Pentax digital spot meter battery question

Hello!

 

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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..

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.

This question was asked before:

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

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

time.

I did a little more research on which batteries are suitable for the

Pentax digital spotmeter, and which is the best.

 

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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.

 

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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.

 

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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.

 

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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.

 

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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.

 

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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.

 

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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.

 

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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.

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.

 

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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.

 

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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."

 

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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.

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.

 

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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.

 

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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.

 

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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.

Yes, discharge curves for this size battery of the same chemistry from

different manufacturers will be almost identical.

 

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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.

 

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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.

 

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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."

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.

 

Lithium has a "flatter" discharge curve than silver oxide, meaning the voltage does not drop as quickly as most batteries (including silver oxide). That's what "gradual voltage falloff as they discharge" meant, wherever you read it.

Lithium has a "flatter" discharge curve than silver oxide, meaning the voltage does not drop as quickly as most batteries (including silver oxide). That's what "gradual voltage falloff as they discharge" meant, wherever you read it.

That is incorrect. A "flat discharge curve" refers to a battery having stable operating voltage during the period when its rated capacity is being used. It does not have anything to do with how quickly the voltage drops after its rated capacity has already been used by a device.

 

In the two decades since this thread was started, most manufacturers of silver oxide PX28 batteries have discontinued them. As a result, I'm unable to find the data sheets for them that were ubiquitous on the Internet back then. However, this one

 

 

shows exactly how flat the discharge curve of a silver oxide battery is. At the end of its rated capacity, a silver oxide battery's voltage drops precipitously.

 

In contrast, the discharge curve of a lithium PX28 exhibits substantial voltage drop no more than halfway through its rated capacity. This data sheet includes the curve:

 

 

Since the Pentax digital spot meter includes no voltage regulation and its readings vary depending on battery voltage, I have sought out and maintain a stock of silver oxide PX28s. There are still a small number available. I relegate lithium PX28s to applications where their operating voltage can vary without negative consequences.

Edited September 9, 2022 by sal_santamaura

If the Pentax meter needs constant voltage to work correctly I wonder why people like it so much to pay a very high price for it. The Minolta counterpart Spotmeter M (which was more expensive when it was new) is significantly less expensive yet it would keep the same reading with the battery voltage from 1.5V down to about 1.1V or so.

It would be rather unusual for a relatively modern meter such as the meter in question to be too picky about voltage. I am surprised if that is the case.

 

Although not an owner of said meter, a main concern for modern meters tend to be battery leaking properties. For example, silver oxide or lithium is vastly superior to alkaline in that regard.

 

Not too many years ago, I got an old Pentax Spotmeter III which was made for mercury 1.35V (and a regular 9V battery for low light). Like many devices using mercury batteries, this meter will not do well with silver oxide or lithium substitutes.

I use a 9V Lithium battery to benefit from the exceptional shelf life and reduce the risk of leaking and have not recorded miss readings due to this change.

 

Luckily simple hearing aid zinc-air cells have almost the perfect discarge profile as a substitute for mercury. https://www.powerstream.com/zinc-air-discharge-curves.htm

 

Disadvantage is that it only last (unused) ~6 month after activation and that it does sweat a little so it must be removed when the device is left unused (normally the sweating will deposit on the adapter often used with hearing aid cells and not on meter parts).

Advantages, besides the favorable discharge profile, is the very low price, less than $2 for 6 pcs (unless you buy the overpriced Wein cells) and availability in almost all pharmacies.

If the Pentax meter needs constant voltage to work correctly I wonder why people like it so much to pay a very high price for it. The Minolta counterpart Spotmeter M (which was more expensive when it was new) is significantly less expensive yet it would keep the same reading with the battery voltage from 1.5V down to about 1.1V or so.

I cannot explain prices on the used market, but reality is that, as I posted much earlier in this thread, the Pentax digital spot meter will change readings by 1/3 - 1/2 EV over the course of a lithium PX28 battery's rated capacity.

 

Personally, I've had my copy for more than 30 years, and wouldn't trade its simplicity, accuracy, precision and ruggedness for any other meter.

Edited September 10, 2022 by sal_santamaura

It would be rather unusual for a relatively modern meter such as the meter in question to be too picky about voltage. I am surprised if that is the case...

The Pentax digital spot meter was introduced in 1977, 45 years ago. You might consider that "relatively modern," but, as I wrote more than 20 years ago in the sixth post of this thread:

 

 

I find the advantages of this meter to more than outweigh its "pickiness about voltage." :)

The Pentax digital spot meter was introduced in 1977, 45 years ago. You might consider that "relatively modern," but, as I wrote more than 20 years ago in the sixth post of this thread:

 

"...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..."

 

I find the advantages of this meter to more than outweigh its "pickiness about voltage." :)

Well, that dosen't change my my opinion, but live and let live.

The Pentax digital spot meter was introduced in 1977, 45 years ago. You might consider that "relatively modern," but, as I wrote more than 20 years ago in the sixth post of this thread:

 

"...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..."

 

I find the advantages of this meter to more than outweigh its "pickiness about voltage." :)

pickiness of 6.0 and 6.2V with an error of 1/3 EV. Useless meter if indeed it's true. I don't have the meter but I don't think it would have that much error with that much voltage difference.

pickiness of 6.0 and 6.2V with an error of 1/3 EV. Useless meter if indeed it's true. I don't have the meter but I don't think it would have that much error with that much voltage difference.

