheerful' tool, this later device is invaluable for an initial evaluation of a products potential to arch and rathole and, within the limitations that are readily apparent, will provide a guide to the size of opening necessary to stimulate initial flow from a settled bed. It is not however a substitute or comparable method with the Jenke test, but reflects a different situation that may well be of less or more interest.

A deep and sensitive understanding is required to recognise the implications of the assumptions, equipment limitations and weaknesses and myriad variations in particle composition and packing structures in relation to powder testing equipment. I have considered this subject for many years and designed interesting devices for measuring tensile and cohesive strength. It has to be accepted that solids flow is a complex phenonenon and take these tests with the limitations they have. The proof of the pudding is in the eating and, if it works, stick with it and your experience will grow. If it's important and your inexperienced, consult an expert. ■