Hello!

I just had my older lr44 battery replaced with a sr44 battery and noticed that there is a 1/3 stop difference with sr44. Also the sr44 seems to give 6.2V when lr44 gave 6 volts. Is there a possibily of damage to the meter by the 0.2V increase? The battery compartment has a small marking that reads 6V..

-- Tatu J Laitinen (laitinentatu@hotmail.com), December 07, 2001

Answers

The Pentax digital spot meter relies on a flat discharge curve from a silver battery for regulated supply voltage. Lithium batteries exhibit a gradual voltage falloff as they discharge, causing the reading discrepancy you noted. No damage, permanent or temporary, results from using either battery; new silver versions (unused, open circuit) typically read closer to 6.35V. The most important thing is to stick with silver and conduct your film speed testing using that type battery. Absolute readings don't matter, as long as they're consistent and your system is calibrated to them.

-- Sal Santamaura (santamaura@earthlink.net), December 07, 2001.

This question was asked before: http://hv.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=003rG1

-- Michael Briggs (MichaelBriggs@Earthlink.net), December 07, 2001.

Well, I didn't see it! I'm glad it was asked (and answered) a second time.

-- Doremus Scudder (ScudderLandreth@compuserve.com), December 08, 2001.

I did a little more research on which batteries are suitable for the Pentax digital spotmeter, and which is the best.

The manual that came with my meter (copyright 1988, printed 1994) lists the power source as "one 6V silver oxide battery or equivalent alkaline battery". I don't think 6V lithium batteries were available to consumers then, so we shouldn't concluded from this that lithium batteries are unsuitable.

Eveready has an excellent website for engineering data: http://data.energizer.com/. Under "Battery Index Guide: Active Batteries" there are datasheets for the A544 Alkaline battery and the L544 Lithium battery. Under "Discontinued Batteries" there is a datasheet for the 544 Silver Oxide Battery.

All three datasheets show the discharge curve, voltage vs time, for a battery subject to a 30,000 ohm load. The initial voltage of all there batteries is 6.4 volts, perhaps 6.5 volts for the Silver Oxide. Conclusion 1: since the Lithium battery has the same max voltage as the other two, the voltage of the Lithium battery will not damage the meter.

All three curves show a decline of voltage with time. The curve for the lithium is almost ideal: there is a small drop in voltage over the first 100 hours, after which the voltage falls very slowly until reaching 5.5 V at 700 hours. Past 700 hours, the voltage rapidly falls. The Silver Oxide battery has a plateau just above 6.0 V for the first 400 hours, then starts to decline. The Alkaline battery shows a continuous and fairly rapid falloff in voltage without any plateau.

Comparing the curves, the Lithium battery has the best curve with little voltage variation for many hours, the Silver Oxide battery has a discharge curve that is almost as good, and the Alkaline battery has a comparitively poor curve. Conclusion 2: the Lithium and Silver Oxide batteries deliver relatively stable voltages, with perhaps the edge to Lithium battery. Conclusion 3: since Pentax thought that the alkaline battery was suitable, and the alkaline battery has the least stable voltage, then all three batteries are suitable. I don't know whether the light reading of the Pentax changes as a function of battery voltage (perhaps I will do some tests). Conclusion 4: If the Pentax meter reading changes with battery voltage, then the Lithium and Silver Oxide are better choices, with the Lithium probably being the best.

Lithium batteries have advantages of long shelf life and excellent cold performance. With the low use time per day and low current consumption of a light meter, shelf life is probably the main determinant of the useful life of the battery. The cold performance will benefit winter time photography.

I guess Eveready discontinued the 544 Silver Oxide battery because they thought the L544 Lithium battery to be superior. The A544 Alkaline, while inferior in technical properties, was probably retained because of its lower cost.

Overall conclusions: for the Pentax digital meter, the alkaline should be avoided. For this application, the few dollars cost savings of the alkaline are not worth the reduced performance. At least from Eveready, the Silver Oxide is no longer made. In any case, the Lithium appears to be superior in terms of voltage stability, long shelf life and cold performance. I have been using the Lithium L544 battery and recommend it. I recommend carrying a spare: when the battery finally runs down, it will happen quickly.