That's not the difference in question. Scroll back up and click on the link to the lithium PX28 data sheet I provided. Look at the discharge curve. Note that, as it provides its rated 160mAh capacity, voltage drops to 5.4V. Plenty of falloff to drive substantial EV error in a meter that relies on flat constant 6V reference battery output.

 

I've always also paid attention to the initial 6.35V open circuit voltage that silver oxide PX28 batteries exhibit. That quickly falls to approximately 6.15V after a small bit of drain and stays stable for the remainder of their service life. Therefore, whenever replacing the silver oxide PX28 in my Pentax digital spot meter, I ensure it powers the LED display for a couple of dozen readings before relying on it to measure light for photography. That's all it takes to get off the starting bump and onto the flat part of its curve.

 

Photography doesn't require absolute accuracy, but it does demand precision. Those using black and white film with a short toe and relying on this meter to determine low value exposure could easily find their negatives with no shadow density if off by 1/2 EV. Testing the entire imaging chain, then being able to replicate one's process without variation, is critical to repeatable results. I maintain that the Pentax digital spot meter, powered by a silver oxide PX28, remains an optimum way to implement such an approach.

Talk about a zombie thread!!! Stone-cold dead for 21 years...So goes PN, I guess.

Talk about a zombie thread!!! Stone-cold dead for 21 years...So goes PN, I guess.

Thre goes that c_watson again. Your contributions, void of anything but your rather sunny character, are really old.

That these age old meters were discussed ages ago, does not mean that people using them today do not need to have good info on them.

Thre goes that c_watson again. Your contributions, void of anything but your rather sunny character, are really old.

That these age old meters were discussed ages ago, does not mean that people using them today do not need to have good info on them.

I didn't know about the battery voltage problem of the Pentax Digital Spot Meter (Pentax newest meter although it's about 40 years old) because I see it very much sought after in the used market. I can't believe people like a meter which has problem with battery voltage.

...I can't believe people like a meter which has problem with battery voltage.

As with everything in photography, it's a technical matter knowledge and understanding of which is necessary to properly use the device. It's not a "problem."

 

I don't merely "like" my Pentax digital spot meter, I find it far superior to more modern meters that, like many overly complicated electronic devices of today, require their users to "serve" them rather than the other way around. Direct, simple operation is a characteristic of great value.

Edited September 12, 2022 by sal_santamaura

Direct, simple operation is a characteristic of great value.

How is using any spotmeter 'direct and simple'?

 

Ansel Adams took up nearly half a book explaining how to use one.

 

OK; you point it at a shadow area and - assuming that the internal baffling of the meter is adequate, and doesn't allow nearby highlights to contaminate the reading - you get an indicated exposure that's going to make that shadow look mid-grey. Then what? You decide that that particular shadow would look nice 4 stops below your, yet to be decided, mid-tone. So you point the meter at your chosen mid-tone and.. Oh, oh! The mid-tone is actually 5 stops brighter than the shadow you pointed at. Now you need to note that the development needs pulling.. assuming you're not shooting colour... or maybe you can live with that particular shadow being blocked. Decisions, decisions!

 

And you haven't even got around to sensitometrically poking about in the highlights yet.

 

Nor fitting the necessary close-up diopter to read from a nearby object.

 

That doesn't sound very direct and simple to me, even if the meter 'only' requires a special type of battery and a push of a button to operate.

Edited September 29, 2022 by rodeo_joe|1

On 9/29/2022 at 3:52 PM, rodeo_joe1 said:

How is using any spotmeter 'direct and simple'?...That doesn't sound very direct and simple to me...

Use of the Pentax spot meter is, unlike other more modern meters, simple and direct.  It doesn't require "serving" a menu (like user-hostile digital cameras do) or a operating multiple controls.

In any case, this thread is about batteries for a Pentax spot meter, not off-topic distracting discussion about the merits of one type meter vs. others.  Might that be a reason why photo.net long ago ceased being the best place to find large format photography information?

On 9/12/2022 at 9:47 AM, sal_santamaura said:

As with everything in photography, it's a technical matter knowledge and understanding of which is necessary to properly use the device. It's not a "problem."

 

I don't merely "like" my Pentax digital spot meter, I find it far superior to more modern meters that, like many overly complicated electronic devices of today, require their users to "serve" them rather than the other way around. Direct, simple operation is a characteristic of great value.

Almost all modern spotmeter can give you the same one LV reading with one push of the button just like the Pentax. The Pentax doesn't have the ability to convert that to aperture and shutter speed electronically so you have to do it with the dial. But one can always do it in his/her head. 

18 minutes ago, BeBu Lamar said:

Almost all modern spotmeter can give you the same one LV reading with one push of the button just like the Pentax. The Pentax doesn't have the ability to convert that to aperture and shutter speed electronically so you have to do it with the dial. But one can always do it in his/her head. 

It's the physical dial that makes a Pentax digital spot meter so useful to those practicing the zone system.  Intentionally placing light readings of different scene areas on specific zones is aided by the graphic nature of the dial scales.  No "doing it one's head" required.  That's especially intuitive if one attaches a zone label to the meter.

For those who, like me, have owned a Zone VI-modified Pentax digital spot meter since the 1980s, its zone label is likely getting a bit tattered by now.  I recently found this replacement

https://www.ebay.com/itm/153857811592?hash=item23d2a3e488:g:aj0AAOSwlgVeZUQ0

and installed it.  I've no connection with the seller other than as a very satisfied customer.