Densiometer Questions

Hello All, I finally acquired a used densiometer (Tobias Model TBX). It seems to respond to changes in density but I would like to check it's linearity across it's range. Would a calibrated step gauge (Kodak, Stouffer) be the thing to get?

It has two internal adjustments and the exterior "zero" adjustment. I assume the internal adjustments are for calibrating the linearity..

If anyone has any information or suggestions on the unit I would appreciate it.

I hope this topic is in the correct area...

Thanks,
John.

JHannon said:

Hello All, I finally acquired a used densiometer (Tobias Model TBX). It seems to respond to changes in density but I would like to check it's linearity across it's range. Would a calibrated step gauge (Kodak, Stouffer) be the thing to get?

John.

Click to expand...

Unless you plan to do very precision work the uncalibrated step wedges made by Stouffer and Kodak are entirely adequate for film testing, and much less expensive than the calibrated versions.

I am not familiar with the Tobias and can not help you with the spefic calibration of the unit. However, nearly all densitometers have a Zero feature which allows you to calibrate the unit back toi Zero by taking a reading with no film between the light and sensor.

John,

a densitometer is usually calibrated to 0,0 logD (or 0,1 logD in case of a reflection model) and a density in the upper rage of the meter. It doesn't have to be the maximum density of the meter. A quality step wedge is usually sufficient and can be perfect, if you know someone who owns a calibrated densitometer who can provide the "real" densities of your wedge.

In my experience the uncalibrated Stouffer step wedges are within 1-2 units of density. My densitometer was just recertified by Xrite and if I can be of help in verifying your readings email me off post.

I personally would not try to adjust the linearity of the instrument until I was sure that it was needed.

You might want to call the mfg and get a calibration target.

It has been years since I used a Tobias, so I hope I remember correctly.

The two "internal" adjustments are in the back if I remember correctly, these are to adjust "zero" and "drift" from a calibrated step wedge.
The zero is obvious, the drift is to make sure you have measurements that agree with the "curve". For example if you have a calibrated step wedge and you read step number 5 at 0.65 density units and your meter reads 0.69 then you adjust the "drift" to make it read 0.65.

The "zero" in the front is used when you want to baseline your readings. IOW you want to read the transmission only with out the b+f.

Since this is "uncalibrated" and they did not sell it to you with the standards, the simplest thing is to take Don on his offer to "calibrate" a step wedge for you. Buy a Stouffers uncalibrated step wedge, send it to Don, have him read the densities and write them on the outside of the packaging envelope and send it back to you. You now have a "calibrated" step wedge that you can use to calibrate your densitometer.

Thanks to everyone for your help. I contacted Donald and I am going to send him a Stouffer 21 step gauge and he will record the readings for me.

That should be a good start to using this instrument.

Thanks all!

John.