-- Michael Briggs (MichaelBriggs@Earthlink.net), December 09, 2001.

Fortunately, Eveready is not the only source of silver oxide batteries in this size, since that chemistry is far superior in the Pentax digital spot meter when consistent readings are important. I use Varta or Panasonic, whichever is available at replacement time.

Before trying lithiums many years ago, I checked with Pentax USA, which advised there would be no damage to my meter. Lithiums exhibit a significantly higher than rated open circuit voltage before any use, and there was concern at that time some equipment could be damaged by it. Pentax said "no problem." Alkalines are also approved by Pentax for this meter.

The essence of this issue is consistency of light readings. I contend that anyone using a digital one degree spot meter is likely to be less than happy with the 1/3 to 1/2 EV change which results from battery voltage drop along the discharge curve of lithiums and alkalines. Saying that a lithium battery "...has little voltage variation for many hours...silver oxide is almost as good..." ignores the meter's great sensitivity to supply voltage. Lithiums continuously drop voltage in use, while silver oxide cells provide a stable reference over their somewhat shorter "service life."

When Pentax designed this meter many years ago, silver cells were ubiquitous, and were apparently relied upon instead of including a voltage regulator. Microcircuits were not quite as inexpensive or available as today, and incorporating an IC regulator was probably overruled to make the product more marketable. My approach is to enjoy the absolutely stable readings a 6V silver cell provides, and simply replace it roughly once a year. Compared to what we spend on other photographic equipment and supplies, it's a small price to pay. Or course, if you're shooting color negatives or something equally forgiving, and don't care about consistency, pop in a lithium. But I can't understand why you're using a Pentax digital spot meter in that case.

-- Sal Santamaura (santamaura@earthlink.net), December 09, 2001.

I suggest that people make their own judgements by examining the datasheets provided by Eveready. I expect that the voltage curves of batteries from other manufacturers will be very similar because the chemistry is the same.

If voltage stability is necessary for the meter, why does Pentax list alkaline batteries as suitable? Regardless of the debate about whether Lithium or Silver Oxide is better, neither is "absolutely stable" and it is very clear that the voltage stability of alkaline batteries is much worse than either. The voltage stability of the alkaline batteries can only be described as terrible.

I can think of four ways to design meter stability: 1) use a voltage-stable battery, like the no-longer available mercury batteries, 2) incorporate a voltage regulator, 3) design the meter to make a relative measurement to normalize away the supply voltage, and 4) have the meter stop working when the voltage drops enough to effect the meter reading.

The typical user will use a battery until the meter stops working. The voltage at which the meter stops working is what really determines the maximum variation in reading (since the three batteries start at almost the same voltage), independent of battery type, assuming that the reading depends on battery voltage, and assuming that the user doesn't replace the battery early.

-- Michael Briggs (MichaelBriggs@Earthlink.net), December 10, 2001.

Yes, discharge curves for this size battery of the same chemistry from different manufacturers will be almost identical.

Voltage stability *is* required for this meter. Pentax's reasons for declaring alkaline batteries "suitable" can only be determined by asking Pentax. Perhaps it doesn't want to restrict consumers to a type of battery that may not be as widely distributed, thereby limiting its market for meters. That would also be consistent with the positive answer I received concerning lithium use.

Possible ways to design a meter are not relevant. This meter, which was designed well over twenty years ago, does rely on the voltage stability of a silver oxide battery, which if memory serves was the only chemistry available in that form factor at that time.

Typical users don't typically use Pentax digital spot meters or ask questions here. I suggest that Tatu and anyone else interested make their own judgements by trying the three battery chemistry types and checking light reading variations at different points along each discharge curve, as I have. Odds are most folks who do so will settle on silver oxide, and will not consider replacing a battery annually to be "early."

-- Sal Santamaura (santamaura@earthlink.net), December 11, 2001.