There are several "densis": http://www.heilandelectronic.de/html...rview_main.htm
"I know that I need to get a densitometer so I can properly EI rate my film ". I also thought I needed a densitometer until I read about a method described by Horenstein in his Beyond Basic Photography. Basically it involves sandwiching a Kodak No. 96 ND 0.10 filter and a developed but unexposed frame and comparing that density with frames exposed to Zone I at various EI's. Not very sophisticated but it sure seems to have worked for me.
I'll add a vote for the Heiland product. I bought one a few years back from Versalab and it has been great. I check it with the calibration strips every so often but as others have said it has not budged. Very solid, and it does both transmission (for negatives) and reflection (for prints) for a reasonable price compared to buying a new X-rite for example. It is a breeze to use also.
A cheap and flexible way to do some simple densitometry without expensive equipment is using step tablets, such as the ones made by Stouffer, for comparative analysis.
In my case I've used mine to objectively get me into the ballpark with a film's exposure and processing variables. Once I have that objective baseline in hand, final adjustments are then made subjectively using my eyes and judgement.
But a more valuable use of the tool for me is ongoing process control after I've settled on a calibration. Every so often when I'm out with the camera I will look for examples of lighter and darker continuous tone objects. Whiteish fences in sun, blackish walls in shade. That sort of thing. I then set the lens to infinity, move close to fill the frame, and make quick highlight (development time) and shadow (film speed) test exposures.
It usually only takes a minute or two and a couple of frames. If necessary I could do the same for expansion and contraction tests, but I rarely do. I'm usually just looking for a quick sanity check.
After development all it takes is a quick check under the densitometer to confirm that the calibration is still valid. Reasonably close density numbers are perfectly acceptable, as I'm not running an analytical lab - just a home darkroom.
A Stouffer step wedge (and Ralph would recommend 31-steps) can be all you need to get started.
Add ANY meter that you can use as a "comparator", and you can create a densitometer precise to one "step". Mike Wilde explains how you can meter your negative and then find the step on the scale that meters about the same. I used to do exactly that.
I was able to get densities of my 35mm negatives by customizing a negative carrier for my enlarger. I cut a small window and sliding slot in my negative carrier to hold the Stouffer scale. I used an Omega CS-10 comparator at the easel to cross-reference significant points of the negative to the Stouffer scale.
This is only precise as your Stouffer scale, for example a 21-step scale is only precise to .15 density.
These are the working steps:
1. Place negative in enlarger and turn it on.
2. Null the comparator on thinnest part of neg.
2. Pull out negative carrier to reveal the first Stouffer scale step.
3. Slide Stouffer scale until comparator indicates null.
4. Note the Stouffer step showing in the window. The corresponding density of the step number is your d-Min.
5. Push in carrier, find darkest spot of interest on the neg and null the comparator.
6. Pull out carrier and null again to find the d-Max Stouffer step.
An advertiser here offers an enlarging meter that I believe works essentially the same but can read densities more directly and precisely.
Not long ago I took a chance on a used TR-524, Transmission and Reflection densitometer. I got it working with little effort, but it was a low-risk purchase for me because I have work experience using and maintaining graphic arts prepress equipment.
I agree with Bill, but looking at the first post again, I think the OP already made up his mind to get a densitometer. Many things can be used to avoid this purchase (step tablets, modified spotmeters and some enlarger meters for example), but nothing beats the real thing.
I have used Agfa, XRite and Heiland. The Agfa was great, but it had problems with measuring inkjet prints due to its 'color blindness'. XRite is very popular and easily available used, but mine needed frequent calibration, and XRite charges a lot of money for factory calibrations. The Heiland served me well, it requires little or no calibration, and Heiland even made me a custom units, because I wanted a special function to read absolute reflection densities.